Creating Android plugins

Introduction

Android plugins are powerful tools to extend the capabilities of the Rebel Engine by tapping into the functionality provided by the Android platform and ecosystem.

Mobile gaming monetization is one such example since it requires features and capabilities that don't belong to the core feature set of a game engine:

  • Analytics

  • In-app purchases

  • Receipt validation

  • Install tracking

  • Ads

  • Video ads

  • Cross-promotion

  • In-game soft & hard currencies

  • Promo codes

  • A/B testing

  • Login

  • Cloud saves

  • Leaderboards and scores

  • User support & feedback

  • Posting to Facebook, Twitter, etc.

  • Push notifications

Android plugin

As a prerequisite, make sure you understand how to set up a custom build environment for Android.

At its core, a Rebel Android plugin is a Android archive library (aar archive file) with the following caveats:

  • The library must have a dependency on the Rebel Engine Android library (rebel-lib.<version>.<status>.aar). A stable version is made available for each Rebel release.

  • The library must include a specifically configured <meta-data> tag in its manifest file.

Building an Android plugin

Prerequisite: Android Studio is strongly recommended as the IDE to use to create Android plugins. The instructions below assumes that you're using Android Studio.

  1. Follow these instructions to create an Android library module for your plugin.

  2. Add the Rebel Engine Android library as a dependency to your plugin module:

  • Download the Rebel Engine Android library (rebel-lib.<version>.<status>.aar).

  • Follow these instructions to add the Rebel Engine Android library as a dependency for your plugin.

  • In the plugin module's build.gradle file, replace implementation with compileOnly for the dependency line for the Rebel Engine Android library.

  1. Create a new class in the plugin module and make sure it extends com.rebeltoobox.plugin.RebelPlugin. At runtime, it will be used to instantiate a singleton object that will be used by the Rebel Engine to load, initialize and run the plugin.

  2. Update the plugin AndroidManifest.xml file:

  • Open the plugin AndroidManifest.xml file.

  • Add the <application></application> tag if it's missing.

  • In the <application> tag, add a <meta-data> tag setup as follow:

    <meta-data
    android:name="com.rebeltoolbox.plugin.v1.[PluginName]"
    android:value="[plugin.init.ClassFullName]" />

    Where PluginName is the name of the plugin, and plugin.init.ClassFullName is the full name (package + class name) of the plugin loading class.

  1. Add the remaining logic for your plugin and run the gradlew build command to generate the plugin's aar file. The build will likely generate both a debug and release aar files. Depending on your need, pick only one version (usually the release one) which to provide your users with.

    It's recommended that the aar filename matches the following pattern: [PluginName]*.aar where PluginName is the name of the plugin in PascalCase (e.g.: MyRebelPlugin.release.aar).

  2. Create a Rebel Android Plugin configuration file to help the system detect and load your plugin:

  • The configuration file extension must be gdap (e.g.: MyPlugin.gdap).

  • The configuration file format is as follow:

    [config]

name="MyPlugin"
binary_type="local"
binary="MyPlugin.aar"

[dependencies]

local=["local_dep1.aar", "local_dep2.aar"]
remote=["example.plugin.android:remote-dep1:0.0.1", "example.plugin.android:remote-dep2:0.0.1"]
custom_maven_repos=["http://repo.mycompany.com/maven2"]

    The config section and fields are required and defined as follow:

    • name: name of the plugin.

    • binary_type: can be either local or remote. The type affects the binary field.

    • binary:

      • If binary_type is local, then this should be the filepath of the plugin aar file.

        • The filepath can be relative (e.g.: MyPlugin.aar) in which case it's relative to the res://android/plugins directory.

        • The filepath can be absolute: res://some_path/MyPlugin.aar.

      • If binary_type is remote, then this should be a declaration for a remote gradle binary (e.g.: com.example:my-plugin:0.0.0).

    The dependencies section and fields are optional and defined as follow:

    • local: contains a list of filepaths to the local .aar binary files the plugin depends on. Similarly to the binary field (when the binary_type is local), the local binaries' filepaths can be relative or absolute.

    • remote: contains a list of remote binary gradle dependencies for the plugin.

    • custom_maven_repos: contains a list of URLs specifying the custom maven repositories required for the plugin's dependencies.

Loading and using an Android plugin

Move the plugin configuration file (e.g.: MyPlugin.gdap) and, if any, its local binary (e.g.: MyPlugin.aar) and dependencies to the Rebel project's res://android/plugins directory.

The Rebel Editor will automatically parse all .gdap files in the res://android/plugins directory and show a list of detected and toggleable plugins in the Android export presets window under the Plugins section.

../../../_images/android_export_preset_plugins_section.png

From your script:

if Engine.has_singleton("MyPlugin"):
    var singleton = Engine.get_singleton("MyPlugin")
    print(singleton.myPluginFunction("World"))

Bundling GDNative resources

An Android plugin can define and provide C/C++ GDNative resources, either to provide and/or access functionality from the game logic. The GDNative resources can be bundled within the plugin aar file which simplifies the distribution and deployment process:

  • The shared libraries (.so) for the defined GDNative libraries will be automatically bundled by the aar build system.

  • Rebel Engine *.gdnlib and *.gdns resource files must be manually defined in the plugin assets directory. The recommended path for these resources relative to the assets directory should be: rebel/plugin/v1/[PluginName]/.

For GDNative libraries, the plugin singleton object must override the com.rebeltoolbox.plugin.RebelPlugin::getPluginGDNativeLibrariesPaths() method, and return the paths to the bundled GDNative libraries config files (*.gdnlib). The paths must be relative to the assets directory. At runtime, the plugin will provide these paths to Rebel Engine core which will use them to load and initialize the bundled GDNative libraries.

Troubleshooting

Rebel crashes upon load

Check adb logcat for possible problems, then:

  • Check that the methods exposed by the plugin used the following Java types: void, boolean, int, float, java.lang.String, com.rebeltoolbox.Dictionary, int[], byte[], float[], java.lang.String[].

  • More complex datatypes are not supported for now.