OpenFL Extensions

For the most part, OpenFL does an excellent job of providing you with the features you need in a platform-independent manner. It tells you the screen size, loads your assets, and even maps Android’s back button to the escape key.

Unfortunately, the OpenFL dev team cannot think of everything. Maybe you want to adjust the screen brightness on Android. Or the music volume. Maybe you want access to the camera on iOS. Or maybe you need to integrate a custom library used internally at your company, which the OpenFL dev team could not possibly have integrated for you.

For simplicity, I’ll be using screen brightness as an example. Setting this can be done in only 1-3 lines of code on both iOS and Android. The catch is, neither of those examples are in Haxe, and there’s no way to convert them to Haxe. If only you’d written the app “normally” rather than using OpenFL, you could just copy-paste those few lines of code, and you’d be done! But no, you wanted luxuries like cross-platform compilation, and now you have to somehow use Haxe code to invoke functions in Objective-C and Java.

Fun Fact

Did you know, when you compile for Android, OpenFL creates a complete, self-contained Android project, and then tells the Android SDK to compile that? And when compiling for iOS, it creates an Xcode project, and then has Xcode do the remaining work?

You can see for yourself by checking your output folder (probably either bin or Export; I’ll assume the former). Dig into bin/android/bin, and you’ll find all the files and folders you’d expect to find in a normal Android project. You could even, if you felt bold enough, modify the project, and compile it directly using the Android SDK. (Warning: Don’t actually do this! OpenFL will almost certainly overwrite your changes.)

The same applies to iOS – after compiling, you can check out bin/ios/bin to see the project that OpenFL created. You could try modifying this too, but again, OpenFL is going to revert your changes. There has to be a better way.

Creating an Extension

The OpenFL team is well aware of this problem, and in their infinite wisdom they created the “extension” feature. Also in their infinite wisdom, they wrote no documentation whatsoever before moving on to the next feature.

Extensions are basically mini-projects consisting of native (or Java) code, as well as Haxe bindings. You include them in your project, and then you can call the native (or Java) code by calling the Haxe bindings. Let’s look at an example.

Start by running the following:

I’m calling it “SetBrightness” because that’s all we’ll be doing here. You can call it something else.

Open up the folder, and you’ll find the following structure:

  • haxelib.json – Allows you to submit to Haxelib.
  • include.xml – Like project.xml in your main project, this tells OpenFL what to do with all the other files here.
  • SetBrightness.hx – Contains Haxe bindings for your Java and C++ functions. At first, everything in here will be sample code.
  • dependencies/
    • android/ – An Android library to be included in your Android builds. This is what lets you include Java code. All files in this folder are processed as templates, so you can use that syntax.
      • build.gradle – A project file for this Android library. Leave this alone unless you’re familiar with Gradle.
      • src/ – Despite the name, you can’t just put source files in here. They actually go in a child folder.
        • main/
          • AndroidManifest.xml – The manifest file for your Android library. If your extension requires permissions, this is the place to put them.
          • java/ – Java source files go here.
            • org/haxe/extension/
              • SetBrightness.java – The recommended place for your Java code. It comes with useful callbacks for monitoring the activity lifecycle, or you can ignore all that and write static functions.
  • ndll/ – When you compile your C++ code, the result will go in here.
    • Linux/
    • Linux64/
    • Mac/
    • Mac64/
    • Windows/
  • project/ – The root folder for your C++ project.
    • Build.xml – Build file for your C++ project. Only files named here (or included from those named here) will be compiled.
    • common/ – C++ source files (but not header files) go here.
      • ExternalInterface.cpp – Registers your C++ functions, allowing SetBrightness.hx to access them.
      • SetBrightness.cpp – Put C++ code here, based on the sample code that starts here.
    • include/ – C++ header files go here.
      • Utils.h – Header file for SetBrightness.cpp. Functions must be declared here in order for ExternalInterface.cpp to access them.

Writing Code for Android

Click through all the folders under dependencies/ until you reach SetBrightness.java. Add the following code:

That’s all well and good, but how do you call this function? The answer… is JNI. *Dramatic thunder crashes* Actually, it’s not that bad if you’re only dealing with one function. Climb your way back to the root SetBrightness/ folder, and add this to SetBrightness.hx:

That’s still a little much. Fortunately, shoe[box] came up with an easier way. Start by including the “inthebox-macros” library in your project, change the package in SetBrightness.hx to org.haxe.extension, and add @:build(ShortCuts.mirrors()) just before the class declaration. Now the code above can be replaced with this:

All that’s left is to include the extension in your project (see below), and you can call SetBrightness.setBrightness(0.8); from Haxe.

Debugging

When you try to use this extension on Android, you’ll run into a few errors. First, a compile error:

This happens because you aren’t compiling an ndll for Android, but by default Lime expects you to. To fix the error, go into include.xml and replace <ndll name="SetBrightness" /> with <ndll name="SetBrightness" unless="android" />.

Next, you’ll get a runtime error:

(The only thing that matters about this error message is that it contains the word “thread.” When developing an OpenFL extension, all thread-related errors have the same solution.)

On Android, some tasks have to be done on the main thread. When you use JNI, it runs on a thread other than the main one. Fortunately, Extension.callbackHandler.post() lets you get back to the main thread.

This function takes a Runnable object, so you’ll have to create one of those. Take all the code in your function, and put it inside the run() function:

(Remember, you only need to do this if you get a thread-related error. Usually, it isn’t worth the trouble.)

And that’s it for Android. On to iOS!

Writing Code for iOS

You’ll notice that the extension is set up for C++ code, but to access system properties like brightness, you need to use Objective-C code. Fortunately, this part’s easy: just change the .cpp file extensions to .mm. You’ll also need to update their names in Build.xml. And because Objective-C is specific to iOS, I suggest disabling them for everything else.

If you need C++ code on other platforms, just disable the C++ files on iOS:

Now to write some actual Objective-C code! Put this in SetBrightness.mm:

Now update Utils.h:

Don’t forget to update ExternalInterface.mm:

Last but not least, create the Haxe bindings in SetBrightness.hx:

Phew! That was a lot of updating. [Edit: inthebox-macros can generate the Haxe code, and I wrote a utility to generate ExternalInterface.mm and Build.xml.]

Time to compile!

Including the Extension

Almost done! All that’s left is to include it in your project.

This can actually be done in two ways. The easy way works if you’re only using one machine, or if the relative path is the same on all machines (for example, if the extension project is contained within your main project). All you have to do is add this to your project.xml or application.xml file:

The slightly harder way is to register it with Haxelib by running the following from the root of the extension:

Then include it: