Big changes coming Lime’s way

Lime hasn’t received a new release version in over a year, even though development has kept going the whole time. As we gear up for the next big release, I wanted to take a look at what’s on the way.

Merged changes

These changes have already been accepted into the develop branch, meaning they’ll be available in the very next release.

Let’s cut right to the chase. We have a bunch of new features coming out:

  • 80f83f6 by joshtynjala ensures that every change submitted to Lime gets tested against Haxe 3. That means everything else in this list is backwards-compatible.
  • #1456 by m0rkeulv, #1465 by me, and openfl#2481 by m0rkeulv enable “streaming” music from a file. That is to say, it’ll only load a fraction of the file into memory at a time, just enough to keep the song going. Great for devices with limited memory!
    • Usage: simply open the sound file with Assets.getMusic() instead of Assets.getSound().
  • #1519 by Apprentice-Alchemist and #1536 by me update Android’s minimum-sdk-version to 21. (SDK 21 equates to Android 5, which is still very old. Only about 1% of devices still use anything older than that.) We’re trying to strike a balance between “supporting every device that ever existed” and “getting the benefit of new features.”
    • Tip: to go back, set <config:android minimum-sdk-version="16" />.
  • #1510 by ninjamuffin99 and Cheemsandfriends adds support for changing audio pitch. Apparently this feature has been missing since OpenFL 4, but now it’s back!
    • Usage: since AS3 never supported pitch, OpenFL probably won’t either. Use Lime’s AudioSource class directly.
  • 81d682d by joshtynjala adds Window.setTextInputRect(), meaning that your text field will remain visible when an onscreen keyboard opens and covers half the app.
  • #1552 by me adds a brand-new JNISafety interface, which helps ensure that when you call Haxe from Java (using JNI), the code runs on the correct thread.
    • Personal story time: back when I started developing for Android, I couldn’t figure out why my apps kept crashing when Java code called Haxe code. In the end I gave up and structured everything to avoid doing that, even at the cost of efficiency. For instance, I wrote my android6permissions library without any callbacks (because that would involve Java calling Haxe). Instead of being able to set an event listener and receive a notification later, you had to actively call hasPermission() over and over (because at least Haxe calling Java didn’t crash). Now thanks to JNISafety, the library finally has the callbacks it always should have had.
  • 2e31ae9 by joshtynjala stores and sends cookies with HTTP requests. Now you can connect to a server or website and have it remember your session.
  • #1509 by me makes Lime better at choosing which icon to use for an app. Previously, if you had both an SVG and an exact-size PNG, there was no way to use the PNG. Nor was there any way for HXP projects to override icons from a Haxelib.
    • Developers: if you’re making a library, set a negative priority to make it easy to override your icon. (Between -1 and -10 should be fine.) If you aren’t making a library and are using HXP, set a positive priority to make your icon override others.

…But that isn’t to say we neglected the bug fixes either:

Pull request #1517 by Apprentice-Alchemist is an enormous change that gets its own section.

For context, HashLink is the new(er) virtual machine for Haxe, intended to replace Neko as the “fast compilation for fast testing” target. While I find compilation to be a little bit slower than Neko, it performs a lot better once compiled.

Prior to now, Lime compiled to HashLink 1.10, which was three years old. Pull request #1517 covers two versions and three years’ worth of updates. From the release notes, we can look forward to:

  • new HashLink CPU Profiler for JIT
  • improved 64 bit support – windows release now 64 bit by default
  • new GC architecture and improvements / bugs fixes
  • support for hot reload
  • better stack primitives for faster haxe throw
  • captured stack for closures (when debugger connected)
  • and more!

(As someone who uses HashLink for rapid testing, I’m always happy to see debugging/profiling improvements.)

Apprentice-Alchemist put a lot of effort into this one, and it shows. Months of testing, responding to CI errors, and making changes in response to feedback.

Perhaps most importantly in the long run, they designed this update to make future updates easier. That paid off on April 28, when HashLink 1.12 came out at 3:21 PM GMT, and Apprentice-Alchemist had the pull request up to date by 5:45!

Pending changes

Lime has several pull requests that are – as of this writing – still in the “request” phase. I expect most of these to get merged, but not necessarily in time for the next release.

Again, let’s start with new features:

…And then the bug fixes:

  • #1529 by arm32x fixes static debug builds on Windows.
  • #1538 by me fixes errors that came up if you didn’t cd to an app’s directory before running it. Now you can run it from wherever you like.
  • #1500 by me modernizes the Android build process. This should be the end of that “deprecated Gradle features were used” warning.

Submodule update

#1531 by me is another enormous change that gets its own section. By far the second biggest pull request I’ve submitted this year.

Lime needs to be able to perform all kinds of tasks, from rendering image files to drawing vector shapes to playing sound to decompressing zip files. It’s a lot to do, but fortunately, great open-source libraries already exist for each task. We can use Cairo for shape drawing, libpng to open PNGs, OpenAL to play sound, and so on.

In the past, Lime has relied on a copy of each of these libraries. For example, we would open up the public SDL repository, download the src/ and include/ folders, and paste their contents into our own repo. Then we’d tell Lime to use our copy as a submodule.

This was… not ideal. Every time we wanted to update, we had to manually download and upload all the files. And if we wanted to try a couple different versions (maybe 1.5 didn’t work and we wanted to see if 1.4 was any better), that’s a whole new download-upload cycle. Plus we sometimes made little customizations unique to our copy repos. Whenever we downloaded fresh copies of the files, we’d have to patch our customizations back in. It’s no wonder no one ever wanted to update the submodules!

If only there was a tool to make this easier. Something that can download files from GitHub and then upload them again. Preferably a tool capable of choosing the correct revision to download, and merging our changes into the downloaded files. I don’t know, some kind of software for controlling versions…

(It’s Git. The answer is Git.)

There was honestly no reason for us to be maintaining our own copy of each repo, much less updating our copies by hand. 20 out of Lime’s 21 submodules have an official public Git repo available, and so I set out to make use of them. It took a lot of refactoring, copying, and debugging. A couple times I had to submit bug reports to the project in question, while other times I was able to work around an issue by setting a specific flag. But I’m pleased to announce that Lime’s submodules now all point to the official repositories rather than some never-updated knockoffs. If we want to use an update, all we have to do is point Lime to the newer commit, and Git will download all the right files.

But I wasn’t quite done. Since it was now so easy to update these libraries, I did so. A couple of them are still a few versions out of date due to factors outside of Lime’s control, but the vast majority are fully up-to-date.

Other improvements that happened along the way:

  • More documentation, so that if anyone else stumbles across the C++ backend, they won’t be as lost as I was.
  • Support for newer Android NDKs. Well, NDK 21 specifically. Pixman’s assembly code prevents us from updating further. (Still better than being stuck with NDK 15 specifically!)
  • Updating zlib fixes a significant security issue (CVE-2018-25032).

Looking forward

OpenFL and Lime are in a state of transition at the moment. We have an expanded leadership team, we’ll be aiming for more frequent releases, and there’s plenty of active discussion of where to go next. Is Lime getting so big that we ought to split it into smaller projects? Maybe! Is it time to stop letting AS3 dictate what OpenFL can and can’t do? Some users say so!

Stay tuned, and we’ll find out soon enough. Or better yet, join the forums or Discord and weigh in!

Status update: winter 2022

In my last status update, I said I’d (1) update Android libraries and then (2) add Infinite Mode to the HTML5 version. As I mentioned in an edit, that first step took a single day. Then the second took… three months. So far.

It all unfolded something like this:

  1. I wanted to add Infinite Mode.
  2. The loading code I used for Explore Mode isn’t up to the task of handling procedurally-generated levels. (Plus I have other plans that require seamless loading.) So I put step 1 on pause in order to write better loading code.
  3. I realized/decided that good loading code needs to support threads. You don’t have to use them, but they should be an option. The good news is, Lime already supports threads. The bad news: not in HTML5. So I put step 2 on hold as I worked to update Lime.
  4. I realized HTML5 threads just aren’t compatible with Lime’s classes, so I took them out and made virtual threads, which are also pretty good.
  5. I stumbled across another way to do HTML5 threads, and realized this new way actually could work in Lime.
  6. I took a bit of a detour, trying to emulate a JavaScript keyword.

Every time I thought I’d reached the bottom of this rabbit hole, I found a way to keep digging. But I’m happy to announce that instead of proceeding with step 6, I’ve turned around and begun the climb back out:

  1. As of today, I finished combining the ideas from steps 3-5 above, which in theory fixes threads in Lime once and for all. I’m sure the community will suggest changes, but I can hopefully shift most of my focus.
  2. Use threads/virtual threads to achieve my vision of flexible and performant loading.
  3. Use this new flexibility to load Infinite Mode’s levels.

I’m also well aware that Run Mobile doesn’t work on Android 12. I’ve been too busy with threads to take a look, but now that I’m (very nearly) done, I really ought to do that next. Update: I did! The fixed version is available on Google Play.

A problem not worth solving

I spent most of Thursday working on a problem. After about ten hours of work, I reverted my changes. Here’s why.

The problem

This past week or so, I’ve been working on a new HTML5Thread” class. One of Lime’s goals is “write once, deploy anywhere,” so I modeled the new class after the existing Thread class. So if you write code based on the Thread class, I want that same code to work with HTML5Thread.

For instance, the Thread class provides the readMessage() function. This can be used to put threads on pause until they’re needed (a useful feature at times), and so I set out to copy the function. In JavaScript, if you want to suspend execution until a message arrives, you use the appropriately-named await keyword.

Just one little problem. The await keyword only works inside an async function, and there’s just no easy way to make those in Haxe. You’d have to modify the JavaScript output file directly, and that’s multiple megabytes of code to sift through.

My solution

At some point I realized that while class functions are difficult, you can make async local functions without too much trouble.

With a workaround in mind, I thought about how best to add it to Lime. As mentioned above, Lime is designed around the assumption that everything should be cross-platform. So if I was going to support async functions in JavaScript, I wanted to support them on all targets.

It would be easy to use: call Async.async() to make an async function, then call Async.await() to wait for something. These functions would then use some combination of JavaScript keywords, Lime Futures, and macro magic to make it happen. You wouldn’t have to worry about the details.

To make a long story short, it almost worked.

The problems with my solution

  1. My solution was too complicated.
  2. My solution wasn’t complicated enough.

My code contained a bunch of little “gotchas”; there were all kinds of things you weren’t allowed to do, and the error message was vague. “Invalid location for a call to Async.await().” Then if you wanted to use the await keyword (which is kind of the point), you had to write JavaScript-specific code (which is against Lime’s design philosophy). Oh, and there were a couple other differences between platforms.

From the user’s point of view, it was too complicated. But the only way to make it simpler would be to add a lot more code under the hood, so in a way, it also wasn’t complicated enough.

Doing this right would take a whole library’s worth of code and documentation, and as it turns out, that library already exists.


In my last post, I quoted Elon Musk saying “First, make your requirements less dumb.” But as I said, I don’t think that’s the right order of operations. I mean, yeah, start out with the best plan you can make. But don’t expect to plan correctly first try. You need perspective before you can get the requirements right, and the easiest way to get perspective is to forge ahead and build the thing.

For instance, when I started this detour, I had no idea there might be valid and invalid locations to call Async.await(). That only occurred to me after I wrote the code, tried experiments like if(Math.random() < 0.5) Async.await(), and saw it all come crashing down. (Actually it wasn’t quite that dramatic. It just ran things out of order.)

Ten hours of work later, I have a lot more perspective. I can see how big this project is, how hard the next steps will be, and how useful users might find it. Putting all that together, I can conclude that this falls firmly in the category of feature creep.

Lime doesn’t need these features at the moment. Not even for my upcoming HTML5Thread class. All that class needs is the async keyword in one specific spot, which only takes eight lines of code. Compare that to my original solution’s 317 lines of code that still weren’t enough!

Basically what I’m saying is, I’m glad I spent the time on this, because I learned worthwhile lessons. I’m also glad I didn’t spend more than a day on it.

Guide to threads in Lime

Disclaimer: this guide focuses on upcoming features, currently only available via pull request.

Concurrent computing

Concurrent computing is a form of computing in which several computations are executed concurrently—during overlapping time periods—instead of sequentially—with one completing before the next starts.

This is a property of a system—whether a program, computer, or a network—where there is a separate execution point or “thread of control” for each process. A concurrent system is one where a computation can advance without waiting for all other computations to complete.

Concurrent computing is a form of modular programming. In its paradigm an overall computation is factored into subcomputations that may be executed concurrently. Pioneers in the field of concurrent computing include Edsger Dijkstra, Per Brinch Hansen, and C.A.R. Hoare.

[Source: English Wikipedia.]

In simpler terms, concurrent execution means two things happen at once. This is great, but how do you do it in OpenFL/Lime?

Choosing the right tool for the job

This guide covers three classes. Lime’s two concurrency classes, and Thread, the standard class they’re based on.

Class Thread Future ThreadPool
Source Haxe Lime Lime
Ease of use ★★★★★ ★★★★☆ ★★★☆☆
Thread safety ★☆☆☆☆ ★★★★☆ ★★★★☆
HTML5 support No Yes Yes

But before you pick a class, first consider whether you should use threads at all.

  • Can you detect any slowdown? If not, threads won’t help, and may even slow things down.
  • How often do your threads interact with the outside world? The more often they transfer information, the slower and less safe they’ll be.

If you have a slow and self-contained task, that’s when you consider using threads.

Demo project

I think a specific example will make this guide easier to follow. Suppose I’m using libnoise to generate textures. I’ve created a feature-complete app, and the core of the code looks something like this:

private function generatePattern(workArea:Rectangle):Void {
    //Allocate four bytes per pixel.
    var bytes:ByteArray = new ByteArray( *;
    //Run getValue() for every pixel.
    for(y in {
        for(x in {
            //getValue() returns a value in the range [-1, 1], and we need
            //to convert to [0, 255].
            var value:Int = + 128 * module.getValue(x, y, 0));
            if(value > 255) {
                value = 255;
            } else if(value < 0) {
                value = 0;
            //Store it as a color.
            bytes.writeInt(value << 16 | value << 8 | value);
    //Draw the pixels to the canvas.
    bytes.position = 0;
    canvas.setPixels(workArea, bytes);

The problem is, this code makes the app lock up. Sometimes for a fraction of a second, sometimes for seconds on end. It all depends on which pattern it’s working on.

(If you have a beefy computer and this looks fine to you, try fullscreen.)

A good user interface responds instantly when the user clicks, rather than locking up. Clearly this app needs improvement, and since the bulk of the work is self-contained, I decide I’ll solve this problem using threads. Now I have two problems.

Using Thread

The easiest option is to use Haxe’s Thread class. Since I know a single function is responsible for the freezing, all I need to do is change how I call that function.

-generatePattern(new Rectangle(0, 0, canvas.width, canvas.height));
+Thread.create(generatePattern.bind(new Rectangle(0, 0, canvas.width, canvas.height)));

View full changes

Thread.create() requires a zero-argument function, so I use bind() to supply the rectangle argument. With that done, create() makes a new thread, and the app no longer freezes.

I’d love to show this in action, but it doesn’t work in HTML5. Sorry.

The downside is, the app now prints a bunch of “null pointer” messages. It turns out I’ve added a race condition.

Thread safety basics

The problem with Haxe’s threads is the fact that they’re just so convenient. You can access any variable from any thread, which is great if you don’t mind all the subtle errors.

My generatePattern() function has two problem variables:

  • module is a class variable, and the main thread updates it with every click. However, generatePattern() assumes module will stay the same the whole time. Worse, module briefly becomes null each time it changes, and that can cause the “null pointer” race condition I mentioned above.
  • canvas is also a class variable, which is modified during generatePattern(). If multiple threads are going at once, it’s possible to modify canvas from two threads simultaneously. canvas is a BitmapData, so I suspect it will merely produce a garbled image. If you do the same to other object types, it could permanently break that object.

Before I go into too much detail, let’s try a simple solution.

-Thread.create(generatePattern.bind(new Rectangle(0, 0, canvas.width, canvas.height)));
+lastCreatedThread = Thread.create(module, generatePattern.bind(new Rectangle(0, 0, canvas.width, canvas.height)));
-private function generatePattern(workArea:Rectangle):Void {
+private function generatePattern(module:ModuleBase, workArea:Rectangle):Void {
    //Allocate four bytes per pixel.
    var bytes:ByteArray = new ByteArray( *;
    //Run getValue() for every pixel.
    for(y in {
        for(x in {
            //getValue() returns a value in the range [-1, 1], and we need
            //to convert to [0, 255].
            var value:Int = + 128 * module.getValue(x, y, 0));
            if(value > 255) {
                value = 255;
            } else if(value < 0) {
                value = 0;
            //Store it as a color.
            bytes.writeInt(value << 16 | value << 8 | value);
+   //If another thread was created after this one, don't draw anything.
+   if(Thread.current() != lastCreatedThread) {
+       return;
+   }
    //Draw the pixels to the canvas.
    bytes.position = 0;
    canvas.setPixels(workArea, bytes);

View full changes

Step one, pass module as an argument. That way, the function won’t be affected when the class variable changes. Step two, enforce a rule that only the last-created thread can modify canvas.

Even then, there’s still at least one theoretical race condition in the above block of code. Can you spot it?

Whether or not you find it isn’t the point I’m trying to make. My point is that thread safety is hard, and you shouldn’t try to achieve it alone. I can spot several types of race condition, and I still don’t trust myself to write perfect code. No, if you want thread safety, you need some guardrails. Tools and design patterns that can take the guesswork out.

My favorite rule of thumb is that every object belongs to one thread, and only that thread may modify that value. And if possible, only that thread should access the value, though that’s less important. Oftentimes, this means making a copy of a value before passing it, so that the receiving thread can own the copy. This rule of thumb means generatePattern() can’t call canvas.setPixels() as shown above, since the main thread owns canvas. Instead, it should send a thread-safe message back and allow the main thread to set the pixels.

And guess what? Lime’s Future and ThreadPool classes provide just the tools you need to do that. In fact, they’re designed as a blueprint for thread-safe code. If you follow the blueprint they offer, and you remember to copy your values when needed, your risk will be vastly reduced.

Using Future

Lime’s Future class is based on the general concept of futures and promises, wherein a “future” represents a value that doesn’t exist yet, but will exist in the future (hence the name).

For instance, BitmapData.loadFromFile() returns a Future<BitmapData>, representing the image that will eventually exist. It’s still loading for now, but if you add an onComplete listener, you’ll get the image as soon as it’s ready.

I want to do pretty much the exact same thing in my sample app, creating a Future<BitmapData> that will wait for the value returned by generatePattern(). For this to work, I need to rewrite generatePattern() so that it actually does return a value.

As discussed under thread safety basics, I want to take both module and workArea as arguments. However, Future limits me to one argument, so I combine my two values into one anonymous structure named state.

-private function generatePattern(workArea:Rectangle):Void {
+private static function generatePattern(state: { module:ModuleBase, workArea:Rectangle }):ByteArray {
    //Allocate four bytes per pixel.
    var bytes:ByteArray = new ByteArray( *;
    //Run getValue() for every pixel.
    for(y in {
        for(x in {
            //getValue() returns a value in the range [-1, 1], and we need
            //to convert to [0, 255].
            var value:Int = + 128 * module.getValue(x, y, 0));
            if(value > 255) {
                value = 255;
            } else if(value < 0) {
                value = 0;
            //Store it as a color.
            bytes.writeInt(value << 16 | value << 8 | value);
-    //Draw the pixels to the canvas.
-    bytes.position = 0;
-    canvas.setPixels(workArea, bytes);
-    bytes.clear();
+    return bytes;

Now I call the function, listen for the return value, and draw the pixels.

-generatePattern(new Rectangle(0, 0, canvas.width, canvas.height));
+future = Future.withEventualValue(generatePattern, { module: module, workArea: new Rectangle(0, 0, canvas.width, canvas.height) }, MULTI_THREADED);
+//Store a copy of future at this point in time.
+var expectedFuture:Future<ByteArray> = future;
+//Add a listener for later.
+future.onComplete(function(bytes:ByteArray):Void {
+   //If another thread was created after this one, don't draw anything.
+   if(future != expectedFuture) {
+       return;
+   }
+   //Draw the pixels to the canvas.
+   bytes.position = 0;
+   canvas.setPixels(new Rectangle(0, 0, canvas.width, canvas.height), bytes);
+   bytes.clear();

View full changes

This event listener always runs on the main thread, meaning only the main thread ever updates canvas, which is super helpful for thread safety. I still check whether another thread was created, but that’s only to make sure I’m drawing the right image, not because there’s a risk of two being drawn at once.

And this time, I can show you an HTML5 demo! Thanks to the use of threads, the app responds instantly after every click.

I should probably also mention that I set Future.FutureWork.maxThreads = 2. This means you can have two threads running at once, but any more will have to wait. Click enough times in a row, and even fast patterns will become slow. Not because they themselves slowed down, but because they’re at the back of the line. The app has to finish calculating all the previous patterns first.

(If the problem isn’t obvious from the small demo, try fullscreen.)

This seems pretty impractical. Why would the app spend all this time calculating the old patterns when it knows it won’t display them? Well, the reason is that you can’t cancel a Future once started. For that, and for other advanced features, you want to use ThreadPool directly instead of indirectly.

Oh yeah, did I mention that Future is built on top of ThreadPool? Hang on while I go check. …Apparently I never mentioned it. Well, Future is built on top of ThreadPool. It tries to provide the same features in a more convenient way, but doesn’t provide all the features. Canceling jobs, sending progress updates,

Using ThreadPool

Thread pools are a common way to make threads more efficient. It takes time to start up and shut down a thread, so why not reuse it instead? Lime’s ThreadPool class follows this basic pattern, though it prioritizes cross-platform compatibility, thread safety, and ease of use over performance.

When using ThreadPool, you’ll also need to be aware of its parent class, WorkOutput, as that’s your ticket to thread-safe message transfer. You’ll receive a WorkOutput instance as an argument (with the benefit that it can’t become null unexpectedly), and it has all the methods you need for communication.

sendComplete() and sendError() convey that your job succeeded/failed. When you call one of them, ThreadPool dispatches onComplete or onError as appropriate, and then initiates the thread recycling process. Don’t call them if you aren’t done!

sendProgress() works differently: you can call it as much as you like, with whatever type of data you like. It has no special meaning other than what you come up with. Unsurprisingly, sendProgress() corresponds to onProgress.

generatePattern() only needs sendComplete(), at least for now.

-private function generatePattern(workArea:Rectangle):Void {
+private static function generatePattern(state: { module:ModuleBase, workArea:Rectangle }, output:WorkOutput):Void {
    //Allocate four bytes per pixel.
    var bytes:ByteArray = new ByteArray( *;
    //Run getValue() for every pixel.
    for(y in {
        for(x in {
            //getValue() returns a value in the range [-1, 1], and we need
            //to convert to [0, 255].
            var value:Int = + 128 * module.getValue(x, y, 0));
            if(value > 255) {
                value = 255;
            } else if(value < 0) {
                value = 0;
            //Store it as a color.
            bytes.writeInt(value << 16 | value << 8 | value);
-   //Draw the pixels to the canvas.
-   bytes.position = 0;
-   canvas.setPixels(workArea, bytes);
-   bytes.clear();
+   output.sendComplete(bytes, [bytes]);

Hmm, what’s up with “sendComplete(bytes, [bytes])“? Looks kind of redundant.

Well, each of the “send” functions takes an optional array argument that improves performance in HTML5. It’s great for transferring ByteArrays and similar packed data containers, but be aware that these containers will become totally unusable. That’s no problem at the end of the function, but be careful if using this with sendProgress().

With generatePattern() updated, the next step to initialize my ThreadPool.

//minThreads = 1, maxThreads = 1.
threadPool = new ThreadPool(1, 1, MULTI_THREADED);
threadPool.onComplete.add(function(bytes:ByteArray):Void {
    //Draw the pixels to the canvas.
    bytes.position = 0;
    canvas.setPixels(new Rectangle(0, 0, canvas.width, canvas.height), bytes);

This time, I didn’t include a “latest thread” check. Instead, I plan to cancel old jobs, ensuring that they never dispatch an onComplete event at all.

-generatePattern(new Rectangle(0, 0, canvas.width, canvas.height));
+jobID =, { module: module, workArea: new Rectangle(0, 0, canvas.width, canvas.height) });

This works well enough in the simplest case, but the full app actually isn’t this simple. The full app actually has several classes listening for events, and they all receive each other’s events. To solve this, they each have to filter.

Allow me to direct your attention to ThreadPool.activeJob. This variable is made available specifically during onComplete, onError, or onProgress events, and it tells you where the event came from.

threadPool.onComplete.add(function(bytes:ByteArray):Void {
+   if( != jobID) {
+       return;
+   }
    //Draw the pixels to the canvas.
    bytes.position = 0;
    canvas.setPixels(new Rectangle(0, 0, canvas.width, canvas.height), bytes);

View full changes

Now, let’s see how the demo looks.

It turns out, setting maxThreads = 1 was a bad idea. Even calling cancelJob() isn’t enough: the app still waits to finish the current job before starting the next. (As before, viewing in fullscreen may make the problem more obvious.)

When a function has already started, cancelJob() does two things: (1) it bans the function call from dispatching events, and (2) it politely encourages the function to exit. There’s no way to force it to stop, so polite requests are all we get. If only generatePattern() was more cooperative.

Virtual threads

Also known as green threads for historical reasons, virtual threads are what happens when you want thread-like behavior in a single-threaded environment.

As it happens, it was JavaScript’s definition of “async” that gave me the idea for this feature. JavaScript’s async keyword runs a function right on the main thread, but sometimes puts that function on pause to let other functions run. Only one thing ever runs at once, but since they take turns, it still makes sense to call them “asynchronous” or “concurrent.”

Most platforms don’t support anything like the async keyword, but we can imitate the behavior by exiting the function and starting it again later. Doesn’t sound very convenient, but unlike some things I tried, it’s simple, it’s reliable, and it works on every platform.

Exiting and restarting forms the basis for Lime’s virtual threads: instead of running a function on a background thread, run a small bit of that function each frame. The function is responsible for returning after a brief period, because if it takes too long the app won’t be able to draw the next frame in time. Then ThreadPool or FutureWork is responsible for scheduling it again, so it can continue. This behavior is also known as “cooperative multitasking” – multitasking made possible by functions voluntarily passing control to one another.

Here’s an outline for a cooperative function.

  1. The first time the function is called, it performs initialization and does a little work.
  2. By the end of the call, it stores its progress for later.
  3. When the function is called again, it checks for stored progress and determines that this isn’t the first call. Using this stored data, it continues from where it left off, doing a little more work. Then it stores the new data and exits again.
  4. Step 3 repeats until the function detects an end point. Then it calls sendComplete() or (if using Future) returns a non-null value.
  5. ThreadPool or FutureWork stops calling the function, and dispatches the onComplete event.

This leaves the question of where you should store that data. In single-threaded mode, you can put it wherever you like. However, this type of cooperation is also useful in multi-threaded mode so that functions can be canceled, and storing data in class variables isn’t always thread safe. Instead, I recommend using the state argument. Which is, incidentally, why I like to call it “state.” It provides the initial input and stores progress.

Typically, state will have some mandatory values (supplied by the caller) and some optional ones (initialized and updated by the function itself). If the optional ones are missing, that indicates it’s the first iteration.

-private static function generatePattern(state: { module:ModuleBase, workArea:Rectangle }, output:WorkOutput):Void {
+private static function generatePattern(state: { module:ModuleBase, workArea:Rectangle, ?y:Int, ?bytes:ByteArray }, output:WorkOutput):Void {
-   //Allocate four bytes per pixel.
-   var bytes:ByteArray = new ByteArray(
- *;
+   var bytes:ByteArray = state.bytes;
+   //If it's the first iteration, initialize the optional values.
+   if(bytes == null) {
+       //Allocate four bytes per pixel.
+       state.bytes = bytes = new ByteArray(
+  *;
+       state.y =;
+   }
+   //Each iteration, determine how much work to do.
+   var endY:Int = state.y + (output.mode == MULTI_THREADED ? 50 : 5);
+   if(endY > {
+       endY =;
+   }
    //Run getValue() for every pixel.
-    for(y in {
+   for(y in state.y...endY) {
        for(x in {
            //getValue() returns a value in the range [-1, 1], and we need
            //to convert to [0, 255].
            var value:Int = + 128 * module.getValue(x, y, 0));
            if(value > 255) {
                value = 255;
            } else if(value < 0) {
                value = 0;
            //Store it as a color.
            bytes.writeInt(value << 16 | value << 8 | value);
+   //Save progress.
+   state.y = endY;
+   //Don't call sendComplete() until actually done.
+   if(state.y >= {
        output.sendComplete(bytes, [bytes]);
+   }

Note that I do more work per iteration in multi-threaded mode. There’s no need to return too often; just often enough to exit if the job’s been canceled. It also incurs overhead in HTML5, so it’s best not to overdo it.

Single-threaded mode is the polar opposite. There’s minimal overhead, and you get better timing if the function is very short. Ideally, short enough to run 5+ times a frame with time left over. On a slow computer, it’ll automatically reduce the number of times per frame to prevent lag.

Next, I tell ThreadPool to use single-threaded mode, and I specify a value of 3/4. This value indicates what fraction of the main thread’s processing power should be spent on this ThreadPool. I’ve elected to take up 75% of it, leaving 25% for other tasks. Since I know those other tasks aren’t very intense, this is plenty.

-threadPool = new ThreadPool(1, 1, MULTI_THREADED);
+threadPool = new ThreadPool(1, 1, SINGLE_THREADED, 3/4);

View full changes

Caution: reduce this number if creating multiple single-threaded ThreadPools. workLoads from different pools add together, and can easily add up to 1 or more. That means 100% (or more) of the available time each frame gets spent on virtual threads, slowing the app down.

In any case, it’s time for another copy of the demo. Since we’re nearing the end, I also went ahead and implemented progress events. Now you can watch the progress in (closer to) real time.

These changes also benefit multi-threaded mode, so I created another multi-threaded version for comparison. With progress events, you can now see the slight pause when it spins up a new web worker (which isn’t that often, since it keeps two of them running).

(For comparison, here they both are in fullscreen: virtual threads, web workers.)

I don’t know, I like them both. Virtual threads have the benefit of being lighter weight, while web workers have the benefit of being real threads, meaning you could run eight in parallel without slowing the main thread.

My advice? Write code that works both ways, as shown in this guide. Keep your options open, since the configuration that works best for a small app may not be what works best for a big one. Good luck out there!

Web Workers in Lime

If you haven’t already read my guide to threads, I suggest starting there.

I’ve spent the last month implementing web worker support in Lime. (Edit: and then I spent another month after posting this.) It turned out to be incredibly complicated, and though I did my best to include documentation in the code, I think it’s worth a blog post too. Let’s go over what web workers are, why you might want to use them, and why you might not want to use them.

To save space, I’m going to assume you’ve heard of threads, race conditions, and threads in Haxe.

About BackgroundWorker and ThreadPool

BackgroundWorker and ThreadPool are Lime’s two classes for safely managing threads. They were added back in 2015, and have stayed largely unchanged since. (Until this past month, but I’ll get to that.)

The two classes fill different roles. BackgroundWorker is ideal for one-off jobs, while ThreadPool is a bit more complex but offers performance benefits when doing multiple jobs in a row.

BackgroundWorker isn’t too different from calling Thread.create() – both make a thread and run a single job. The main difference is that BackgroundWorker builds in safety features.

Recently, Haxe added its own thread pool implementations: FixedThreadPool has a constant number of threads, while ElasticThreadPool tries to add and remove threads based on demand. Lime’s ThreadPool does a combination of the two: you can set the minimum and maximum number of threads, and it will vary within that range based on demand. Plus it offers structure and safety features, just like BackgroundWorker. On the other hand, ThreadPool lacks ElasticThreadPool‘s threadTimeout feature, so threads will exit instantly if they don’t have a job to do.

I always hate reinventing the wheel. Why does Lime need a ThreadPool class when Haxe already offers two? (Ignoring the fact that Lime’s came first.) Just because of thread safety? There are other ways to achieve that.

If only Haxe’s thread pools worked in JavaScript…

Web workers

Mozilla describes web workers as “a simple means for web content to run scripts in background threads.” “Simple” is a matter of perspective, but they do allow you to create background threads in JavaScript.

Problem is, they have two fundamental differences from Haxe’s threads, which is why Haxe doesn’t include them in ElasticThreadPool and FixedThreadPool.

  • Web workers use source code.
  • Web workers are isolated.

Workers use source code

Web workers execute a JavaScript file, not a JavaScript function. Fortunately, it is usually possible to turn a function back into source code, simply by calling toString(). Usually. Let’s start with how this works in pure JavaScript:

function add(a, b) {
    return a + b;

console.log(add(1, 2)); //Output: 3
console.log(add.toString()); //Output:
//function add(a, b) {
//    return a + b;

That first log() call is just to show the function working. The second shows that we get the function source code as a string. It even preserved our formatting!

If we look at the examples, we find that it goes to great lengths to preserve the original formatting.

toString() input toString() output
function f(){} "function f(){}"
class A { a(){} } "class A { a(){} }"
function* g(){} "function* g(){}"
a => a "a => a"
({ a(){} }.a) "a(){}"
({ [0](){} }[0]) "[0](){}"
Object.getOwnPropertyDescriptor({ get a(){} }, "a").get "get a(){}"
Object.getOwnPropertyDescriptor({ set a(x){} }, "a").set "set a(x){}"
Function.prototype.toString "function toString() { [native code] }"
(function f(){}.bind(0)) "function () { [native code] }"
Function("a", "b") "function anonymous(a\n) {\nb\n}"

That’s weird. In two of those cases, the function body – the meat of the code – has been replaced with “[native code]”. (That isn’t even valid JavaScript!) As the documentation explains:

If the toString() method is called on built-in function objects or a function created by Function.prototype.bind, toString() returns a native function string

In other words, if we ever call bind() on a function, we can’t get its source code, meaning we can’t use it in a web worker. And wouldn’t you know it, Haxe automatically calls bind() on certain functions.

Let’s try writing some Haxe code to call toString(). Ideally, we want to write a function in Haxe, have Haxe translate it to JavaScript, and then get its JavaScript source code.

class Test {
    static function staticAdd(a, b) {
        return a + b;
    function add(a, b) {
        return a + b;
    static function main() {
        var instance = new Test();
        trace(staticAdd(1, 2));
        trace(instance.add(2, 3));
        #if js
        trace((cast staticAdd).toString());
        trace((cast instance.add).toString());
    inline function new() {}

If you try this code, you’ll get the following output:

Test.hx:15: 3
Test.hx:16: 5
Test.hx:18: staticAdd(a,b) {
        return a + b;
Test.hx:19: function() {
    [native code]

The first two lines prove that both functions work just fine. staticAdd is printed exactly like it appears in the JavaScript file. But instance.add is all wrong. Let’s look at the JS source to see why:

static main() {
    let instance = new Test();

Yep, there it is. Haxe inserted a call to $bind(), a function that – perhaps unsurprisingly – calls bind().

Turns out, Haxe always inserts $bind() when you try to refer to an instance function. This is in fact required: otherwise, the function couldn’t access the instance it came from. But it also means we can’t use instance functions in web workers. Or can we?

After a lot of frustration and effort, I came up with ThreadFunction. Read the source if you want details; otherwise, the one thing to understand is that it can only remove the $bind() call if you convert to ThreadFunction ASAP. If you have a variable (or function argument) representing a function, that variable (or argument) must be of type ThreadFunction.

//Instead of this...
class DoesNotWork {
    public var threadFunction:Dynamic -> Void;
    public function new(threadFunction:Dynamic -> Void) {
        this.threadFunction = threadFunction;
    public function runThread():Void {
        new BackgroundWorker().run(threadFunction);

// want to do this.
class DoesWork {
    public var threadFunction:ThreadFunction<Dynamic -> Void>;
    public function new(threadFunction:ThreadFunction<Dynamic -> Void>) {
        this.threadFunction = threadFunction;
    public function runThread():Void {
        new BackgroundWorker().run(threadFunction);

class Main {
    private static function main():Void {
        new DoesWork(test).runThread(); //Success
        new DoesNotWork(test).runThread(); //Error
    private static function test(_):Void {
        trace("Hello from a background thread!");

Workers are isolated

Once we have our source code, creating a worker is simple. We take the string and add some boilerplate code, then construct a Blob out of this code, then create a URL for the blob, then create a worker for that URL, then send a message to the worker to make it start running. Or maybe it isn’t so simple, but it does work.

Web workers execute a JavaScript source file. The code in the file can only access other code in that file, plus a small number of specific functions and classes. But most of your app resides in the main JS file, and is off-limits to workers.

This is in stark contrast to Haxe’s threads, which can access anything. Classes, functions, variables, you name it. Sharing memory like this does of course allow for race conditions, but as mentioned above, BackgroundWorker and ThreadPool help prevent those.

For a simple example:

class Main {
    private static var luckyNumber:Float;
    private static function main():Void {
        luckyNumber = Math.random() * 777;
        new BackgroundWorker().run(test);
    private static function test(_):Void {
        trace("Hello from a background thread!");
        trace("Your lucky number is: " + luckyNumber);

On most targets, any thread can access the Main.luckyNumber variable, so test() will work. But in JavaScript, neither Main nor luckyNumber will have been defined in the worker’s file. And even if they were defined in that file, they’d just be copies. The value will be wrong, and the main thread won’t receive any changes made.

So… how do you transfer data?

Passing messages

I’ve glossed over this so far, but takes up to two arguments. The first, of course, is the ThreadFunction to run. The second is a message to pass to that function, which can be any type. (And if you need multiple values, you can pass an array.)

Originally, BackgroundWorker was designed to be run multiple times, each time reusing the same function but working on a new set of data. It wasn’t well-optimized (ThreadPool is much more appropriate for that) nor well-tested, but it was very convenient for implementing web workers.

See, web workers also have a message-passing protocol, allowing us to send an object to the background thread. You know, an object like‘s second argument:

class Main {
    private static var luckyNumber:Float;
    private static function main():Void {
        luckyNumber = Math.random() * 777;
        new BackgroundWorker().run(test, luckyNumber);
    private static function test(luckyNumber:Float):Void {
        trace("Hello from a background thread!");
        trace("Your lucky number is: " + luckyNumber);

The trick is, instead of trying to access Main.luckyNumber (which is on the main thread), test() takes an argument, which is the same value except copied to the worker thread. You can actually transfer a lot of data this way:

new BackgroundWorker().run(test, {
    luckyNumber: Math.random() * 777,
    imageURL: "",
    cakeRecipe: File.getContent("cake.txt"),
    calendar: Calendar.getUpcomingEvents(10)

Bear in mind that your message will be copied using the structured clone algorithm, a deep copy algorithm that cannot copy functions. This sets limits on what kinds of messages you can pass. You can’t pass a function without first converting it to ThreadFunction, nor can you pass an object that contains functions, such as a class instance.

Copying your message is key to how JavaScript prevents race conditions: memory is never shared between threads, so two threads can’t accidentally access the same memory location at the wrong time. But if there’s no sharing, how does the main thread get any information back from the worker?

Returning results

Web workers don’t just receive messages, they can send them back. The rules are the same: everything is copied, no functions, etc.

The BackgroundWorker class provides three functions for this, each representing something different. sendProgress() for status updates, sendError() if something goes horribly wrong, and sendComplete() for the final product. (You may recall that workers don’t normally have access to Haxe functions, but these three are inlined. Inline functions work fine.)

It’s at about this point we need to talk about another problem with copying data. One common reason to use background threads is to process large amounts of data. Suppose you produce 10 MB of data, and you want to pass it back once finished. Your computer is going to have to make an exact copy of all that data, and it’ll end up taking 20 MB in all. Don’t get me wrong, it’s doable, but it’s hardly ideal.

It’s possible to save both time and memory using transferable objects. If you’ve stored your data in an ArrayBuffer, you can simply pass a reference back to the main thread, no copying required. The worker thread loses access to it, and then the main thread gains access (because unlike Haxe, JavaScript is very strict about sharing memory).

ArrayBuffer can be annoying to use on its own, so it’s fortunate that all the wrappers are natively available. By “wrappers,” I’m talking about Float32Array, Int16Array, UInt8Array, and so on. As long as you can represent your data as a sequence of numbers, you should be able to find a matching wrapper.

Transferring a buffer looks like this: backgroundWorker.sendComplete(buffer, [buffer]). I know that looks redundant, and at first I thought maybe backgroundWorker.sendComplete(null, [buffer]) could work instead. But the trick is, the main thread will only receive the first argument (a.k.a. the message). If the message doesn’t contain some kind of reference to buffer, then the main thread won’t have any way to access buffer.

But the two arguments don’t have to be identical. You can pass a wrapper (e.g., an Int16Array) as the message, and transfer the buffer inside: backgroundWorker.sendComplete(int16Array, [int16Array.buffer]). The Int16Array numeric properties (byteLength, byteOffset, and length) will be copied, but the underlying buffer will be moved instead.

Great! You now know how to transfer an ArrayBuffer. But what if you have your own class instances you want to use? It’s time to take another look at that.

Using classes in web workers

(Still working on this part; check back later.)

Supporting 64-bit devices

When I left off last week, I was told that I needed to upload my app in app bundle format, instead of APK format. The documentation there may seem intimidating, but clicking through the links eventually brought me to instructions for building an app bundle with Gradle. (There are other ways to do it, but I’m already using Gradle.) It’s as simple as swapping the command I give to Gradle – instead of assembleRelease, I run bundleRelease. And it seems to work. At least, Google Play accepts the bundle.

But then Google gives me another error. I’ve created a 32-bit app, and from now on Google requires 64-bit support. I do like 64-bit code in theory, but at this stage it’s also kind of scary. I’ll need to mess with the C++ compile process, which I’m not familiar with. And I’m stuck with old versions of Lime and hxcpp, so even if they’ve added 64-bit support, I can’t use that.

Initially, I got a bit of false hope, as the documentation says “Enabling builds for your native code is as simple as adding the arm64-v8a and/or x86_64, depending on the architecture(s) you wish to support, to the ndk.abiFilters setting in your app’s ‘build.gradle’ file.” I did that, and it seemed to work. It compiled and uploaded, at least, but it turned out the app wouldn’t start because it couldn’t find

I knew I’d seen that error ages ago, but wasn’t sure where. Eventually after a lot of trial and error, I tracked it down to the ndk.abiFilters setting. Yep, that was it. Attempting to support 64-bit devices just breaks it for everybody, and the reason is that I don’t actually have any 64-bit shared libraries (a.k.a. .so files). This means I need to:

  1. Track down all the shared libraries the app uses.
  2. Compile 64-bit versions of each one.
  3. Include them in the build.

And I have only about a week to do it.

Tracking down shared libraries

The 32-bit app ends up using seven shared libraries:,,,,,, and

Three of them (regexp, std, and zlib) are from hxcpp, located in hxcpp’s bin/Android folder. lime-legacy is from Lime, naturally, and is found in Lime’s lime-private/ndll/Android folder. ApplicationMain is my own code, and is found inside my project’s bin/android/obj folder. From each of these locations, the shared libraries are copied into my Android project, specifically into app/src/main/jniLibs/armeabi.

The “adcolony” and “js” libraries are slightly different. Both of those are downloaded during when Gradle compiles the Android app. Obviously the former is from AdColony, and I think the latter is too. Since both have 64-bit versions, I don’t think I need to worry about them.

Interestingly, Lime already has a few different versions of, but no 64-bit version. If I had a 64-bit version, it would go in app/src/main/jniLibs/arm64-v8a, where Gradle will look for it. (Though since I’m using an older Gradle plugin than 4.0, I may have to figure out how to include CMake IMPORTED targets, whatever that means.)

As far as I can tell, that’s a complete list. C++ is the main problem when dealing with 32- and 64-bit support. On Android, C++ code has to go in a shared object. All shared objects go inside the lib folder in an APK, and the above is a complete list of the contents of my app’s lib folder. So that’s everything. I hope.

Compiling hxcpp’s libraries

How you compile a shared library varies depending on where it’s from, so I’ll take them one at a time.

I looked at hxcpp first, and found specific instructions. Run neko build.n android from a certain folder to recompile the shared libraries. This created several versions of,, and, including 64-bit versions. Almost no effort required.

Next, I got started working on, but it took a while. Eventually, I realized I needed to test a simple hypothesis before I wasted too much time. Let’s review the facts:

  • When I compile for 32-bit devices only, everything works. Among other things, is found.
  • When I compile for 32-bit and 64-bit devices, it breaks. is not found.
  • Even though it can’t be found, is present in the APK, inside lib/armeabi. (That’s the folder with code for 32-bit devices.)
  • lib/arm64-v8a (the one for 64-bit devices) contains only and

My hypothesis: because the arm64-v8a folder exists, my device looks only in there and ignores armeabi. If I put there, the app should find it. If not, then I’m not going to be able to use either.

Test #1: The 64-bit version of is (Unsurprisingly.) Let’s add it to the app under that name. I don’t think this will work, but I can at least make sure it ends up in the APK. Result: made it into the APK, and then the app crashed because it couldn’t find

Test #2: Actually name it the correct thing. This is the moment of truth: when the app crashes (because it will crash), what will the error message be? Result: made it into the APK, and then the app crashed because it couldn’t find Success! That means it found the library I added.

Test #3: Add and This test isn’t so important, but I have the files sitting around, so may as well see what happens. My guess is, is next. Result: could not find, as I guessed.

(For the record, I’m not doing any of this the “right” way, which means if I clear my build folder or switch to another machine, it’ll stop working. But I’ll get to that later.)

Compiling Lime’s library

Like hxcpp, Lime comes with instructions, but unlike hxcpp, they didn’t work first try. From the documentation you’d think lime rebuild android -64 would do it, but that’s for Intel processors (Android typically uses Arm). So the correct command is lime rebuild android -arm64, but even that doesn’t work.

Turns out, AndroidPlatform only compiles for three specific 32-bit architectures, and ignores any others you request. I’m going to need to add a 64-bit option there.

Let’s jump forwards in time and see what the latest version of AndroidPlatform looks like. …What do you know, it now supports 64-bit architectures. Better yet, the rest of the code is practically unchanged (they renamed a class, but that’s about it). Since it’s so similar, I should be able to copy over the new code, adjusting only the class name. Let’s give that a try…

…and I figured out why the 64-bit option wasn’t included yet. The compiler immediately crashes with a message that it can’t find stdint.h. Oh, and the error occurred inside the stdint.h file. So it went looking for stdint.h, and found it, but then stdint.h told it to find stdint.h, and it couldn’t find stdint.h. Makes sense, right?

According to the tech support cheat sheet, what you do is search the web for a few words related to the problem, then follow any advice. When I did, I found someone who had the same bug (including the error message pointing to stdint.h), and the accepted solution was to target Android 21 because that’s the first one that supports 64-bit. Following that advice, I did a find and replace, searching all of Lime’s files for “android-9” and replacing with “android-21”. And it worked.

As expected, fixing one problem just exposed another. I got an error about how casting from a pointer to int loses precision. I’m certain this is only the first of many, many similar errors, since all this code was designed around 32 bit pointers. It should be fixable, in one of several ways. As an example of a bad way to fix it, I tried changing int to long. A long can hold a 64-bit pointer, but it’s overkill on 32-bit devices, and it’s even possible that the mismatch would cause subtle errors.

But hey, with that change, the compile process succeeded. Much to my surprise. I was expecting an endless stream of errors from all the different parts of the code that aren’t 64-bit-compatible, but apparently those all turned into warnings, so I got them all at once instead of one at a time. These warnings ended up falling into three distinct groups.

  • Three warnings came from Lime-specific code. After consulting the modern version of this code, I made some educated guesses about how to proceed. First, cast values to intptr_t instead of int, because the former will automatically adjust for 64 bits. (Actually I went with uintptr_t, but it probably doesn’t matter.) Second, when the pointer is passed to Haxe code, pass it as a Float value, because in Haxe that’s a 64-bit value. Third, acknowledge that step 2 was very weird and proceed anyway, hoping it doesn’t matter.
  • A large number of warnings came from OpenAL (an open-source audio library, much like how OpenGL is an open-source graphics library and OpenFL is an open-source Flash library). I was worried that I’d have to fix them all by hand, but eventually I stumbled across a variable to toggle 32- vs. 64-bit compilation. But luckily, the library already supported 64 bits, and I just had to enable it. (Much safer than letting me implement it.)
  • cURL produced one warning – apparently it truncates a pointer to use as a random seed. I don’t know if that’s a good idea, but I do know the warning is irrelevant. srand works equally well if you give it a full 32-bit pointer or half of a 64-bit pointer.

Ignoring the cURL warning, the build proceeded smoothly. Four down, one to go.

Copying files the correct way

As I mentioned earlier, I copied hxcpp’s libraries by hand, which is a temporary measure. The correct way to copy them into the app is through Lime, specifically AndroidPlatform.hx. Like last time I mentioned that file, this version only supports 32-bit architectures, but the latest version supports more. Like before, my plan is copy the new version of the function, and make a few updates so it matches the old.

Then hit compile, and if all goes well, it should copy over the 64-bit version of the four shared libraries I’ve spent all week creating. And if I’m extra lucky, they’ll even make it into the APK. Fingers crossed, compiling now…

Compilation done. Let’s see the results. In the Android project, we look under jniLibs/arm64-v8a, and find:


Hey, cool, five out of four libraries were copied successfully. (Surprise!)

I might’ve glossed over this at the start of this section, but AndroidPlatform.hx is what compiles When I enabled the arm64 architecture to make it copy all the libraries, it also compiled my Haxe code for 64 bits and copied that in. On the first try, too.

Hey look at that, I have what should be a complete APK. Time to install it. Results: it works! The main menu shows up (itself a huge step), and not only that, I successfully started the game and played for a few seconds.

More audio problems

And then it crashed when I turned the music on, because apparently OpenAL still doesn’t work. There was a stack trace showing where the null pointer error happened, but I had to dig around some more to figure out where the variable was supposed to be set. (It was actually in a totally different class, and that class had printed an error message but I’d ignored the error message because it happened well before the crash.)

Anyway, the problem was it couldn’t open, even though that library does exist on the device. And dlerror() returned nothing, so I was stumped for a while. I wrote a quick if statement to keep it from crashing, and resigned myself to a silent game for the time being.

After sleeping on it, I poked around a little more. Tried several things without making progress, but then I had the idea to try loading the library in Java. Maybe Java could give me a better error message. And it did! “dlopen failed: "/system/lib/" is 32-bit instead of 64-bit.” Wait, why does my 64-bit phone not have 64-bit libraries? Let me take another look… aha, there’s also a /system/lib64 folder containing another copy of I bet that’s the one I want. Results: it now loads the library, but it freezes when it tries to play sound.

It didn’t take too long to track the freeze down to a threading issue. It took a little longer to figure out why it refused to create this particular thread. It works fine in the 32-bit version, but apparently “round robin” scheduling mode is disallowed in 64-bit code. Worse, there’s very little information to be found online, and what little there is seems to be for desktop machines with Intel processors, not Android devices with Arm processors. My solution: use the default scheduling mode/priority instead of round robin + max priority. This seems to work on my device, and the quality seems unaffected. Hopefully it holds up on lower-end devices too.


And that’s about it for this post. It was a lot of work, and I’m very glad it came together in the end.

Looking back… very little of this code will be useful in the future. These are stopgap measures until Runaway catches up. Once that happens, I can use the latest versions of Lime and OpenFL, which support 64-bit apps (and do a better job of it than I did here). I will be happy once I can consign this code to the archives and never deal with it again.


Work like this is never wasted. The code may not be widely useful, but I’ve learned a lot about Android, C++, and how the two interact. I’ve learned about Lime, too, digging deeper into its build process, its native code, and several of its submodules. (Which will definitely come in handy because I’m one of the ones responsible for its future.)

The app is just about ready for release, with about a week to spare. But I still have a couple things to tidy up, and Kongregate would like some more time to test, so we’re going to aim for the middle of next week, still giving us a few days to spare.

Updating the Google Play Billing Library

As I’ve mentioned a couple posts ago, I need to update this specific library by November. Google was nice enough to provide a migration guide to help with the process, though due to my own setup, it isn’t quite as easy as that.

As I start writing this post, I’ve just spent several hours restructuring Java code, and I anticipate several more. I’ve been tabbing back and forth between the guide linked above and the API documentation, going back and forth as to class structure. I’m trying to track in-app purchases (you know, the ones bought with actual money), but this is made harder by the fact that Google doesn’t, you know, track those purchases.

The problem with purchases on Google Play

Let me introduce you to the concept of “consuming” purchases. On the Play Store, each in-app purchase has two states: “owned” and “not owned.” If owned, you can’t buy it again. This is a problem for repeatable purchases (like in Run Mobile), because you’re supposed to be able to buy each option as much as you like. Google’s solution is this: games should record the purchase, then “consume” it, setting it back to “not owned” and allowing the player to buy it again.

I know for a fact Google keeps track of how many times each player has purchased each item, but that information is not available to the game. I get a distressingly high number of reports that people lost their data; would’ve been nice to have a backup, but nope.

There’s one upside. In the new version of the library, Google allows you to look up the last time each item was bought. So that’s a partial backup; even if all the other data was deleted, the game can award that one purchase. (Actually up to four purchases, one per price point.) That’ll be the plan, anyway.

The problem with templates

Templates are a neat little feature offered by Haxe and enhanced by Lime. As I’ve mentioned before, Lime doesn’t create Android apps directly; it actually creates an Android project and allows the official Android SDK to build the app. Normally, Lime has its own set of templates to build the project from, but you can set your own if you prefer.

That’s how I write Java code. I write Java files and then tell Lime “these are templates; copy them into the project.” Now, Lime doesn’t do a direct copy; it actually does some processing first. This processing step means I get to adjust settings. Sometimes I mark code as being only for debug builds, or only for the Amazon version of the app. (Yeah, it’s on Amazon.)

Essentially, this functions as conditional compilation, which is a feature I use extensively in Haxe code. When I saw the opportunity to do the same thing in Java, I jumped on it.

Problem is, the template files are not (quite) valid Java code. This makes most code helpers nigh-unusable. Well, as far as I know. Since I never expected any to work, I didn’t try very hard to make it happen. Instead, I just coded without any quality of life features (code completion, instant error checking, etc.). Guess what happens when you don’t have quality of life features? Yep, your life quality suffers.

Use the best tools for each part: Xcode to make iOS code, Android Studio to make Android code and VSCode (or your other favourite Haxe editor) to make Haxe code

—Some quality advice that I never followed.

You know, I’ve always hated working on the “mobile” parts of Run Mobile. I’d do it, but only reluctantly, and it’d always be slow going, no matter how simple and straightforward everything seemed on paper. When I was done and could get back to Haxe, it’d feel like a weight off my chest. In retrospect, I think the lack of code completion was a big part of it. (The other part being that billing is outside my comfort zone.)

I’m not going to change up my development process just yet. There’s too much riding on me getting this done, and not nearly enough time to change my development process. But eventually, I’m going to see about writing the Java code in Android Studio.


After even more hours of work, I’ve started to get the hang of the new library, rewritten hundreds of lines of code, and fixed a few dozen compile errors. I’ve removed a few features along the way, but hopefully nothing that impacts end users.

In retrospect, the conversion guide wasn’t very helpful. It provided “before” and “after” code examples, but the “before” code looked nothing like my code, so I couldn’t be sure what to replace with what. The API docs were far more useful – since everything centers on BillingClient, I could always start reading there.

As of right now, the app compiles, but that’s it. When launched, it immediately hangs, probably because of an infinite loop somewhere. Once that’s fixed, it’s on to the testing phase.

Coding in multiple languages

Quick: what programming language is Run 3 written in? Haxe, of course. (It’s even in the title of my blog.) But that isn’t all. Check out this list:

  • Haxe
  • Neko
  • Java
  • JavaScript
  • ActionScript
  • C++
  • Objective-C
  • Python
  • Batch
  • Bash
  • Groovy

Those are the programming languages that are (or were) involved in the development of Run 3/Run Mobile. Some are on the list because of Haxe’s capabilities, others because of Haxe’s limits.

Haxe’s defining feature is its ability to compile to other languages. This is great if you want to write a game for multiple platforms. JavaScript runs in browsers, C++ runs on desktop, Neko compiles quickly during testing, ActionScript… well, we don’t talk about ActionScript anymore. And that’s why those four are on the list.

Batch and Bash are good at performing simple file operations. Copying files, cleaning up old folders, etc. That’s also why Python is on the list: I have a Python file that runs after each build and performs simple file operations. Add 1 to the build count, create a zip file from a certain folder, etc. Honestly it doesn’t make much difference which language you use for the simple stuff, and I don’t remember why I chose Python. Nowadays I’d definitely choose Haxe for consistency.

The rest are due to mobile apps. Android apps are written in Java or Kotlin, then compiled using Groovy. Haxe does compile to Java, but it has a reputation for being slow. Therefore, OpenFL tries to use as much C++ as possible, only briefly using Java to get the game up and running.

iOS is similar: apps are typically written in Objective-C or Swift, and Haxe doesn’t compile to either of those. But you can have a simple Objective-C file start the app, then switch to C++.

Even leaving aside Python, Batch, and Bash, that’s a lot of languages. Some of them are independent, but others have to run at the same time and even interact. How does all that work?

Source-to-source compilation

Let’s start with source-to-source compilation (the thing I said was Haxe’s defining feature) and what it means. Suppose I’m programming in Haxe and compiling to JavaScript.

Now, by default Haxe code can only call other Haxe code. Say there’s a fancy JavaScript library that does calligraphy, and I want to draw some large shiny letters. If I was writing JavaScript, I could call the drawCalligraphy() function no problem, but not in Haxe.

To accomplish the same thing in Haxe, I need some special syntax to insert JavaScript code. Something like this:

//Haxe code (what I write)
var fruitOptions = ["apple", "orange", "pear", "lemon"];
var randomFruit = fruitOptions[ * fruitOptions.length)];

//JS code (generated when the above is compiled)
let fruitOptions = ["apple","orange","pear","lemon"];
let randomFruit = fruitOptions[Math.random() * fruitOptions.length | 0];

Note how similar the Haxe and JavaScript functions end up being. It almost feels like I shouldn’t need the special syntax at all. As you can see from the final line of code, function calls are left unchanged. If I typed drawCalligraphy(randomFruit) in Haxe, it would become drawCalligraphy(randomFruit) in JavaScript, which would work perfectly. Problem is, it doesn’t compile. drawCalligraphy isn’t a Haxe function, so Haxe throws an error.

Well, that’s where externs come in. By declaring an “extern” function, I tell Haxe “this function will exist at runtime, so don’t throw compile errors when you see it.” (As a side-effect, I’d better type the function name right, because Haxe won’t check my work.)

tl;dr: Since Haxe creates code in another programming language, you can talk to other code in that language. If you compile to JS, you can talk to JS.

Starting an iOS app

Each Android or iOS app has a single defined “entry point,” which has to be a Java/Kotlin/Objective-C/Swift file. Haxe can compile to (some of) these, but there’s really no point. It’s easier and better to literally type out a Java/Kotlin/Objective-C/Swift file, which is exactly what Lime does.

I’ve written about this before, but as a refresher, Lime creates an Xcode project as one step along the way to making an iOS app. At this point, the Haxe code has already been compiled into C++, in a form usable on iOS. Lime then copy-pastes in some Objective-C files and an Xcode project file, which Xcode compiles to make a fully-working iOS app. (And it’s a real project; you could even edit it in Xcode, though that isn’t recommended.)

And that’s enough to get the app going. When compiled side-by-side, C++ and Objective-C++ can talk to one another, as easily as JavaScript can communicate with JavaScript. (the Objective-C entry point) calls a C++ function, which calls another C++ function, and so on until eventually one of them calls the compiled Haxe function. Not as simple as it could be, but it has the potential to be quite straightforward.

Unlike Android.

Shared libraries

A shared library or shared object is a file that is intended to be shared by executable files and further shared object files. Modules used by a program are loaded from individual shared objects into memory at load time or runtime, rather than being copied by a linker when it creates a single monolithic executable file for the program.

Traditionally, shared library/object files are toolkits. Each handles a single task (or group of related tasks), like network connections or 3D graphics. The “shared” part of the name means many different programs can use the library at once, which is great if you have a dozen programs connecting to the net and don’t want to have to download a dozen copies of the network connection library.

I mention this to highlight that Lime does something odd when compiling for Android. All of your Haxe-turned-C++ code goes in one big shared object file named But this “shared” object never gets shared. It’s only ever used by one app, because, well, it is that app. Everything outside of is essentially window dressing; it’s there to get the C++ code started. I’m not saying Lime is wrong to do this (in fact, the NDK documentation tells you to do it), I’m just commenting on the linguistic drift.

To get the app started, Lime loads the shared object and then passes the name of the main C++ function to SDL, which loads the function and then calls it. Bit roundabout, but whatever works.

tl;dr: A shared library is a pre-compiled group of code. Before calling a function, you need two steps: load the library, then load the function. On Android, one of these functions is basically “run the entire app.”

Accessing Android/iOS features from Haxe

If your Haxe code is going into a shared object, then tools like externs won’t work. How does a shared object send messages to Java/Objective-C? I’ve actually answered this one before with examples, but I didn’t really explain why, so I’ll try to do that.

  • On Android, you call JNI.createStaticMethod() to get a reference to a single Java function, as long as the Java function is declared publicly. Once you have this reference, you can call the Java function. If you want more functions, you call JNI (Java Native Interface) multiple times.
  • On iOS, you call CFFI.load() to get a reference to a single C (Objective or otherwise) function, as long as the C function is a public member of a shared library. Once you have this reference, you can call the C function. If you want more functions, you call CFFI (C Foreign Function Interface) multiple times.

Gotta say, there are a lot of similarities, and I’m guessing that isn’t a coincidence. Lime is actually doing a lot of work under the hood in both cases, with the end goal of keeping them simple.

But wait a minute. Why is iOS using shared libraries all of a sudden? We’re compiling to C++ and talking to Objective-C; shouldn’t extern functions be enough? In fact, they are enough. Shared libraries are optional here, though recommended for organization and code consistency.

You might also note that last time I described calling a shared library, it took extra steps (load the library, load the function, call the function). This is some of the work Lime does under the hood. The CFFI class combines the “load library” and “load function” steps into one, keeping any open libraries for later use. (Whereas C++ doesn’t really do “convenience.”)

tl;dr: On Android, Haxe code can call Java functions using JNI. iOS extensions are designed to mimic this arrangement, though you use CFFI instead of JNI.

Why I wrote this post

Looking back after writing this, I have to admit it’s one of my less-informative blog posts. I took a deep dive into how Lime works, yes, but very little here is useful to an average OpenFL/Lime user. If you want to use CFFI or JNI, you’d be better off reading my old blog post instead.

Originally, this post was supposed to be a couple paragraphs leading into to another Android progress report. (And I’d categorized it under “development,” which is hardly accurate.) But the more I wrote, the clearer it became that I wasn’t going to get to the progress report. I almost abandoned this post, but I was learning new things, so I decided to put it out there.

(For instance, it had never occurred to me that CFFI was optional on iOS. It may well be the best option, but since it is just an option rather than mandatory, I’ll want to double-check.)

Why I haven’t updated Run Mobile in ages (part 1)

Google has announced a November 1 deadline to update Play Store apps, and I’ve been keeping an eye on that one. We’re now getting close enough to the deadline that I’m officially giving up on releasing new content with the update, and instead I’ll just release the same version with the new required library.

But why did it take this long for me to decide that? Why didn’t I do this a year ago when Google made their announcement, and keep working in the meantime? To answer that question, this blog post will document my journey to make a single small change to Run Mobile. The first step, of course, is to make sure it still compiles.

I should mention that I have two computers that I’ve used to compile Android apps. A Linux desktop, and a Windows laptop.

The Linux machine:

  • Is where I do practically all my work nowadays.
  • Performs well.
  • Is the one you’ve seen if you’ve watched my streams.
  • Has never, if I recall correctly, compiled a release copy of Run Mobile.

The Windows machine:

  • Hasn’t seen use in years.
  • Is getting old and slow; probably needs a fresh install.
  • Has the exact code I used to compile the currently-released version of Run Mobile.

Compiling on Windows

I tried this second, so let’s talk about it first. That makes sense, right?

Well, I found that I had some commits I hadn’t uploaded. Figured I’d do that real quick, and it turns out that Git is broken somehow. Not sure why, but it always rejects my SSH key. I restarted the machine, reuploaded the key to GitHub, tried both command line and TortiseGit, and even tried GitHub’s app which promises that you don’t have to bother with SSH keys. Everything still failed. At some point I’ll reinstall Git, but that’s for later. My goal here is to compile.

Fortunately, almost all of my code was untouched since the last time I compiled, and so I compiled to Neko. No dice. There were syntax errors, null pointers, and a dozen rendering crashes. Oh right, I never compiled this version for Neko, because I always targeted Flash instead.

So I stopped trying to fix Neko, and compiled for Android. And… well, there certainly were errors, but I’ll get to them later. Eventually, I fixed enough errors that it compiled. Hooray!

But for some reason I couldn’t get the machine to connect to my phone, so I couldn’t install and test the app. Tried multiple cables, multiple USB ports… nothing. And that was the last straw. This laptop was frustrating enough when Git and adb worked.

Compiling on Linux

Since Git does work on this machine, I was easily able to roll my code back. I’d even made a list of what libraries needed to be rolled back, and how far. (This is far from the first time I’ve had to do this.)

With all my code restored, I tried compiling. Result: lime/graphics/cairo/Cairo.hx:35: characters 13-21 : Unexpected operator. A single error, presumably hiding many more. Out of curiosity, I checked the file in question, expecting to see a plus sign in a weird spot, or an extra pair of square brackets around something, or who knows. Instead I found the word “operator”. (Once again I have fallen victim to the use-mention distinction.) Apparently Haxe 4.0.0 made “operator” a keyword, and Lime had to work around it.

Right, right. I’d gone back to old versions of my code, but I hadn’t downgraded Haxe. I’d assumed doing so would be difficult, possibly requiring me to download an untrustworthy file. This was the point in the process when I tried to compile on Windows instead. As explained above, that fell through, so I came back and discovered I could get it straight from the Haxe Foundation. (I’d been looking in the wrong place.) Once I reverted Haxe, that first error went away.

But that was only error #1. Fixing it revealed a new one, and fixing that revealed yet another. Rinse and repeat for an unknown number of times. Plus side, it’s simpler to keep track of a single error at a time than 20 at once. Minus side, there were a lot of errors.

  1. Type not found: haxe.Exception – Apparently I hadn’t downgraded all my libraries. After some file searching, I found two likely culprits and downgraded both.
  2. Cannot create closure with more than 5 arguments – I’ve never seen this one before, and neither has Google. I never even knew that function bindings were closures. Also, I’m not sure how addRectangle.bind(104, 0x000000, 0) has more than 5 arguments (perhaps it counts the optional arguments). But this wasn’t worth a lot of time, so I used an anonymous function to do the same thing.
  3. Invalid field access : lastIndexOf – This often comes up when a string is null. Can’t search for the last index of a letter if the string isn’t there. Fortunately I’d already run into this bug on Windows and knew the solution. Haxe 3.4 tells you to use Sys.programPath() instead of Sys.executablePath(), except programPath is broken.
  4. Extern constructor could not be inlined – Another error stemming from an old version of Haxe, this comes up when you cache data between compilations. It can be fixed by updating Haxe (not an option in my case) or turning off VSHaxe’s language server.
  5. Invalid field access : __s – Another null pointer error I’d already seen. But it was at this point that I remembered not to try to compile for Neko, so I turned my focus to Android instead.
  6. You need to run "lime setup android" before you can use the Android target – So of course that wasn’t going to be easy either. Apparently I’d never told Lime where to find some crucial files. (Also apparently I’d never downloaded the [NDK](, meaning I’ve never used this machine to compile for Android.)
  7. Type not found : Jni – Wait, I (vaguely) remember that class. Why is it missing? _One search later…_ Aha, it’s still there, it’s just missing some tweaks I made on the Windows computer. This is all for some debug info that rarely matters, so I removed it for the time being.
  8. arm-linux-androideabi-g++: not found – Uh oh. This is an error in hxcpp, a library that I try very hard to avoid dealing with. Android seems to have retired the “standalone toolchains” feature this old version of hxcpp uses, and I’ve long since established that newer versions of hxcpp are incompatible. Well, I tried using HXCPP_VERBOSE to get more info, and while it helped with a few hidden errors, I spent way too long digging into hxcpp without making much progress. Instead, I went all the way back to NDK r18.
  9. 'typeinfo' file not found – Another C++ error, great. Seems I’m not the first OpenFL dev to run into this one, which is actually good because it lets me know which NDK version I actually need: r15c. The Android SDK manager only goes as far back as r16, so I did a manual download.
  10. gradlew: not found – It might be another “not found” error, but make no mistake, this is huge progress. All the C++ files (over 99% of the game) compiled successfully, and it had reached the Gradle/Java portion, something I’m far more familiar with. Not that I needed to be because, someone else already fixed it. The only reason I was still seeing the error is because I couldn’t use newer versions of OpenFL. One quick copy-paste later, and…
  11. Cannot convert URL '[Kongregate SDK file]' to a file. – No kidding it can’t find the Kongregate SDK; I hard-coded a file path that only exists on Windows. In retrospect I should have used a relative path, but for now I hard-coded a different path. Then, to make absolutely certain I had the right versions, I copied the Kongregate SDK (and other Java libraries) from my laptop.
  12. Could not GET '[...]'. Received status code 403 from server: Forbidden – There were nine or ten of these errors, each with a different url. Apparently Bintray has been shut down, which means everyone has found somewhere else to host their files. I looked up the new URLs and plugged them in. Surprisingly, the new urls worked first try.

And, finally, it compiled.

Closing thoughts

And that’s why I haven’t updated Run Mobile in ages. Every time I try to compile, I have to wade through a slew of bugs. Not usually this many, but there’s always something, and I’ve learned to associate updating mobile with frustration.

I was hoping to avoid this whole process. I’d hoped to finish Runaway, allowing me to use the latest versions of OpenFL, hxcpp, and the Android SDK. But there just wasn’t enough time.

Don’t get me wrong, it feels good to have accomplished this. But as a reminder, I’ve made zero improvements thus far. I haven’t copied over any new content, I haven’t updated any libraries, I haven’t even touched the Google Play Billing Library (you know, the one that must be updated by November). I’ve spent two weeks just trying to get back what I already had.

Maybe I’m being too pessimistic here. I have, in fact, made progress since February 2018. My code now compiles on Linux, unlike in 2018. My 2018 code relied on Bintray, which is now gone. And it’s possible that new content may have been included without me even trying.

And that’s enough for today. Join me next time, on my journey to make a single small change to Run Mobile.

Haxe has too many ECS frameworks

Ever since I learned about it, I’ve wanted to use the entity component system (ECS) pattern to make games. Used properly, the pattern leads to clean, effective, and in my opinion cool-looking code. So when I was getting started my game engine, I looked for an ECS library that to build off of. And I found plenty.

The Haxe community is prolific and enthusiastic, releasing all kinds of libraries completely for free. That’s great, but it’s also a bit of a problem. Instead of working together to build a few high-quality libraries, everyone decided to reinvent the wheel.

xkcd: Standards

It did occur to me that I was preparing to reinvent the wheel, but no one had built a game engine capable of what I wanted, so I went ahead with it. Eventually I realized that’s probably what all the other developers were thinking too. Maybe there’s a reason for the chaos.

Let’s take a look at (eleven of) the available frameworks. What distinguishes each one?

Or if you want to see the one I settled on, skip to Echoes.


Let’s start at the beginning. Ash was one of the first ECS frameworks for Haxe, ported from an ActionScript 3 library of the same name. Makes sense: Haxe was originally based on AS3, and most of the early developers came from there.

Richard Lord, who developed the AS3 version, also wrote some useful blog posts on what an ECS architecture is and why you might want to use it.

Objectively, Ash is a well-designed engine. However, it’s held back by having started in ActionScript. Good design decisions there (such as using linked list nodes for performance) became unnecessary in Haxe, but the port still kept them in an effort to change as little as possible. This means it takes a bunch of typing to do anything.

//You have to define a "Node" to indicate which components you're looking for; in this case Position and Motion.
class MovementNode extends Node<MovementNode>
    public var position:Position;
    public var motion:Motion;
//Then systems use this node to find matching entities.
private function updateNode(node:MovementNode, time:Float):Void
    var position:Position = node.position;
    var motion:Motion = node.motion;

    position = node.position;
    motion = node.motion;
    position.position.x += motion.velocity.x * time;
    position.position.y += motion.velocity.y * time;

It honestly isn’t that bad for one example, but extra typing adds up.


ECX seems to be focused on performance, though I can’t confirm or debunk this.

As far as usability goes, it’s one step better than Ash. You can define a collection of entities (called a “Family” instead of a “Node”) in a single line of code, right next to the function that uses it. Much better organized.

class MovementSystem extends System {
    //Define a family.
    var _entities:Family<Transform, Renderable>;
    override function update() {
        //Iterate through all entities in the family.
        for(entity in _entities) {


Eskimo is the programmer’s third attempt at a framework, and it shows in the features available. You can have entirely separate groups of entities, as if they existed in different worlds, so they’ll never accidentally interact. It can notify you when any entity gains or loses components (and you can choose which components you want to be notified of). Like in ECX, you can create a collection of components (here called a View rather than a Family) in a single line of code:

var viewab = new View([ComponentA, ComponentB], entities);
for (entity in viewb.entities) {
    trace('Entity id: ${}');

The framework has plenty of flaws, but its most notable feature is the complete lack of macros. Macros are a powerful feature in Haxe that allow you to run code at compile time, which makes programming easier and may save time when the game is running.

Lacking macros (as well as the “different worlds” thing I mentioned) slows Eskimo down, and makes it so you have to type out more code. Not as much code as in Ash, but it’s still inconvenient.

Honestly, though, I’m just impressed. Building an ECS framework without macros is an achievement, even though the framework suffers for it. Every single one of the other frameworks on this list uses macros, for syntax sugar if nothing else. Even Ash uses a few macros, despite coming from AS3 (which has no macros).


edge (all lowercase) brings an amazing new piece of syntax sugar:

class UpdateMovement implements ISystem {
    function update(pos:Position, vel:Velocity) {
        pos.x += vel.vx,
        pos.y += vel.vy;

You no longer have to create a View yourself, or iterate through that view, or type out entity.get(Position) every time you want to access the Position component. Instead, just define an update function with the components you want. edge will automatically give you each entity’s position and velocity. You don’t even have to call entity.get(Position) or anything; that’s already done. This saves a lot of typing when you have a lot of systems to write.

edge also provides most of the other features I’ve mentioned so far. Like in Eskimo, you can separate entities into different “worlds” (called Engines), and you can receive notifications when entities gain or lose components. You can access Views if needed/preferred, and it only takes a line of code to set up. Its “World” and “Phase” classes are a great way to organize systems, and the guiding principles are pretty much exactly how I think the ECS pattern should work.

Have I gushed enough about this framework yet? Because it’s pretty great. Just one small problem.

A system’s update function must be named update. A single class can only have one function with a given name. Therefore, each system can only have one update function. If you want to update two different groups of entities, you need two entire systems. So the syntax sugar doesn’t actually save that much typing, because you have to type out an entire new class declaration for each function.

Eventually, the creator abandoned edge to work on edge 2. This addresses the “one function per system” problem, though sadly in its current state it loses all the convenience edge offered. (And the lack of documentation makes me think it was abandoned midway.)


Baldrick is notable because it was created specifically in response to edge. Let’s look at the creator’s complaints, to see what others care about.

• It requires thx.core, which pulls a lot of code I don’t need

That’s totally fair. Unnecessary dependencies are annoying.

• It hasn’t been updated in a long time, and has been superceded by the author by edge2

It’s always concerning when you see a library hasn’t been updated in a while. This could mean it’s complete, but usually it means it’s abandoned, and who knows if any bugs will be fixed. I don’t consider this a deal-breaker myself, nor do I think edge 2 supersedes it (yet).

• Does a little bit too much behind-the-scenes with macros (ex: auto-creating views based on update function parameters)

Oh come on, macros are great! And the “auto-creating views” feature is edge’s best innovation.

• Always fully processes macros, even if display is defined (when using completion), slowing down completion

I never even thought about this, but now that they mention it, I have to agree. It’s a small but significant oversight.

• Isn’t under my control so isn’t easily extendable for my personal uses

It’s… open source. You can make a copy that you control, and if you can submit your changes back to the main project. If you’re making changes that aren’t worth submitting, you’re probably making the wrong changes. Probably.

• Components and resources are stored in an IntMap (rather than a StringMap like in edge)

This is actually describing what Baldrick does, but it still mentions something edge does wrong. StringMap isn’t terrible, but Baldrick’s IntMap makes a lot more sense.

Anyway, Baldrick looks well-built, and it’s building on a solid foundation, but unfortunately it’s (quite intentionally) missing the syntax sugar that I liked so much.

var movingEntities:View<{pos:Position, vel:Velocity}> = new View();

public function process():Void {
    for(entity in positionEntities) { +=; +=;

That seems like more typing than needed – Compare that to edge, which only requires you to type pos.x. I suppose it could be worse, but that doesn’t mean I’d want to use it.

Oh, and as far as I can tell, there’s no way to get delta time. That’s inconvenient.


Short for “experimental entity component system,” exp-ecs is inspired by Ash but makes better use of Haxe. It does rely on several tink libraries (comparable to edge’s dependency on thx.core). The code looks pretty familiar by now, albeit cleaner than average:

@:nodes var nodes:Node<Position, Velocity>;

override function update(dt:Float) {
    for(node in nodes) {
        node.position.x += node.velocity.x * dt;
        node.position.y += node.velocity.y * dt;

Not bad, even if it isn’t edge.

Under the hood, it looks like component tracking is slower than needed. tink_core‘s signals are neat and all, but the way they’re used here means every time a component is added, the entity will be checked against every node in existence. This won’t matter in most cases, but it could start to add up in large games with lots of systems.

Ok, I just realized how bad that explanation probably was, so please enjoy this dramatization of real events instead, featuring workers at a hypothetical entity factory:

Worker A: Ok B, we just added a Position component. Since each node needs to know which entities have which components, we need to notify them.

Worker B: On it! Here’s a node for entities with Hitboxes; does it need to be notified?

Worker A: Nope, the entity doesn’t have a Hitbox.

Worker B: Ok, here’s a node that looks for Acceleration and Velocity; does it need to be notified?

Worker A: No, the entity doesn’t have Acceleration. (It has a Velocity, but that isn’t enough.)

Worker B: Next is a node that looks for Velocity and Position; does it need to be notified?

Worker A: Yes! The entity has both Velocity and Position.

Worker B: Here’s a node that needs both Position and Appearance; does it need to be notified?

Worker A: No, this is an invisible entity, lacking an Appearance. (It has a Position, but that isn’t enough.)

Worker B: Ok, next is a node for entities with Names; does it need to be notified?

Worker A: It would, but it already knows the entity’s Name. No change here.

Worker B: Next, we have…

This process continues for a while, and most of it is totally unnecessary. We just added a Position component, so why are we wasting time checking dozens or hundreds of nodes that don’t care about Position? None of them will have changed. Sadly, exp-ecs just doesn’t have any way to keep track. It probably doesn’t matter for most games, but in big enough projects it could add up.

(Please note that exp-ecs isn’t the only framework with this issue, it’s just the one I checked to be sure. I suspect the majority do the same thing.)

On the plus side, I have to compliment the code structure. There’s no ECS framework in existence whose code can be understood at a glance, but in my opinion exp-ecs comes close. (Oh, and the coding style seems to perfectly match my own, a coincidence that’s never happened before. There was always at least one small difference. So that’s neat.)


Cog is derived from exp-ecs, and calls itself a “Bring Your Own Entity” framework. You’re supposed to integrate the Components class into your own Entity class (and you can call your class whatever you like), and now your class acts like an entity. I don’t buy it. Essentially their Components class is the Entity class, they’re just trying to hide it.

As far as functionality, it unsurprisingly looks a lot like exp-ecs:

@:nodes var movers:Node<position, velocity>;
override public function step(dt:Float) {
    for (node in movers) {
        node.position.x += node.velocity.x * dt;
        node.position.y += node.velocity.y * dt;

I was pleasantly surprised to note that it has component events (the notifications I talked about for Eskimo and edge). If Cog had existed when I started building Runaway, I would have seriously considered using it. In the end I’d probably have rejected it for lack of syntax sugar, but only barely.


Awe is a pseudo-port of Artemis, an ECS framework written in Java. I’m not going to dig deep into it, because this is the example code:

var world ={
    systems: [new InputSystem(), new MovementSystem(), new RenderSystem(), new GravitySystem()],
    components: [Input, Position, Velocity, Acceleration, Gravity, Follow],
    expectedEntityCount: ...
var playerArchetype =, Position, Velocity, Acceleration, Gravity);
var player = world.createEntityFromArchetype(playerArchetype);

Java has a reputation for being verbose, and this certainly lives up to that. I can look past long method names, but I can’t abide by having to list out every component in advance, nor having to count entities in advance, nor having to define each entity’s components when you create that entity. What if the situation changes and you need new components? Just create a whole new entity I guess? This engine simply isn’t for programmers like me.

That said, the README hints at something excellent that I haven’t seen elsewhere…

@Packed This is a component that can be represented by bytes, thus doesn’t have any fields whose type is not primitive.

…efficient data storage. With all the restrictions imposed above, I bet it takes up amazingly little memory. Sadly this all comes at the cost of flexibility. It reminds me of a particle system, packing data tightly, operating on a set number of particles, and defining the limits of the particles’ capabilities in advance.


OSIS combines entities, components, systems, and network support. The networking is optional, but imposes a limitation of 64 component types that applies no matter what. (I’ve definitely already exceeded that.) I don’t have the time or expertise to discuss the can of worms that is networking, so I’ll leave it aside.

Also notable is the claim that the library “avoids magic.” That means nothing happens automatically, and all the syntax sugar is gone:

var entitySet:EntitySet;

public override function init()
    entitySet = em.getEntitySet([CPosition, CMonster]);

public override function loop()

    for(entity in entitySet.entities)
        var pos = entity.get(CPosition);
        pos.x += 0.1;

I have to admit this is surprisingly concise, and the source code seems well-written. The framework also includes less-common features like component events and entity worlds (this time called “EntityManagers”).

I still like my syntax sugar, I need more than 64 components, and I don’t need networking, so this isn’t the library for me.


According to, GASM is the most popular haxe library with the “ecs” tag. However, I am an ECS purist, and as its README states:

Note that ECS purists will not consider this a proper ECS framework, since components contain the logic instead of systems. If you are writing a complex RPG or MMO, proper ECS might be worth looking in to, but for more typical small scale web or mobile projects I think having logic in components is preferable.

Listen, if it doesn’t have systems, then don’t call it “ECS.” Call it “EC” or something.

It seems to be a well-built library, better-supported than almost anything else on this list. However, I’m not interested in entities and components without systems, so I chose to keep looking.

Ok, so what did I go with?


Echoes’ original creator described it as a practice project, created to “learn the power of macros.” Inspired by several others on the list, it ticked almost every single one of my boxes.

It has syntax sugar like edge’s (minus the “one function per system” restriction), no thx or tink dependencies, yes component events, convenient system organization, and a boatload of flexibility. Despite deepcake’s (the creator’s) modesty, this framework has a lot to it. It received 400+ commits even before I arrived, and is now over 500. (Not a guarantee of quality, but it certainly doesn’t hurt.)

Echoes’ performance

I haven’t seriously tested Echoes’ speed, but deepcake (the original dev) made speed a priority, and I can tell that it does several things right. It uses IntMap to store components, it keeps track of which views care about which components (meaning it’s the first one I’m sure doesn’t suffer from the problem I dramatized in the exp-ecs section), and it does not let you separate entities into “worlds.” The latter is a good feature to have, but incurs a small performance hit. I’m not saying it’s good to be missing a feature, I’m just saying it’s faster (and I haven’t needed it yet).

Echoes’ flexible components

Let’s talk about how components work. In every other framework I’ve discussed thus far, a component must be a class, and it must extend or implement either “Component” or “IComponent,” respectively. There’s a very specific reason for these restrictions, but they still get in the way.

For instance, say you wanted to work with an existing library, such as—oh, I don’t know—Away3D. Suppose that Away3D had a neat little Mesh class, representing a 3D model that can be rendered onscreen. Suppose you wanted an entity to have a Mesh component. Well, Mesh already extends another class and cannot extend Component. It can implement IComponent, but that’s inconvenient, and you’d have to edit Mesh.hx. (Which falls squarely in the category of “edits you shouldn’t have to make.”) Your best bet is to create your own MeshComponent class that wraps Mesh, and that’s just a lot of extra typing.

In Echoes, almost any Haxe type can be a component. That Mesh? Valid complement, no extending or implementing necessary. An abstract type? Yep, it just works. That anonymous structure? Well, not directly, but you can wrap it in an abstract type. Or if wrapping it in an abstract is too much work, make a typedef for it. (Note: typedefs don’t work in deepcake’s build, but they were the very first thing I added, specifically because wrapping things in abstracts is too much work.)

All this is accomplished through some slightly questionable macro magic. Echoes generates a lot of extra classes as a means of data storage. For instance, Position components would be stored in a class named ContainerOfPosition. Echoes does this both to get around the “extend or implement” restriction, and because it assumes that it’ll make lookups faster. This may well be true (as long as the compiler is half-decent), it’s just very unusual.

Echoes: conclusion

I settled on Echoes for the syntax sugar and the component events. At the time, the deciding factor was component events, and I hadn’t realized any other libraries offered those. So… whoops.

I don’t regret my choice, at all. The syntax sugar is great, abstract/typedef support is crucial, and the strange-seeming design decisions hold up better than I first though. I know I found one I’m happy with, but it’s a shame that all the others fall short in one way or another…?