This is an Android sample that uses NativeActivity with native_app_glue, which enables building NDK apps without having to write any Java code. In practice most apps, even games which are predominantly native code, will need to call some Java APIs or customize their app's activity further. While it may save you a small amount of effort during prototyping, it may result in a difficult migration later. It's also worth noting that some of the code in this sample is spent undoing the work of native_app_glue to create a class very similar to Activity.

The more modern approach to this is to use GameActivity, which has all the same benefits as NativeActivity and native_app_glue, while also making it easier to include Java code in your app without a rewrite later. It's also source compatible. However, it still has all the problems explained in the warning above, and in practice neither NativeActivity nor GameActivity is the recommended app development model.

The app here is intentionally quite simple, aiming to show the core event and draw loop necessary for an app using native_app_glue without any extra clutter. It uses AChoreographer to manage the update/render loop, and uses ANativeWindow and AHardwareBuffer to update the screen with a simple color clear.

The interesting sections of code in this sample are in three files: [AndroidManifest.xml], [CMakeLists.txt], and main.cpp. Each of those files has code comments explaining the portions relevant to using NativeActivity, but the high level details of the app are explained here.

This app uses NativeActivity rather than its own child class of Activity or AppCompatActivity. This is specified in the <activity /> declaration in the manifest.

Apps which use NativeActivity are typically written using native_app_glue, which adapts the Android activity lifecycle code to look more like a desktop program with a main() function and an event loop. This is set up in the app's CMakeLists.txt file.

When using native_app_glue with a version script, you must export ANativeActivity_onCreate. This sample does this in libnative-activity.map.txt.

This is a fairly simple application, so all of the code is in a single file, main.cpp. The entry point for an app using native_app_glue is android_main(). That function is the best place to start reading in this file to learn how the sample works, then follow through to the definition of engine_handle_cmd and Engine.