iris: add the android-app cdylib crate (I2, part 3)

iris/android-app is the concrete app RUST.md's I2 is judged against:
JNI_OnLoad, a Client implementing AndroidAppState, and new_view_peer
wrapping iris::android::new_peer's generic function in the plain
function pointer register_view_class needs. Its UI is tabs-ui::build,
unchanged from the winit example.

Deliberately excluded from the iris workspace (iris/Cargo.toml's new
`exclude`): android-view needs the NDK sysroot to link, so folding this
crate in would break `cargo build --workspace --all-targets` on the
host. It resolves as its own single-crate workspace instead, built
with `cd iris/android-app && cargo ndk -t x86_64 -P 26 build`.

Verified: cross-compiles and clippys clean for x86_64-linux-android
API 26; the host iris workspace (build/clippy/fmt/19 tests) is
unaffected. Not yet built: the Gradle shell (IrisView.java,
MainActivity, AndroidManifest) to actually install and run this on the
emulator -- next in RUST.md's I2.

Co-Authored-By: Claude Sonnet <noreply@anthropic.com>

.gitignore

@@ -26,3 +26,4 @@ sessions/
 
 # iris, the in-house UI library, is vendored at iris/ and built by cargo.
 iris/target/
+iris/android-app/target/

iris/Cargo.toml

@@ -49,6 +49,13 @@ tabs-ui = { path = "tabs-ui" }
 
 [workspace]
 members = ["core", "macro", "tabs-ui"]
+# android-app pulls in android-view, which needs the NDK sysroot to link
+# -- excluded so `cargo build --workspace --all-targets` on the host stays
+# buildable. Cross-compile it from its own directory (its own single-crate
+# workspace, since it has no `[workspace]` table of its own and this
+# exclusion stops it inheriting this one): `cd android-app && cargo ndk
+# -t x86_64 -P 26 build`.
+exclude = ["android-app"]
 
 [workspace.package]
 version = "0.1.0"

iris/android-app/Cargo.toml

@@ -0,0 +1,29 @@
+[package]
+name = "iris-android-app"
+version = "0.1.0"
+edition = "2024"
+
+# Deliberately outside the `iris` workspace (see that Cargo.toml's
+# `[workspace] exclude`): this crate exists only to be cross-compiled with
+# `cargo ndk` for the emulator/a phone, and pulls in android-view, which
+# needs the NDK sysroot to link. Folding it into the main workspace would
+# make `cargo build --workspace --all-targets` -- the host command RUST.md
+# and AGENTS.md both require to stay clean -- try to link a cdylib against
+# libraries that do not exist on this machine. See RUST.md's I2.
+
+[lib]
+name = "main"
+crate-type = ["cdylib"]
+
+[dependencies]
+iris = { path = "../" }
+tabs-ui = { path = "../tabs-ui" }
+android-view = { git = "https://github.com/rust-mobile/android-view.git", rev = "bec6c62a96cef8239b0fd7fedeef9b184d02e3a1" }
+android_logger = "0.15.0"
+log = "0.4.28"
+
+[profile.release]
+panic = "abort"
+
+[profile.dev]
+panic = "abort"

iris/android-app/src/lib.rs

@@ -0,0 +1,87 @@
+//! The android-view demo app: iris's `tabs` widget tree (`tabs_ui::build`,
+//! shared with the winit example) running through
+//! `iris::android`'s `ViewPeer`. This is RUST.md's I2 pass condition made
+//! concrete -- there is no UI here beyond what `tabs-ui` already draws.
+//!
+//! `JNI_OnLoad` and `new_view_peer` mirror android-view's own demo
+//! (`~/src/android-view/demo/src/lib.rs`): the only android-view-specific
+//! plumbing a real app needs is registering its `View` subclass and
+//! wrapping `iris::android::new_peer`'s generic function in a concrete
+//! `extern "system" fn`, since `register_view_class` wants a plain
+//! function pointer.
+
+use android_view::{
+    Context, View,
+    jni::{
+        JNIEnv, JavaVM,
+        sys::{JNI_VERSION_1_6, JavaVM as RawJavaVM, jint, jlong},
+    },
+    register_view_class,
+};
+use iris::android::{AndroidAppState, AndroidRsc, AndroidUiState, HasAndroidUiState};
+use iris::prelude::*;
+use log::LevelFilter;
+use std::ffi::c_void;
+
+/// The app's `View` subclass, matching the Java side's package --
+/// `app/src/main/java/dev/iris/android/demo/IrisView.java`.
+const VIEW_CLASS: &str = "dev/iris/android/demo/IrisView";
+
+pub struct Client {
+    ui_state: AndroidUiState,
+}
+
+impl HasAndroidUiState for Client {
+    fn android_state(&self) -> &AndroidUiState {
+        &self.ui_state
+    }
+    fn android_state_mut(&mut self) -> &mut AndroidUiState {
+        &mut self.ui_state
+    }
+}
+
+impl AndroidAppState for Client {
+    fn new(mut ui_state: AndroidUiState, rsc: &mut AndroidRsc<Self>) -> Self {
+        // `widgets.info` is the winit example's frame-debug readout, kept
+        // current from `DefaultAppState::window_event` -- android-view has
+        // no per-frame hook to drive the equivalent from here yet, so it
+        // is left at its built "" text rather than wired to nothing.
+        let _ = tabs_ui::build(rsc, &mut ui_state);
+        Self { ui_state }
+    }
+
+    fn back_pressed(&mut self, _rsc: &mut AndroidRsc<Self>, _render: &mut UiRenderState) -> bool {
+        // Nothing in the tabs example has a back stack of its own to pop --
+        // declining lets the activity finish, which is the same "no
+        // handler" behaviour the default impl gives. Present as an
+        // explicit override (rather than relying on the default) so a
+        // reader checking "does the back gesture reach this app" finds an
+        // answer here rather than nothing.
+        false
+    }
+}
+
+extern "system" fn new_view_peer<'local>(
+    env: JNIEnv<'local>,
+    view: View<'local>,
+    context: Context<'local>,
+) -> jlong {
+    iris::android::new_peer::<Client>(env, view, context)
+}
+
+/// # Safety
+/// Interacting with JNI at load time is always unsafe at some level --
+/// mirrors android-view's own demo, which carries the same comment.
+#[unsafe(no_mangle)]
+pub unsafe extern "system" fn JNI_OnLoad(vm: *mut RawJavaVM, _: *mut c_void) -> jint {
+    android_logger::init_once(
+        android_logger::Config::default()
+            .with_max_level(LevelFilter::Debug)
+            .with_tag("iris-android-app"),
+    );
+    let vm = unsafe { JavaVM::from_raw(vm) }.unwrap();
+    let mut env = vm.get_env().unwrap();
+    register_view_class(&mut env, VIEW_CLASS, new_view_peer);
+    iris::android::register_native_methods(&mut env, VIEW_CLASS);
+    JNI_VERSION_1_6
+}