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)

To make it possible for application crates to avoid explicitly
depending on the `android-activity` crate (and avoid version conflicts)
this re-exports the android-activity crate under:

  `winit::platform::android::activity::*`

This also adds `android-native-activity` and `android-game-activity`
features that set the corresponding android-activity features.

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

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

.github/workflows/ci.yml

@@ -33,7 +33,7 @@ jobs:
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest,   }
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest, options: --no-default-features, features: x11 }
           - { target: x86_64-unknown-linux-gnu, os: ubuntu-latest, options: --no-default-features, features: "wayland,wayland-dlopen" }
-          - { target: aarch64-linux-android,    os: ubuntu-latest,  cmd: 'apk --' }
+          - { target: aarch64-linux-android,    os: ubuntu-latest,  cmd: 'apk --', features: "android-native-activity" }
           - { target: x86_64-apple-darwin,      os: macos-latest,    }
           - { target: x86_64-apple-ios,         os: macos-latest,    }
           - { target: aarch64-apple-ios,        os: macos-latest,    }

CHANGELOG.md

@@ -26,6 +26,7 @@ And please only add new entries to the top of this list, right below the `# Unre
 - On macOS, added support for `WindowEvent::ThemeChanged`.
 - **Breaking:** Removed `WindowBuilderExtWindows::with_theme` and `WindowBuilderExtWayland::with_wayland_csd_theme` in favour of `WindowBuilder::with_theme`.
 - **Breaking:** Removed `WindowExtWindows::theme` in favour of `Window::theme`.
+- **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))
 
 # 0.27.4
 

Cargo.toml

@@ -41,6 +41,8 @@ wayland-dlopen = ["sctk/dlopen", "wayland-client/dlopen"]
 wayland-csd-adwaita = ["sctk-adwaita", "sctk-adwaita/ab_glyph"]
 wayland-csd-adwaita-crossfont = ["sctk-adwaita", "sctk-adwaita/crossfont"]
 wayland-csd-adwaita-notitle = ["sctk-adwaita"]
+android-native-activity = [ "android-activity/native-activity" ]
+android-game-activity = [ "android-activity/game-activity" ]
 
 [dependencies]
 instant = { version = "0.1", features = ["wasm-bindgen"] }
@@ -59,7 +61,7 @@ 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 = "0.4.0-beta.1"
 
 [target.'cfg(any(target_os = "ios", target_os = "macos"))'.dependencies]
 objc2 = "=0.3.0-beta.3"

README.md

@@ -99,36 +99,93 @@ 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 = "0.4"`   |
+| 0.27  | `ndk-glue = "0.7"`           |
+| 0.26  | `ndk-glue = "0.5"`           |
+| 0.25  | `ndk-glue = "0.3"`           |
+| 0.24  | `ndk-glue = "0.2"`           |
+
+The recommended way to avoid a conflict with the glue version is to avoid explicitly
+depending on the `android-activity` crate, and instead consume the API that
+is re-exported by Winit under `winit::platform::android::activity::*`
 
 Running on an Android device needs a dynamic system library, add this to Cargo.toml:
 
 ```toml
-[[example]]
-name = "request_redraw_threaded"
+[lib]
+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` | `android-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`] | `android-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
+winit = { version = "0.28", features = [ "android-game-activity" ] }
+
+[target.'cfg(target_os = "android")'.dependencies]
+android_logger = "0.11.0"
+```
+
+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")]
+use winit::platform::android::activity::AndroidApp;
+
+#[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`
+2. Enable the `"android-native-activity"` feature for Winit: `winit = { version = "0.28", features = [ "android-native-activity" ] }`
+3. 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).
+4. 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 +36,43 @@ 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
+    }
+}
+
+/// Re-export of the `android_activity` API
+///
+/// Winit re-exports the `android_activity` API for convenience so that most
+/// applications can rely on the Winit crate to resolve the required version of
+/// `android_activity` and avoid any chance of a conflict between Winit and the
+/// application crate.
+///
+/// Unlike most libraries there can only be a single implementation
+/// of the `android_activity` glue crate linked with an application because
+/// it is responsible for the application's `android_main()` entry point.
+///
+/// Since Winit depends on a specific version of `android_activity` the simplest
+/// way to avoid creating a conflict is for applications to avoid explicitly
+/// depending on the `android_activity` crate, and instead consume the API that
+/// is re-exported by Winit.
+///
+/// For compatibility applications should then import the `AndroidApp` type for
+/// their `android_main(app: AndroidApp)` function like:
+/// ```rust
+/// #[cfg(target_os="android")]
+/// use winit::platform::android::activity::AndroidApp;
+/// ```
+pub mod activity {
+    pub use android_activity::*;
+}