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Android#

On Android, your screens are drawn with Android views: a Button is an android.widget.Button, a TextInput an EditText, a ScrollView a ScrollView. The app that leptoskit new generates is a small Kotlin activity that runs your screens as WebAssembly, so it can also replace them over the air with code push. Text is measured with the same fonts Android draws it with, so each label and button gets exactly the space it needs.

The generated app runs your screens' code on a thread of its own, not Android's main thread. Updating state, laying the screen out, and measuring text happen there, and the main thread only applies the result to the views. A slow update delays what you draw next, but scrolling, animations, and touches keep running meanwhile.

What you need#

  • Java 17 or newer, the Android SDK with platform 36, CMake, and an NDK. Set ANDROID_HOME to the SDK; ANDROID_NDK_HOME picks a specific NDK.
  • The Rust targets for your device or emulator, and for WebAssembly:
rustup target add aarch64-linux-android x86_64-linux-android wasm32-unknown-unknown

Develop with live reload#

leptoskit dev --platform android

dev builds the app, installs it on every attached phone and emulator, and opens it. With nothing attached, it boots your first emulator and leaves it running for next time. When you save, it rebuilds your interface and the running app swaps it in. It finds adb and the emulator through ANDROID_HOME. Building and serving covers the rest.

Build and run#

leptoskit build --platform android

This builds an arm64 debug APK, for phones and for emulators on Apple silicon, at android/app/build/outputs/apk/debug/app-debug.apk. Install it on a running emulator or a connected phone:

adb install -r android/app/build/outputs/apk/debug/app-debug.apk

For an x86-64 emulator, build leptoskit native build android-x86_64 instead. Add --release for a release APK, and an App Bundle beside it for Google Play. Release builds are signed with your upload key when the LEPTOSKIT_ANDROID_KEYSTORE, LEPTOSKIT_ANDROID_KEYSTORE_PASSWORD, LEPTOSKIT_ANDROID_KEY_ALIAS, and LEPTOSKIT_ANDROID_KEY_PASSWORD variables are set, and unsigned when they aren't. leptoskit publish play uploads to Google Play; see Publishing to the stores.

You can also open the android/ folder in Android Studio and run it from there. Build once with the CLI first, and again after you change your app or its plugins: leptoskit native prepare android regenerates the Rust inputs without running Gradle.

The android/ folder is yours: the activity, the manifest, the Gradle files, and Cargo.toml for native Rust dependencies such as runtime plugins. The CLI keeps its generated files in .leptoskit/android/, including the R8 keep rules that release builds need for the native library; put rules for your own code in android/app/proguard-rules.pro.

Test it#

Each project includes an instrumented test that taps Add one and waits for Count: 1. Update it when you change the home screen, then run it with an emulator running:

cd android
./gradlew connectedDebugAndroidTest

How it differs from macOS#

  • Generated apps also draw Switch, Checkbox, Radio, Slider, LinearProgressIndicator, and ActivityIndicator with Android controls. Switches and checkboxes report their checked state; radio options select within the group your screen controls, and sliders report values on their configured step grid. Tapping outside an editor clears focus and dismisses the keyboard. The keyboard moves to the next enabled field, or submits and dismisses from the last field. System Back is not yet connected to the file router; use an in-app Back button.
  • Rive uploads native vector frames to a reusable Android bitmap. The generated host schedules through Choreographer, stops when idle or offscreen, and treats disabled system animations as reduced motion. Native playback rejects text, image, layout, script, and audio assets.
  • In the backend crate, a Custom widget kind with no registered native implementation stops the app. Other platforms render an empty container instead.
  • TalkBack gets a smaller set of roles than macOS's accessibility API.
  • Italic text and per-edge borders (border-t, border-l-2, …) paint through Android views the same way they do on the web.
  • Sign-in uses Credential Manager for passkeys once your site vouches for the app's signing certificate. Other providers open in Chrome's Auth Tab, which needs Chrome 137 or later as the default browser; with another default browser, provider sign-in ends with the outcome unsupported. Sign in with Apple uses Apple's web sign-in, in the same Auth Tab.
  • Image loads local files and android.resource:// URIs, not URLs.
  • The generated app keeps your screen clear of the status and navigation bars. The bars take your root view's background, as a browser paints the page's background across the whole window, with light icons on a dark one. A root without an opaque background leaves the theme's white bars.
  • With the backend crate, the host reports the window insets for the system bars and display cutouts, and the safe-area utilities pad by them.

Ship an update without the Play Store#

The Play Store build carries the Kotlin activity, the interpreter, and a copy of your screens. To change the screens afterwards, publish a code push release for Android:

leptoskit native bundle --target android --out dist/android
leptoskit codepush release --from dist/android --platform android --channel production

The app downloads the release when the user asks for an update, tries it, and keeps it only if it renders. Anything that needs new native code, such as a new plugin or permission, still needs a Play Store release. Code push covers signing, publishing, and rollback.

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