Ferrous-Solitaire/docs/ANDROID.md

Android build — developer setup

This doc captures the toolchain install + build invocation for the Android target. Steps are runnable on a fresh Debian 13 (trixie) box; later sections document what's known to compile, what's stubbed, and the next milestones.

Status (2026-05-07): First working APK at fb8b2ac. 54 MB debug-signed ferrous-solitaire.apk for x86_64-linux-android. Has NOT yet been verified to launch on a device or emulator — that's the next milestone.

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1. Toolchain install (Debian 13 / trixie)

Run as one block. Will pull ~15-20 GB of disk between APT, the SDK, the NDK, the system image, and Rust target sysroots. Requires sudo.

# 1. JDK 21 (Android tooling needs JDK 17+; Debian 13 default is 21).
sudo apt update && sudo apt install -y openjdk-21-jdk-headless unzip wget

# 2. SDK directory + Google's cmdline-tools bootstrap.
export ANDROID_HOME="$HOME/Android/Sdk"
mkdir -p "$ANDROID_HOME/cmdline-tools"
wget -O /tmp/cmdline-tools.zip \
  https://dl.google.com/android/repository/commandlinetools-linux-11076708_latest.zip
unzip -q /tmp/cmdline-tools.zip -d "$ANDROID_HOME/cmdline-tools"
mv "$ANDROID_HOME/cmdline-tools/cmdline-tools" "$ANDROID_HOME/cmdline-tools/latest"
rm /tmp/cmdline-tools.zip

# 3. Persist env vars.
{
  echo ''
  echo '# Android dev'
  echo 'export ANDROID_HOME="$HOME/Android/Sdk"'
  echo 'export ANDROID_NDK_HOME="$ANDROID_HOME/ndk/26.3.11579264"'
  echo 'export JAVA_HOME="$(dirname $(dirname $(readlink -f $(which java))))"'
  echo 'export PATH="$PATH:$ANDROID_HOME/cmdline-tools/latest/bin:$ANDROID_HOME/platform-tools:$ANDROID_HOME/emulator"'
} >> ~/.bashrc
source ~/.bashrc

# 4. Accept SDK licences (interactive prompts answered by `yes |`).
yes | sdkmanager --licenses

# 5. Platform packages — ~5 GB.
sdkmanager \
  "platform-tools" \
  "platforms;android-34" \
  "build-tools;34.0.0" \
  "ndk;26.3.11579264" \
  "emulator" \
  "system-images;android-34;google_apis;x86_64"

# 6. AVD for testing (one-time).
echo no | avdmanager create avd \
  -n bevy_test \
  -k "system-images;android-34;google_apis;x86_64" \
  -d pixel_7

# 7. Rust cross-compile targets.
rustup target add \
  aarch64-linux-android \
  armv7-linux-androideabi \
  x86_64-linux-android \
  i686-linux-android

# 8. cargo-apk.
cargo install cargo-apk

Sanity:

java --version | head -1            # openjdk 21.0.x
adb --version | head -1             # 35.x or higher
sdkmanager --list_installed | head  # build-tools, emulator, ndk, platforms, system-images
avdmanager list avd | head          # bevy_test
rustup target list --installed | grep android  # 4 targets
cargo apk --help | head -5

If sdkmanager --version errors with JAVA_HOME is not set, the env section in step 3 didn't apply to your shell — source ~/.bashrc again or open a new terminal.

Optional: emulator runtime libs

The Android emulator is dynamically linked against X11/GL/audio. If emulator -list-avds works but emulator -avd bevy_test complains about libX11.so.6, install:

sudo apt install -y \
  libx11-6 libxcursor1 libxrandr2 libxi6 libxinerama1 libxxf86vm1 \
  libgl1 libnss3 libpulse0 libxcomposite1

Headless emulator launch:

emulator -avd bevy_test -no-window -gpu swiftshader_indirect &
adb wait-for-device && adb devices
# Stop later:
# adb -s emulator-5554 emu kill

Headless + software rendering is fine for "does it boot" smoke tests but useless for perf measurement — use a physical Pixel-class device over USB for real numbers.

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2. Build the APK

cargo apk build -p solitaire_app --target x86_64-linux-android

Output:

target/debug/apk/ferrous-solitaire.apk

Targets shipped via [package.metadata.android].build_targets in solitaire_app/Cargo.toml:

Target	Use
aarch64-linux-android	Real phones (modern 64-bit ARM)
armv7-linux-androideabi	Older 32-bit ARM phones
x86_64-linux-android	The bevy_test AVD on this dev box

Build any of them with --target <triple>.

Known cosmetic warning

After the APK is signed cargo-apk panics with:

thread 'main' panicked: Bin is not compatible with Cdylib

This happens AFTER the APK is on disk and signed. cargo-apk tries to also wrap the desktop [[bin]] target alongside the [lib]. The APK is valid — the panic is cosmetic. Always use --lib, which is the canonical build command (see CLAUDE.md §15.1):

cargo apk build -p solitaire_app --lib

Root cause: upstream cargo-apk bug — it does not skip [[bin]] targets when building for Android. No in-repo fix is possible; --lib is the accepted workaround.

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3. Install + run

Physical device:

adb devices                                    # confirm connection
adb install target/debug/apk/ferrous-solitaire.apk
adb shell am start -n com.ferrousapp.solitaire/android.app.NativeActivity
adb logcat | grep -iE "RustStdoutStderr|solitaire|panic"

Emulator:

emulator -avd bevy_test -no-window -gpu swiftshader_indirect &
adb wait-for-device
adb install target/debug/apk/ferrous-solitaire.apk
# ... same start + logcat steps as above.

If adb install errors with INSTALL_FAILED_NO_MATCHING_ABIS, the emulator is x86_64 but the APK was built for arm — rebuild with the x86_64-linux-android target, or add an x86_64 system image to the AVD.

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4. What's wired vs. what's stubbed

The first build pass (commit fb8b2ac) gates four desktop-only crates / call sites so the workspace cross-compiles. Each gate is documented at its call site.

Surface	Desktop	Android
Bevy windowing	x11 + wayland	android-native-activity (NativeActivity glue)
Clipboard ("Copy share link")	arboard writes URL	Toast surfaces the URL inline
OS keychain (JWT tokens)	keyring v4 → Secret Service / Keychain / Credential Store	Stub returning KeychainUnavailable; sync requires fresh login each launch
App entry point	bin target → solitaire_app::run()	cdylib target loaded by NativeActivity

What's NOT yet ported / not yet measured:

• dirs::data_dir() returns None on Android. Callers in solitaire_data/src/storage.rs, progress.rs, replay.rs, achievements.rs, settings.rs all need an Android-aware helper (likely /data/data/com.ferrousapp.solitaire/files).
• Touch UX pass — hit-target sizes, modal scaling on small screens, app lifecycle (suspend / resume), font scaling.
• Android Keystore via JNI for auth_tokens.
• JNI ClipboardManager for share links.
• Google Play Games sign-in (the solitaire_gpgs crate referenced in older docs doesn't yet exist).

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5. Iteration loop

# Edit code…
cargo build -p solitaire_app                           # desktop sanity
cargo clippy --workspace --all-targets -- -D warnings  # gate
cargo test --workspace                                 # gate
cargo apk build -p solitaire_app --target x86_64-linux-android --lib
adb install -r target/debug/apk/ferrous-solitaire.apk    # `-r` reinstalls
adb logcat -c && adb shell am start -n com.ferrousapp.solitaire/android.app.NativeActivity
adb logcat | grep -iE "RustStdoutStderr|solitaire"

adb logcat is the canonical way to see Bevy / Rust panic output — they end up in the RustStdoutStderr tag.