Android: rework backend to use android_activity crate

This updates the Android backend to use the android_activity crate instead
of ndk-glue. This solves a few issues:
1. The backend is agnostic of the application's choice of Activity base
   class
2. Winit is no longer responsible for handling any Java synchronization
   details, since these are encapsulated by the design of
   android_activity
3. The backend no longer depends on global / static getters for state
   such as the native_window() which puts it in a better position to
   support running multiple activities within a single Android process.
4. Redraw requests are flagged, not queued, in a way that avoids taking
   priority over user events (resolves rust-windowing#2299)

Addresses: PR rust-windowing#1892
Addresses: PR rust-windowing#2307
Addresses: PR rust-windowing#2343

Addresses: rust-windowing#2293
Resolves: rust-windowing#2299

CHANGELOG.md

@@ -21,6 +21,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - On Wayland, `wayland-csd-adwaita` now uses `ab_glyph` instead of `crossfont` to render the title for decorations.
 - On Wayland, a new `wayland-csd-adwaita-crossfont` feature was added to use `crossfont` instead of `ab_glyph` for decorations.
 - On Wayland, if not otherwise specified use upstream automatic CSD theme selection.
+- **Breaking:** On Android, switched to using [`android-activity`](https://github.com/rib/android-activity) crate as a glue layer instead of [`ndk-glue](https://github.com/rust-windowing/android-ndk-rs/tree/master/ndk-glue). See [README.md#Android](https://github.com/rust-windowing/winit#Android) for more details. ([#2444](https://github.com/rust-windowing/winit/pull/2444))
 
 # 0.27.3
 

Cargo.toml

@@ -59,7 +59,7 @@ simple_logger = { version = "2.1.0", default_features = false }
 [target.'cfg(target_os = "android")'.dependencies]
 # Coordinate the next winit release with android-ndk-rs: https://github.com/rust-windowing/winit/issues/1995
 ndk = "0.7.0"
-ndk-glue = "0.7.0"
+android-activity = "0.2"
 
 [target.'cfg(any(target_os = "ios", target_os = "macos"))'.dependencies]
 objc2 = "=0.3.0-beta.3"

README.md

@@ -99,36 +99,77 @@ book].
 
 #### Android
 
-This library makes use of the [ndk-rs](https://github.com/rust-windowing/android-ndk-rs) crates, refer to that repo for more documentation.
+The Android backend builds on (and exposes types from) the [`ndk`](https://docs.rs/ndk/0.7.0/ndk/) crate.
 
-The `ndk-glue` version needs to match the version used by `winit`. Otherwise, the application will not start correctly as `ndk-glue`'s internal `NativeActivity` static is not the same due to version mismatch.
+Native Android applications need some form of "glue" crate that is responsible for defining the main entry point for your Rust application as well as tracking various life-cycle events and synchronizing with the main JVM thread.
 
-`winit` compatibility table with `ndk-glue`:
+Winit uses the [android-activity](https://github.com/rib/android-activity) as a glue crate (prior to `0.28` it used [ndk-glue](https://github.com/rust-windowing/android-ndk-rs/tree/master/ndk-glue).
 
-| winit |       ndk-glue       |
-| :---: | :------------------: |
-| 0.24  | `ndk-glue = "0.2.0"` |
-| 0.25  | `ndk-glue = "0.3.0"` |
-| 0.26  | `ndk-glue = "0.5.0"` |
-| 0.27  | `ndk-glue = "0.7.0"` |
+The version of the glue crate that your application depends on _must_ match the version that Winit depends on because the glue crate is responsible for your application's main entrypoint. If Cargo resolves multiple versions they will clash.
+
+`winit` glue compatibility table:
+
+| winit |       ndk-glue               |
+| :---: | :--------------------------: |
+| 0.28  | `android-activity = "0.3.0"` |
+| 0.27  | `ndk-glue = "0.7.0"`         |
+| 0.26  | `ndk-glue = "0.5.0"`         |
+| 0.25  | `ndk-glue = "0.3.0"`         |
+| 0.24  | `ndk-glue = "0.2.0"`         |
+
+_Note: There is unfortunately no ergonomic way to configure Cargo so that it understands that the glue crate is special on Android and to give a clear error if multiple versions are resolved. With `ndk-glue` you will get a runtime failure caused by clashing, global static variables. With `android-activity` you will see a compile-time error due to missing feature flags._
 
 Running on an Android device needs a dynamic system library, add this to Cargo.toml:
 
 ```toml
 [[example]]
-name = "request_redraw_threaded"
+name = "main"
 crate-type = ["cdylib"]
 ```
 
-And add this to the example file to add the native activity glue:
+All Android applications are based on an `Activity` subclass and the `android-activity` crate is designed to support different choices for this base class. You application must specify the base class it needs via a feature flag:
+
+| Base Class       | Feature Flag      |  Notes  |
+| :--------------: | :---------------: | :-----: |
+| `NativeActivity` | `native-activity` | Built-in to Android - making it possible to build some tests/demos without needing to compile any JVM code. Can give a false sense of convenience because it's often not really possible to avoid needing a build system that can compile some JVM code, to at least subclass `NativeActivity` |
+| [`GameActivity`] | `game-activity`   | Derives from [`AndroidAppCompat`] which is a defacto standard `Activity` base class that helps support a wider range of Android versions. Will offer integration with [`GameTextInput`] library for soft keyboard support. Requires a build system that can compile Java and fetch Android dependencies from a Maven repository (with [Gradle] being the defacto standard build system for Android applications) |
+
+[`GameActivity`]: https://developer.android.com/games/agdk/game-activity
+[`GameTextInput`]: https://developer.android.com/games/agdk/add-support-for-text-input
+[`AndroidAppCompat`]: https://developer.android.com/reference/androidx/appcompat/app/AppCompatActivity
+[Gradle]: https://developer.android.com/studio/build
+
+For example, add this to Cargo.toml:
+```toml
+[target.'cfg(target_os = "android")'.dependencies]
+android_logger = "0.11.0"
+android-activity = { version = "0.3", features = [ "game-activity" ] }
+```
+
+And, for example, define an entry point for your library like this:
 ```rust
-#[cfg_attr(target_os = "android", ndk_glue::main(backtrace = "on"))]
-fn main() {
-    ...
+#[cfg(target_os = "android")]
+#[no_mangle]
+fn android_main(app: AndroidApp) {
+    use winit::platform::android::EventLoopBuilderExtAndroid;
+
+    android_logger::init_once(android_logger::Config::default().with_min_level(log::Level::Trace));
+
+    let event_loop = EventLoopBuilder::with_user_event()
+        .with_android_app(app)
+        .build();
+    _main(event_loop);
 }
 ```
 
-And run the application with `cargo apk run --example request_redraw_threaded`
+For more details, refer to these `android-activity` [example applications](https://github.com/rib/android-activity/tree/main/examples).
+
+##### Converting from `ndk-glue` to `android-activity`
+
+If your application is currently based on `NativeActivity` via the `ndk-glue` crate and building with `cargo apk` then the minimal changes would be:
+1. Remove `ndk-glue` from your `Cargo.toml` and add dependency on `android-activity` that specifies the `"native-activity"` feature like: `android-activity = { version = "0.3", features = [ "native-activity" ] }`
+2. Add an `android_main` entrypoint (as above), instead of using the '`[ndk_glue::main]` proc macro from `ndk-macros` (optionally add a dependency on `android_logger` and initialize logging as above).
+3. Pass a clone of the `AndroidApp` that your application receives to Winit when building your event loop (as shown above).
 
 #### MacOS
 

src/event_loop.rs

@@ -97,8 +97,18 @@ impl<T> EventLoopBuilder<T> {
     ///   `WINIT_UNIX_BACKEND`. Legal values are `x11` and `wayland`.
     ///   If it is not set, winit will try to connect to a Wayland connection, and if that fails,
     ///   will fall back on X11. If this variable is set with any other value, winit will panic.
+    /// - **Android:** Must be configured with an `AndroidApp` from `android_main()` by calling
+    ///     [`.with_android_app(app)`] before calling `.build()`.
     ///
     /// [`platform`]: crate::platform
+    #[cfg_attr(
+        target_os = "android",
+        doc = "[`.with_android_app(app)`]: crate::platform::android::EventLoopBuilderExtAndroid::with_android_app"
+    )]
+    #[cfg_attr(
+        not(target_os = "android"),
+        doc = "[`.with_android_app(app)`]: #only-available-on-android"
+    )]
     #[inline]
     pub fn build(&mut self) -> EventLoop<T> {
         static EVENT_LOOP_CREATED: OnceCell<()> = OnceCell::new();

src/platform/android.rs

@@ -1,9 +1,9 @@
 use crate::{
-    event_loop::{EventLoop, EventLoopWindowTarget},
+    event_loop::{EventLoop, EventLoopBuilder, EventLoopWindowTarget},
     window::{Window, WindowBuilder},
 };
-use ndk::configuration::Configuration;
-use ndk_glue::Rect;
+
+use android_activity::{AndroidApp, ConfigurationRef, Rect};
 
 /// Additional methods on [`EventLoop`] that are specific to Android.
 pub trait EventLoopExtAndroid {}
@@ -17,15 +17,15 @@ pub trait EventLoopWindowTargetExtAndroid {}
 pub trait WindowExtAndroid {
     fn content_rect(&self) -> Rect;
 
-    fn config(&self) -> Configuration;
+    fn config(&self) -> ConfigurationRef;
 }
 
 impl WindowExtAndroid for Window {
     fn content_rect(&self) -> Rect {
         self.window.content_rect()
     }
 
-    fn config(&self) -> Configuration {
+    fn config(&self) -> ConfigurationRef {
         self.window.config()
     }
 }
@@ -36,3 +36,17 @@ impl<T> EventLoopWindowTargetExtAndroid for EventLoopWindowTarget<T> {}
 pub trait WindowBuilderExtAndroid {}
 
 impl WindowBuilderExtAndroid for WindowBuilder {}
+
+pub trait EventLoopBuilderExtAndroid {
+    /// Associates the `AndroidApp` that was passed to `android_main()` with the event loop
+    ///
+    /// This must be called on Android since the `AndroidApp` is not global state.
+    fn with_android_app(&mut self, app: AndroidApp) -> &mut Self;
+}
+
+impl<T> EventLoopBuilderExtAndroid for EventLoopBuilder<T> {
+    fn with_android_app(&mut self, app: AndroidApp) -> &mut Self {
+        self.platform_specific.android_app = Some(app);
+        self
+    }
+}