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Update README.md

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This tries to refresh some of the information in the README, providing
some more clarity on what version of the GameActivity Jetpack library is
required (if using the game-activity backend) and removing the older
information about how to port crates from ndk-glue to android-activity.
This commit is contained in:
Robert Bragg
2026-03-23 14:48:09 +00:00
parent f17b25b673
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@@ -13,7 +13,7 @@ It's comparable to [`android_native_app_glue.c`][ndk_concepts]
for C/C++ applications and is an alternative to the [ndk-glue] crate. for C/C++ applications and is an alternative to the [ndk-glue] crate.
`android-activity` provides a way to load your crate as a `cdylib` library via `android-activity` provides a way to load your crate as a `cdylib` library via
the `onCreate` method of your Android `Activity` class; run an `android_main()` the `onCreate` method of your Android `Activity` class; run an `android_main`
function in a separate thread from the Java main thread and marshal events (such function in a separate thread from the Java main thread and marshal events (such
as lifecycle events and input events) between Java and your native thread. as lifecycle events and input events) between Java and your native thread.
@@ -29,9 +29,9 @@ applications.
[ndk-glue]: https://crates.io/crates/ndk-glue [ndk-glue]: https://crates.io/crates/ndk-glue
[agdk]: https://developer.android.com/games/agdk/overview [agdk]: https://developer.android.com/games/agdk/overview
## Example ## Quick Start
Cargo.toml **Cargo.toml:**
```toml ```toml
[dependencies] [dependencies]
@@ -47,16 +47,20 @@ _Note: that you will need to either specify the **"native-activity"** feature or
**"game-activity"** feature to identify which `Activity` base class your **"game-activity"** feature to identify which `Activity` base class your
application is based on_ application is based on_
lib.rs **lib.rs:**
```rust ```rust
use std::sync::OnceLock;
use android_activity::{AndroidApp, InputStatus, MainEvent, PollEvent}; use android_activity::{AndroidApp, InputStatus, MainEvent, PollEvent};
// - Called on a dedicated Activity main loop thread, spawned after `android_on_create` returns
// - May be called multiple times if your Activity is destroyed and recreated.
// - Note: this symbol has a "Rust" ABI (default), not "C" ABI.
#[unsafe(no_mangle)] #[unsafe(no_mangle)]
fn android_main(app: AndroidApp) { fn android_main(app: AndroidApp) {
// `android_main` is tied to your `Activity` lifecycle, not your application lifecycle // `android_main` is tied to your `Activity` lifecycle, not your application lifecycle
// and so it may be called multiple times if your activity is destroyed and recreated. // and so it may be called multiple times if your Activity is destroyed and recreated.
// //
// Use a `OnceLock` or similar to ensure that you don't attempt to initialize global state // Use a `OnceLock` or similar to ensure that you don't attempt to initialize global state
// multiple times. // multiple times.
@@ -77,7 +81,7 @@ fn android_main(app: AndroidApp) {
// be associated with any `Activity` and it's methods will effectively be no-ops. // be associated with any `Activity` and it's methods will effectively be no-ops.
// //
// You should return from `android_main` and if your `Activity` gets recreated then // You should return from `android_main` and if your `Activity` gets recreated then
// a new `AndroidApp` will be passsed to a new invocation of `android_main`. // a new `AndroidApp` will be passed to a new invocation of `android_main`.
MainEvent::Destroy => { return; } MainEvent::Destroy => { return; }
_ => {} _ => {}
} }
@@ -101,11 +105,36 @@ cargo apk run
adb logcat example:V *:S adb logcat example:V *:S
``` ```
_Note: although `cargo apk` is convenient for this quick start example, it's
generally recommended that you should use a more-standard, Gradle-based build
system for your Android application and use something like `cargo ndk` for
building your Rust code into a `cdylib` that is then packaged via Gradle._
## Full Examples
See [this collection of
examples](https://github.com/rust-mobile/rust-android-examples) (based on both
`GameActivity` and `NativeActivity`).
Each example is a standalone Android Studio project that can serve as a
convenient template for starting a new project.
For the examples based on middleware frameworks (Winit or Egui) they also
aim to demonstrate how it's possible to write portable code that will run on
Android and other systems.
## Optional `android_on_create` entry point ## Optional `android_on_create` entry point
`android-activity` also supports an optional `android_on_create` entry point that gets called from the `android-activity` also supports an optional `android_on_create` entry point
`Activity.onCreate()` callback before `android_main()` is called, allowing for doing some setup work on the Java main that gets called from the `Activity.onCreate()` callback before `android_main()`
thread before the main Rust code starts running. is called.
`android_on_create` is called from the Java main / UI thread before the
`android_main` thread is spawned.
Considering that many Android SDK APIs (such as `android.view.View`) must be
accessed from the main thread, `android_on_create` can be a good place to do any
setup work that needs to be done on the Java main thread.
For example: For example:
@@ -131,80 +160,192 @@ fn android_on_create(state: &android_activity::OnCreateState) {
} }
``` ```
## Full Examples _(Note: there is also an `AndroidApp::run_on_java_main_thread()` method that
gives another way to run code on the Java main thread for some use cases)_
See [this collection of examples](https://github.com/rust-mobile/rust-android-examples) (based on both `GameActivity` and `NativeActivity`).
Each example is a standalone project that may also be a convenient templates for starting a new project.
For the examples based on middleware frameworks (Winit and or Egui) they also aim to demonstrate how it's possible to write portable code that will run on Android and other systems.
## Should I use NativeActivity or GameActivity? ## Should I use NativeActivity or GameActivity?
To learn more about the `NativeActivity` class that's shipped with Android see [here](https://developer.android.com/ndk/guides/concepts#naa). To learn more about the `NativeActivity` class that's shipped with Android see
[here](https://developer.android.com/ndk/guides/concepts#naa).
To learn more about the `GameActivity` class that's part of the [Android Game Developer's Kit][agdk] and also see a comparison with `NativeActivity` see [here](https://developer.android.com/games/agdk/game-activity) To learn more about the `GameActivity` class that's part of the [Android Game
Developer's Kit][agdk] and also see a comparison with `NativeActivity` see
[here](https://developer.android.com/games/agdk/game-activity)
Generally speaking, if unsure, `NativeActivity` may be more convenient to start with since you may not need to compile/link any Java or Kotlin code. Generally speaking, if unsure, `NativeActivity` may be more convenient to start
with since you may not need to compile/link any Java or Kotlin code, but
GameActivity is likely to be the better longer-term choice, due to being based
on `AppCompatActivity` and having built in support for input methods (such as
onscreen keyboards).
It's expected that the `GameActivity` backend will gain more sophisticated input handling features over time (such as for supporting input via onscreen keyboards or game controllers) and only `GameActivity` is based on the [`AppCompatActivity`] subclass which you may want in some situations to help with compatibility across devices. ### NativeActivity
Even if you start out using `NativeActivity` for the convenience, it's likely that most moderately complex applications will eventually need to define their own `Activity` subclass (either subclassing `NativeActivity` or `GameActivity`) which will require compiling at least a small amount of Java or Kotlin code. This is generally due to Android's design which directs numerous events via the `Activity` class which can only be processed by overloading some associated Activity method. - Good for: Simple apps, quick prototyping, limited text input support
- Setup: Just add the feature flag
- Limitations: No built-in input method support (can only receive physical key
events from soft keyboards that typically only allows basic ascii input)
## Switching from ndk-glue to android-activity The unique advantage of the `NativeActivity` class is that it's shipped as part
of the Android OS and so you can use it without needing to compile or link any
Java or Kotlin code.
### Winit-based applications `NativeActivity` is technically the only way to build a native Android
application purely in Rust without any Java or Kotlin code at all.
Firstly; if you have a [Winit](https://crates.io/crates/winit) based application and also have an explicit dependency on `ndk-glue` your application will need to remove its dependency on `ndk-glue` for the 0.28 release of Winit which will be based on android-activity (Since glue crates, due to their nature, can't be compatible with alternative glue crates). The most significant limitation of `NativeActivity` is that it doesn't have
built-in support for input methods (such as onscreen keyboards) and so it's
often not a good choice for applications that need to support text input.
Winit-based applications can follow the [Android documentation](https://docs.rs/winit/latest/winit/platform/android/index.html) guidance for advice on how to switch over. Most Winit-based applications should aim to remove any explicit dependency on a specific glue crate (so not depend directly on `ndk-glue` or `android-activity` and instead rely on Winit to pull in the right glue crate). The main practical change will then be to add a `#[unsafe(no_mangle)]fn android_main(app: AndroidApp)` entry point. Since some soft keyboards will deliver physical key events for basic ascii input
then `NativeActivity` can enable basic text input for prototyping but this is
unlikely to be sufficient for production applications.
See the [Android documentation](https://docs.rs/winit/latest/winit/platform/android/index.html) for more details and also see the [Winit-based examples here](https://github.com/rust-mobile/rust-android-examples). For advanced use cases, it would be possible to provide custom `InputConnection`
support in conjunction with `NativeActivity` but this isn't something that
`android-activity` provides out of the box currently.
### Middleware crates (i.e. not applications) ### GameActivity
If you have a crate that would be considered a middleware library (for example using JNI to support access to Bluetooth, or Android's Camera APIs) then the crate should almost certainly remove any dependence on a specific glue crate because this imposes a strict compatibility constraint that means the crate can only be used by applications that use that exact same glue crate version. - Good for: Apps needing text input, modern AndroidX features
- Setup requirements:
- Add gradle dependency: `androidx.games:games-activity:4.4.0`
- Enable the `game-activity` feature in Cargo.toml
- **Important**: Do NOT enable prefab support [details here](#don't-compile-and-link-the-upstream-gameactivity-prefab-c-glue-layer)
- Provides: IME support, AppCompatActivity features
Middleware libraries can instead look at using the [ndk-context](https://crates.io/crates/ndk-context) crate as a means for being able to use JNI without making any assumptions about the applications overall architecture. This way a middleware crate can work with alternative glue crates (including `ndk-glue` and `android-activity`) as well as work with embedded use cases (i.e. non-native applications that may just embed a dynamic library written in Rust to implement certain native functions). `GameActivity` has built in support for input methods via the `GameTextInput`
library and so is a better choice for applications that need to support text
input.
### Other, non-Winit-based applications `GameActivity` allows you to update the `ImeOptions` and actions associated with
the soft keyboard as well as receive IME span updates for tracking the user's
text input state.
The steps to switch a simple standalone application over from `ndk-glue` to `android-activity` (still based on `NativeActivity`) should be: `GameActivity` is based on the [`AppCompatActivity`] class, which is a standard
Jetpack / AndroidX class that offers a lot of built-in functionality to help
with compatibility across different Android versions and devices.
1. Remove `ndk-glue` from your Cargo.toml ### Game Activity Library Version
2. Add a dependency on `android-activity`, like `android-activity = { version="0.6", features = [ "native-activity" ] }`
3. Optionally add a dependency on `android_logger = "0.13.0"`
4. Update the `main` entry point to look like this:
```rust `android-activity` currently supports the [`GameActivity` 4.4.0 Jetpack
use android_activity::AndroidApp; library](https://developer.android.com/jetpack/androidx/releases/games) and is
backwards compatible with the previous `4.0.0` stable release. We can't
guarantee that the next 4.x stable release will be compatible, but it's fairly
likely that it will be.
#[no_mangle] Your Android package should depend on `androidx.games:games-activity:4.4.0` from
fn android_main(app: AndroidApp) { Google's Maven repository.
android_logger::init_once(android_logger::Config::default().with_min_level(log::Level::Info));
Read the upstream [GameActivity getting
started](https://developer.android.com/games/agdk/game-activity/get-started)
guide for more details on how to add the GameActivity library to your project.
#### Don't compile and link the upstream GameActivity 'prefab' (C++ glue) layer
**Important**: Do _not_ follow upstream instructions to enable native prefab
support for `GameActivity` that will compile and link the upstream C++ glue
layer as part of your build. The upstream glue layer is not directly compatible
with `android-activity` which provides its own native glue layer that integrates
with Rust.
I.e. you do _not_ need to enable prefabs via your `build.gradle` file:
```gradle
buildFeatures {
prefab true
} }
``` ```
See this minimal [NativeActivity Mainloop](https://github.com/rust-mobile/android-activity/tree/main/examples/na-mainloop) for more details about how to poll for events. and do _not_ add a snippet like this to your `CMakeLists.txt` file:
There is is no `#[ndk_glue::main]` replacement considering that `android_main()` entry point needs to be passed an `AndroidApp` argument which isn't compatible with a traditional `main()` function. Having an Android specific entry point also gives a place to initialize Android logging and handle other Android specific details (such as building an event loop based on the `app` argument) ```cmake
find_package(game-activity REQUIRED CONFIG)
target_link_libraries(${PROJECT_NAME} PUBLIC log android
game-activity::game-activity_static)
```
### Design Summary / Motivation behind android-activity ### Planning to Implement an Activity Subclass
Prior to working on android-activity, the existing glue crates available for building standalone Rust applications on Android were found to have a number of technical limitations that this crate aimed to solve: It's not possible to subclass an Activity from Rust / JNI code alone.
1. **Support alternative Activity classes**: Prior glue crates were based on `NativeActivity` and their API precluded supporting alternatives. In particular there was an interest in the [`GameActivity`] class in conjunction with it's [`GameTextInput`] library that can facilitate onscreen keyboard support. This also allows building applications based on the standard [`AppCompatActivity`] base class which isn't possible with `NativeActivity`. Finally there was interest in paving the way towards supporting a first-party `RustActivity` that could be best tailored towards the needs of Rust applications on Android. Keep in mind that Android's design directs many events via the `Activity` class
2. **Encapsulate IPC + synchronization between the native thread and the JVM thread**: For example with `ndk-glue` the application itself needs to avoid race conditions between the native and Java thread by following a locking convention) and it wasn't clear how this would extend to support other requests (like state saving) that also require synchronization. which can only be processed by overloading some associated `Activity` method, so
3. **Avoid static global state**: Keeping in mind the possibility of supporting applications with multiple native activities there was interest in having an API that didn't rely on global statics to track top-level state. Instead of having global getters for state then `android-activity` passes an explicit `app: AndroidApp` argument to the entry point that encapsulates the state connected with a single `Activity`. if you want to handle those events then you will need to implement an `Activity`
subclass and overload the relevant methods.
Most moderately complex applications will eventually need to define their own
`Activity` subclass (either subclassing `NativeActivity` or `GameActivity`)
which will require compiling at least a small amount of Java or Kotlin code.
_At the end of the day, Android's application programming model is fundamentally
based around a Java VM running Java/Kotlin code that can optionally call into
native code (not the other way around)._
## Design Summary / Motivation behind android-activity
Prior to working on `android-activity`, the existing glue crates available for
building standalone Rust applications on Android were found to have a number of
technical limitations that this crate aimed to solve:
1. **Support alternative Activity classes**: Prior glue crates were based on
`NativeActivity` and their API precluded supporting alternatives. In
particular there was an interest in the [`GameActivity`] class in conjunction
with its [`GameTextInput`] library that can facilitate onscreen keyboard
support. This also allows building applications based on the standard
[`AppCompatActivity`] base class which isn't possible with `NativeActivity`.
Finally there was interest in paving the way towards supporting a first-party
`RustActivity` that could be best tailored towards the needs of Rust
applications on Android.
2. **Encapsulate IPC + synchronization between the native thread and the JVM thread**:
For example with `ndk-glue` the application itself needs to avoid
race conditions between the native and Java thread by following a locking
convention) and it wasn't clear how this would extend to support other
requests (like state saving) that also require synchronization.
3. **Avoid static global state**: Keeping in mind the possibility of supporting
applications with multiple native activities there was interest in having an
API that didn't rely on global statics to track top-level state. Instead of
having global getters for state then `android-activity` passes an explicit
`app: AndroidApp` argument to the entry point that encapsulates the state
connected with a single `Activity`.
It's possible to write an application with `android-activity` that can
gracefully handle repeated create -> run -> destroy cycles of the `Activity`
due to its avoidance of global state. Theoretically you could even run
multiple `Activity` instances at the same (though since `NativeActivity` and
`GameActivity` were designed for fullscreen games, that only need a single
Activity, this is not a common use case).
[`GameTextInput`]: https://developer.android.com/games/agdk/add-support-for-text-input [`GameTextInput`]: https://developer.android.com/games/agdk/add-support-for-text-input
[`AppCompatActivity`]: https://developer.android.com/reference/androidx/appcompat/app/AppCompatActivity [`AppCompatActivity`]: https://developer.android.com/reference/androidx/appcompat/app/AppCompatActivity
## MSRV ## MSRV
We aim to (at least) support stable releases of Rust from the last three months. Rust has a 6 week release cycle which means we will support the last three stable releases. We aim to (at least) support stable releases of Rust from the last three months.
For example, when Rust 1.69 is released we would limit our `rust-version` to 1.67. Rust has a 6 week release cycle which means we will support the last three
stable releases. For example, when Rust 1.69 is released we would limit our
`rust-version` to 1.67.
We will only bump the `rust-version` at the point where we either depend on a new features or a dependency has increased its MSRV, and we won't be greedy. In other words we will only set the MSRV to the lowest version that's _needed_. We will only bump the `rust-version` at the point where we either depend on a
new features or a dependency has increased its MSRV, and we won't be greedy. In
other words we will only set the MSRV to the lowest version that's _needed_.
MSRV updates are not considered to be inherently semver breaking (unless a new feature is exposed in the public API) and so a `rust-version` change may happen in patch releases. MSRV updates are not considered to be inherently semver breaking (unless a new
feature is exposed in the public API) and so a `rust-version` change may happen
in patch releases.
## Game Activity Library Versioning Policy
Any single release of `android-activity` will support a specific version of the
Game Activity Jetpack / AndroidX library (documented above).
The required version of the Game Activity library does not form part of our Rust
semver contract, since it doesn't affect the public Rust API of
`android-activity`.
This means that a new patch release of `android-activity` may update the
required version of `GameActivity`, which may require users to update how they
package their application.
This is similar to how MSRV updates work, where new toolchain requirements can
affect how you build your application but that change is orthogonal to the
public API of the crate.