iris: add an android-view backend beside winit (I2, part 1)

Relocates the platform-neutral halves of the winit `default` backend
(WidgetState, CursorState/sense, Tasks, Selector/Selectable's focus
handling) into shared crate-root modules so both backends can use them
without duplication, generalizes Tasks' redraw nudge behind a
RequestRedraw trait instead of a concrete winit::window::Window, and
adds iris/src/android/: a second backend on android-view's ViewPeer --
wgpu on the view's surface, touch as a mouse-like cursor, an
InputConnection bridge onto TextEdit (I1's parley editor), and a
window-insets side channel since android-view has no hook for it.
winit's own Android support pulls in android-activity without a
selected backend feature, so `default`/`android` are now target-gated
rather than both compiled in; confirmed by cross-compiling before this
split (cargo ndk failed inside android-activity) and after (clean).

Host build/clippy/fmt/tests and the android (x86_64, API 26)
cross-compile of the iris crate are all clean; the winit tabs example
still renders via run-headless.sh. Not yet exercised: an actual
android-app crate and Gradle shell to run this on the emulator -- next
in RUST.md's I2.

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

iris/Cargo.lock

@@ -88,6 +88,20 @@ version = "0.2.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "fc7eb209b1518d6bb87b283c20095f5228ecda460da70b44f0802523dea6da04"
 
+[[package]]
+name = "android-view"
+version = "0.1.0"
+source = "git+https://github.com/rust-mobile/android-view.git?rev=bec6c62a96cef8239b0fd7fedeef9b184d02e3a1#bec6c62a96cef8239b0fd7fedeef9b184d02e3a1"
+dependencies = [
+ "dpi",
+ "jni",
+ "ndk",
+ "num_enum",
+ "send_wrapper",
+ "smallvec",
+ "ui-events",
+]
+
 [[package]]
 name = "android_system_properties"
 version = "0.1.5"
@@ -739,15 +753,6 @@ version = "0.2.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "77ce24cb58228fbb8aa041425bb1050850ac19177686ea6e0f41a70416f56fdb"
 
-[[package]]
-name = "font-types"
-version = "0.10.1"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "39a654f404bbcbd48ea58c617c2993ee91d1cb63727a37bf2323a4edeed1b8c5"
-dependencies = [
- "bytemuck",
-]
-
 [[package]]
 name = "font-types"
 version = "0.12.4"
@@ -771,7 +776,7 @@ dependencies = [
  "objc2-core-text",
  "objc2-foundation 0.3.2",
  "parlance",
- "read-fonts 0.41.0",
+ "read-fonts",
  "roxmltree",
  "smallvec",
  "windows",
@@ -933,7 +938,7 @@ checksum = "c03d949a14aa089bbb282f7dd76a498a7f684428e4257202efc119ec010376f9"
 dependencies = [
  "bitflags 2.10.0",
  "bytemuck",
- "read-fonts 0.41.0",
+ "read-fonts",
  "smallvec",
 ]
 
@@ -1171,12 +1176,14 @@ dependencies = [
 name = "iris"
 version = "0.1.0"
 dependencies = [
+ "android-view",
  "arboard",
  "image",
  "iris-core",
  "iris-macro",
  "parley",
  "pollster",
+ "send_wrapper",
  "swash",
  "tokio",
  "wgpu",
@@ -1255,6 +1262,16 @@ dependencies = [
  "wasm-bindgen",
 ]
 
+[[package]]
+name = "keyboard-types"
+version = "0.8.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "0fbe853b403ae61a04233030ae8a79d94975281ed9770a1f9e246732b534b28d"
+dependencies = [
+ "bitflags 2.10.0",
+ "serde",
+]
+
 [[package]]
 name = "khronos-egl"
 version = "6.0.0"
@@ -1971,7 +1988,7 @@ dependencies = [
  "linebender_resource_handle",
  "parlance",
  "parley_data",
- "skrifa 0.44.0",
+ "skrifa",
 ]
 
 [[package]]
@@ -2314,16 +2331,6 @@ dependencies = [
  "crossbeam-utils",
 ]
 
-[[package]]
-name = "read-fonts"
-version = "0.36.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5eaa2941a4c05443ee3a7b26ab076a553c343ad5995230cc2b1d3e993bdc6345"
-dependencies = [
- "bytemuck",
- "font-types 0.10.1",
-]
-
 [[package]]
 name = "read-fonts"
 version = "0.41.0"
@@ -2331,7 +2338,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "046a7d674daf459825b32f5062056d6882db0d2f5a479fbd76ccfc870ac18709"
 dependencies = [
  "bytemuck",
- "font-types 0.12.4",
+ "font-types",
  "once_cell",
 ]
 
@@ -2455,6 +2462,12 @@ dependencies = [
  "tiny-skia",
 ]
 
+[[package]]
+name = "send_wrapper"
+version = "0.6.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "cd0b0ec5f1c1ca621c432a25813d8d60c88abe6d3e08a3eb9cf37d97a0fe3d73"
+
 [[package]]
 name = "serde"
 version = "1.0.228"
@@ -2506,16 +2519,6 @@ dependencies = [
  "quote",
 ]
 
-[[package]]
-name = "skrifa"
-version = "0.39.0"
-source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "9c9eb0b904a04d09bd68c65d946617b8ff733009999050f3b851c32fb3cfb60e"
-dependencies = [
- "bytemuck",
- "read-fonts 0.36.0",
-]
-
 [[package]]
 name = "skrifa"
 version = "0.44.0"
@@ -2523,7 +2526,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "819ab7d62b1d3e72d9d9dea5650bac30424f9111364bb94928dbf5ecad1baa68"
 dependencies = [
  "bytemuck",
- "read-fonts 0.41.0",
+ "read-fonts",
 ]
 
 [[package]]
@@ -2614,7 +2617,7 @@ version = "0.2.10"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6c2499c2d826531388872b2268718aed907a39bd785ab0dcfe57fab26283f92e"
 dependencies = [
- "skrifa 0.39.0",
+ "skrifa",
  "yazi",
  "zeno",
 ]
@@ -2823,6 +2826,16 @@ version = "0.25.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "d2df906b07856748fa3f6e0ad0cbaa047052d4a7dd609e231c4f72cee8c36f31"
 
+[[package]]
+name = "ui-events"
+version = "0.1.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "c4c2cc34489c685d4e7a1a1f97b7b4416c5aa789892114ae77df6cd2a60f0ec4"
+dependencies = [
+ "dpi",
+ "keyboard-types",
+]
+
 [[package]]
 name = "unicode-ident"
 version = "1.0.22"

iris/Cargo.toml

@@ -10,13 +10,36 @@ iris-core = { workspace = true }
 iris-macro = { workspace = true }
 parley = { workspace = true }
 swash = { workspace = true }
-winit = { workspace = true }
-arboard = { workspace = true, features = ["wayland-data-control"] }
 pollster = { workspace = true }
 wgpu = { workspace = true }
 image = { workspace = true }
 tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread"] }
 
+# winit everywhere except Android; android-view (below) is what stands in
+# for it there. Both backends live in this crate (see `src/android/mod.rs`'s
+# doc comment) but are never compiled together: winit's own Android support
+# pulls in `android-activity`, which panics at compile time unless one of
+# its own backend features is picked, and picking one is exactly what
+# `iris-core` was kept free of (RUST.md's I0b). Confirmed by trying it
+# 2026-09-05: `cargo ndk -t x86_64 -P 26 build -p iris` failed inside
+# `android-activity` itself with "Either game-activity or native-activity
+# must be enabled" before this split existed.
+[target.'cfg(not(target_os = "android"))'.dependencies]
+winit = { workspace = true }
+arboard = { workspace = true, features = ["wayland-data-control"] }
+
+# Pinned to the exact commit RUST.md's E1 (2026-09-04) measured on this
+# emulator -- real Vulkan rendering, a working `InputConnection`, and the
+# accesskit-detach abort, all against this rev specifically. Advancing it
+# wants re-running E1's checks, the same reason the nightly toolchain pin
+# is dated rather than floating.
+[target.'cfg(target_os = "android")'.dependencies]
+android-view = { git = "https://github.com/rust-mobile/android-view.git", rev = "bec6c62a96cef8239b0fd7fedeef9b184d02e3a1" }
+# Not re-exported by android-view (only `jni` and `ndk` are), and needed
+# for `android/insets.rs`'s own id -> state map -- the same reason
+# android-view's own `PEER_MAP` carries one.
+send_wrapper = "0.6.0"
+
 [dev-dependencies]
 tokio = { workspace = true, features = ["sync", "rt", "rt-multi-thread", "time"] }
 

iris/src/android/attr.rs

@@ -0,0 +1,25 @@
+use crate::attr::{FocusHost, recent_click};
+use crate::prelude::*;
+
+use super::view::HasAndroidUiState;
+
+impl<T: HasAndroidUiState> FocusHost for T {
+    fn recent_click(&mut self) -> bool {
+        recent_click(&mut self.android_state_mut().last_click)
+    }
+
+    fn set_focus(&mut self, id: Option<WeakWidget<TextEdit>>) {
+        self.android_state_mut().focus = id;
+    }
+
+    fn focus_gained(&mut self, region: Option<PixelRegion>) {
+        // Showing the keyboard is a JNI call (`InputMethodManager.showSoftInput`),
+        // and this runs deep inside the platform-agnostic sensor dispatch
+        // with no `CallbackCtx` in reach -- `IrisViewPeer::after_input`
+        // (`view.rs`) is what actually makes the call, right after the
+        // sensor pass that got here returns.
+        if region.is_some() {
+            self.android_state_mut().pending_show_keyboard = true;
+        }
+    }
+}

iris/src/android/ime.rs

@@ -0,0 +1,272 @@
+//! `InputConnection`, implemented directly against a focused `TextEdit`
+//! rather than against a stand-in editor the way android-view's own demo
+//! does over its `parley::PlainEditor` -- I1 already put parley behind
+//! `TextEdit`, so this is that same bridge, just wired to iris's widget
+//! instead of a bespoke one. Follows `demo/src/lib.rs`'s
+//! `impl InputConnection for DemoViewPeer`, which is where RUST.md's E1
+//! found the shape this needs (`text_before_cursor` is what gets Gboard's
+//! suggestion strip to read real words out of the buffer).
+//!
+//! Two things the demo tracks that this does not, both noted rather than
+//! silently dropped: a real "composing region" distinct from the
+//! selection (`set_composing_region` here just moves the caret, since
+//! `TextEdit` has no third range to hold one), and batch-edit coalescing
+//! (`begin`/`end_batch_edit` are no-ops -- a redraw mid-batch costs a frame
+//! it does not need to, not correctness).
+
+use crate::prelude::*;
+use android_view::{
+    CAP_MODE_SENTENCES, CallbackCtx, EditorInfo, IME_FLAG_NO_ENTER_ACTION, IME_FLAG_NO_EXTRACT_UI,
+    IME_FLAG_NO_FULLSCREEN, INPUT_TYPE_CLASS_TEXT, INPUT_TYPE_TEXT_FLAG_AUTO_CORRECT,
+    INPUT_TYPE_TEXT_FLAG_CAP_SENTENCES, INPUT_TYPE_TEXT_FLAG_MULTI_LINE, InputConnection,
+    caps_mode,
+};
+use std::borrow::Cow;
+
+use super::view::{AndroidAppState, IrisViewPeer};
+
+/// Byte offset -> UTF-16 code unit offset, the unit every `InputConnection`
+/// method speaks in (Java strings are UTF-16). `TextEdit` is byte-indexed
+/// throughout since I1 moved it to parley -- see `edit.rs`'s doc comment on
+/// `text()` -- so every crossing of this boundary goes through here rather
+/// than through ad hoc counting at each call site.
+fn byte_to_utf16(text: &str, byte_idx: usize) -> usize {
+    text[..byte_idx].encode_utf16().count()
+}
+
+fn utf16_to_byte(text: &str, utf16_idx: usize) -> usize {
+    let mut utf16_len = 0;
+    for (byte_idx, ch) in text.char_indices() {
+        if utf16_len >= utf16_idx {
+            return byte_idx;
+        }
+        utf16_len += ch.len_utf16();
+    }
+    text.len()
+}
+
+impl<State: AndroidAppState> IrisViewPeer<State> {
+    fn focus(&self) -> Option<WeakWidget<TextEdit>> {
+        self.state.android_state().focus
+    }
+}
+
+impl<State: AndroidAppState> InputConnection for IrisViewPeer<State> {
+    fn on_create_input_connection<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        out_attrs: &EditorInfo<'local>,
+    ) {
+        // Set once per `InputConnection`, not per field -- Android calls
+        // this when the view (not a particular widget) attaches to an
+        // IME. `MULTI_LINE`/`AUTO_CORRECT`/`CAP_SENTENCES` cover both the
+        // tabs example's composer and a plain single-line field well
+        // enough that no per-field variant is worth the extra state yet.
+        out_attrs.set_input_type(
+            &mut ctx.env,
+            INPUT_TYPE_CLASS_TEXT
+                | INPUT_TYPE_TEXT_FLAG_CAP_SENTENCES
+                | INPUT_TYPE_TEXT_FLAG_AUTO_CORRECT
+                | INPUT_TYPE_TEXT_FLAG_MULTI_LINE,
+        );
+        out_attrs.set_ime_options(
+            &mut ctx.env,
+            IME_FLAG_NO_FULLSCREEN | IME_FLAG_NO_EXTRACT_UI | IME_FLAG_NO_ENTER_ACTION,
+        );
+        if let Some(focus) = self.focus() {
+            let text = &self.rsc[focus];
+            let sel = text.selection_range().unwrap_or(0..0);
+            let start = byte_to_utf16(text.text(), sel.start) as i32;
+            let end = byte_to_utf16(text.text(), sel.end) as i32;
+            out_attrs.set_initial_sel_start(&mut ctx.env, start);
+            out_attrs.set_initial_sel_end(&mut ctx.env, end);
+            let caps = caps_mode(
+                &mut ctx.env,
+                text.text(),
+                start as usize,
+                CAP_MODE_SENTENCES,
+            );
+            out_attrs.set_initial_caps_mode(&mut ctx.env, caps);
+        }
+    }
+
+    fn text_before_cursor<'slf>(
+        &'slf mut self,
+        _ctx: &mut CallbackCtx,
+        n: i32,
+    ) -> Option<Cow<'slf, str>> {
+        if n < 0 {
+            return None;
+        }
+        let focus = self.focus()?;
+        let text = &self.rsc[focus];
+        let sel = text.selection_range()?;
+        let end_16 = byte_to_utf16(text.text(), sel.start);
+        let start_16 = end_16.saturating_sub(n as usize);
+        let start = utf16_to_byte(text.text(), start_16);
+        Some(Cow::Borrowed(&text.text()[start..sel.start]))
+    }
+
+    fn text_after_cursor<'slf>(
+        &'slf mut self,
+        _ctx: &mut CallbackCtx,
+        n: i32,
+    ) -> Option<Cow<'slf, str>> {
+        if n < 0 {
+            return None;
+        }
+        let focus = self.focus()?;
+        let text = &self.rsc[focus];
+        let sel = text.selection_range()?;
+        let len_16 = byte_to_utf16(text.text(), text.text().len());
+        let start_16 = byte_to_utf16(text.text(), sel.end);
+        let end_16 = (start_16 + n as usize).min(len_16);
+        let end = utf16_to_byte(text.text(), end_16);
+        Some(Cow::Borrowed(&text.text()[sel.end..end]))
+    }
+
+    fn selected_text<'slf>(&'slf mut self, _ctx: &mut CallbackCtx) -> Option<Cow<'slf, str>> {
+        let focus = self.focus()?;
+        Some(Cow::Owned(self.rsc[focus].selected_text()?))
+    }
+
+    fn cursor_caps_mode(&mut self, ctx: &mut CallbackCtx, req_modes: u32) -> u32 {
+        let Some(focus) = self.focus() else {
+            return 0;
+        };
+        let text = &self.rsc[focus];
+        let Some(caret) = text.caret() else {
+            return 0;
+        };
+        let off = byte_to_utf16(text.text(), caret);
+        caps_mode(&mut ctx.env, text.text(), off, req_modes)
+    }
+
+    fn delete_surrounding_text(
+        &mut self,
+        ctx: &mut CallbackCtx,
+        before_length: i32,
+        after_length: i32,
+    ) -> bool {
+        let Some(focus) = self.focus() else {
+            return false;
+        };
+        let text = &self.rsc[focus];
+        let Some(sel) = text.selection_range() else {
+            return false;
+        };
+        let content = text.text();
+        let start_16 =
+            byte_to_utf16(content, sel.start).saturating_sub(before_length.max(0) as usize);
+        let len_16 = byte_to_utf16(content, content.len());
+        let end_16 = (byte_to_utf16(content, sel.end) + after_length.max(0) as usize).min(len_16);
+        let start = utf16_to_byte(content, start_16);
+        let end = utf16_to_byte(content, end_16);
+        focus.edit(&mut self.rsc).delete_byte_range(start, end);
+        self.after_input(ctx);
+        true
+    }
+
+    fn delete_surrounding_text_in_code_points(
+        &mut self,
+        ctx: &mut CallbackCtx,
+        before_length: i32,
+        after_length: i32,
+    ) -> bool {
+        // Approximated as UTF-16 units rather than Unicode scalar values --
+        // the two differ only outside the Basic Multilingual Plane, which
+        // this widget tree does not exercise today. Worth revisiting if a
+        // field ever needs to edit emoji or other astral-plane text well.
+        self.delete_surrounding_text(ctx, before_length, after_length)
+    }
+
+    fn set_composing_text(
+        &mut self,
+        ctx: &mut CallbackCtx,
+        text: &str,
+        _new_cursor_position: i32,
+    ) -> bool {
+        let Some(focus) = self.focus() else {
+            return false;
+        };
+        // The IME re-sends its whole composition on every keystroke;
+        // `compose_len` (chars, not bytes -- `TextEditCtx::replace`'s unit)
+        // is what lets `replace` remove exactly what it inserted last time.
+        // The same shape as `default::DefaultApp`'s `Ime::Preedit` handling
+        // for winit.
+        let compose_len = self.state.android_state().compose_len;
+        focus.edit(&mut self.rsc).replace(compose_len, text);
+        self.state.android_state_mut().compose_len = text.chars().count();
+        self.after_input(ctx);
+        true
+    }
+
+    fn set_composing_region(&mut self, _ctx: &mut CallbackCtx, _start: i32, _end: i32) -> bool {
+        // `TextEdit` has no separate composing range to move -- see this
+        // module's doc comment. Declining (rather than moving the caret,
+        // which would surprise a caller expecting only a style change)
+        // is the safer approximation.
+        false
+    }
+
+    fn finish_composing_text(&mut self, ctx: &mut CallbackCtx) -> bool {
+        self.state.android_state_mut().compose_len = 0;
+        self.after_input(ctx);
+        true
+    }
+
+    fn set_selection(&mut self, ctx: &mut CallbackCtx, start: i32, end: i32) -> bool {
+        let Some(focus) = self.focus() else {
+            return false;
+        };
+        let text = &self.rsc[focus];
+        let content = text.text();
+        // Collapsed to `end`: `TextEditCtx` has no range-selection setter
+        // yet (nothing before I2 needed one), so an IME-driven selection
+        // lands the caret at its focus end rather than spanning both.
+        let byte = utf16_to_byte(content, end.max(0) as usize);
+        focus.edit(&mut self.rsc).set_cursor_byte(byte);
+        let _ = start;
+        self.after_input(ctx);
+        true
+    }
+
+    fn perform_editor_action(&mut self, _ctx: &mut CallbackCtx, _editor_action: i32) -> bool {
+        // `IME_FLAG_NO_ENTER_ACTION` above asks the IME not to offer one;
+        // nothing here needs handling it yet.
+        false
+    }
+
+    fn begin_batch_edit(&mut self, _ctx: &mut CallbackCtx) -> bool {
+        true
+    }
+
+    fn end_batch_edit(&mut self, _ctx: &mut CallbackCtx) -> bool {
+        true
+    }
+
+    fn send_key_event<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        event: &android_view::KeyEvent<'local>,
+    ) -> bool {
+        let key_code = event.key_code(&mut ctx.env);
+        let handled = super::input::on_key(
+            &mut self.rsc,
+            &mut self.state,
+            &mut ctx.env,
+            key_code,
+            event,
+        );
+        if handled {
+            self.after_input(ctx);
+        }
+        handled
+    }
+
+    fn request_cursor_updates(&mut self, _ctx: &mut CallbackCtx, _cursor_update_mode: i32) -> bool {
+        // No cursor-anchor UI to feed -- see RUST.md's I2 notes on what
+        // this backend does not do yet.
+        false
+    }
+}

iris/src/android/input.rs

@@ -0,0 +1,39 @@
+use crate::prelude::*;
+use android_view::{jni::JNIEnv, ndk::event::Keycode};
+
+use super::view::{AndroidAppState, AndroidRsc};
+
+/// Hardware/synthesized key handling for the field that currently has
+/// focus. Most typing on Android goes through the IME's `InputConnection`
+/// (`android/ime.rs`) instead -- this only sees what a soft keyboard still
+/// sends as a real `KeyEvent` in "not fullscreen" mode (Backspace, Enter,
+/// the arrow keys on a physical keyboard) plus whatever `unicode_char`
+/// reports for a plain key press. Returns whether anything used the event.
+pub(super) fn on_key<'local, State: AndroidAppState>(
+    rsc: &mut AndroidRsc<State>,
+    state: &mut State,
+    env: &mut JNIEnv<'local>,
+    key_code: Keycode,
+    event: &android_view::KeyEvent<'local>,
+) -> bool {
+    let Some(focus) = state.android_state().focus else {
+        return false;
+    };
+    let mut text = focus.edit(rsc);
+    match key_code {
+        Keycode::Del => text.backspace(false),
+        Keycode::ForwardDel => text.delete(false),
+        Keycode::DpadLeft => text.motion(Motion::Left, false),
+        Keycode::DpadRight => text.motion(Motion::Right, false),
+        Keycode::DpadUp => text.motion(Motion::Up, false),
+        Keycode::DpadDown => text.motion(Motion::Down, false),
+        Keycode::MoveHome => text.motion(Motion::LineStart, false),
+        Keycode::MoveEnd => text.motion(Motion::LineEnd, false),
+        Keycode::Enter | Keycode::NumpadEnter => text.newline(),
+        _ => match event.unicode_char(env) {
+            Some(c) if !c.is_control() => text.insert(&c.to_string()),
+            _ => return false,
+        },
+    }
+    true
+}

iris/src/android/insets.rs

@@ -0,0 +1,129 @@
+//! Window insets, fed in from outside `ViewPeer`.
+//!
+//! android-view's registered native methods (`view.rs` in that crate) cover
+//! touch, keys, focus, the surface and the IME -- there is nothing for
+//! `View.onApplyWindowInsets`, because android-view's own demo does not
+//! need it. The back gesture needed no new plumbing at all: with no
+//! `OnBackPressedCallback` registered, Android still delivers it as an
+//! ordinary `KEYCODE_BACK` `KeyEvent` through the ordinary key path (see
+//! `view.rs`'s `on_key_down`), which is the legacy behaviour every app gets
+//! by default and is enough for "the back gesture as an event". Insets have
+//! no such stand-in, so this module registers one more native method by
+//! hand, on the app's own `View` subclass rather than on android-view's.
+//!
+//! The peer id android-view hands back from `register_view_peer` is opaque
+//! outside that crate (`with_peer` is `pub(crate)` there), so there is no
+//! way to reach an existing `IrisViewPeer` from a JNI entry point we define
+//! ourselves. Instead of forking android-view to add a hook, `new_peer`
+//! (`view.rs`) inserts the *same* id into this module's own map, pointing
+//! at a plain `Rc<RefCell<Shared>>` cloned into `AndroidUiState` too --
+//! so writing here is reading there, with no dependency in either
+//! direction on the other's internals.
+
+use android_view::{
+    View,
+    jni::{
+        JNIEnv, NativeMethod,
+        descriptors::Desc,
+        objects::JClass,
+        sys::{jint, jlong},
+    },
+};
+use std::{
+    cell::RefCell,
+    collections::HashMap,
+    ffi::c_void,
+    rc::Rc,
+    sync::{Mutex, OnceLock},
+};
+
+use send_wrapper::SendWrapper;
+
+#[derive(Clone, Copy, Default, Debug, PartialEq, Eq)]
+pub struct Insets {
+    pub left: i32,
+    pub top: i32,
+    pub right: i32,
+    pub bottom: i32,
+    /// The keyboard's own inset (`WindowInsetsCompat.Type.ime()`), separate
+    /// from `bottom` (the system bars): a layout wants to know about the
+    /// keyboard specifically, since it usually means "make room" rather
+    /// than "stay clear of a corner".
+    pub ime_bottom: i32,
+}
+
+#[derive(Default)]
+pub struct Shared {
+    pub insets: Insets,
+}
+
+type SharedMap = HashMap<jlong, SendWrapper<Rc<RefCell<Shared>>>>;
+
+fn map() -> &'static Mutex<SharedMap> {
+    static MAP: OnceLock<Mutex<SharedMap>> = OnceLock::new();
+    MAP.get_or_init(Default::default)
+}
+
+/// Called from `view::new_peer` with the same id android-view's
+/// `register_view_peer` returned, so a later `apply_window_insets` call
+/// (keyed on that id by Java, which only ever sees the one long) reaches
+/// the same `Shared` cell `AndroidUiState` reads from.
+pub(super) fn register(id: jlong, shared: Rc<RefCell<Shared>>) {
+    map().lock().unwrap().insert(id, SendWrapper::new(shared));
+}
+
+extern "system" fn unregister_insets<'local>(
+    _env: JNIEnv<'local>,
+    _view: View<'local>,
+    peer: jlong,
+) {
+    map().lock().unwrap().remove(&peer);
+}
+
+extern "system" fn apply_window_insets<'local>(
+    mut env: JNIEnv<'local>,
+    view: View<'local>,
+    peer: jlong,
+    left: jint,
+    top: jint,
+    right: jint,
+    bottom: jint,
+    ime_bottom: jint,
+) {
+    if let Some(shared) = map().lock().unwrap().get(&peer) {
+        shared.borrow_mut().insets = Insets {
+            left,
+            top,
+            right,
+            bottom,
+            ime_bottom,
+        };
+    }
+    // Insets can change (the keyboard opening) with no resize and no
+    // touch, so nothing else here would otherwise ask for a frame.
+    view.post_frame_callback(&mut env);
+}
+
+/// Registers `applyWindowInsetsNative` on the app's own `View` subclass.
+/// Called once from `JNI_OnLoad` alongside `android_view::register_view_class`.
+pub fn register_native_methods<'local, 'other_local>(
+    env: &mut JNIEnv<'local>,
+    class: impl Desc<'local, JClass<'other_local>>,
+) {
+    env.register_native_methods(
+        class,
+        &[
+            NativeMethod {
+                name: "applyWindowInsetsNative".into(),
+                sig: "(JIIIII)V".into(),
+                fn_ptr: apply_window_insets as *mut c_void,
+            },
+            NativeMethod {
+                name: "unregisterInsetsNative".into(),
+                sig: "(J)V".into(),
+                fn_ptr: unregister_insets as *mut c_void,
+            },
+        ],
+    )
+    .unwrap();
+}

iris/src/android/mod.rs

@@ -0,0 +1,42 @@
+//! iris's second windowing backend: `android-view` (a `SurfaceView` plus a
+//! JNI `ViewPeer`) instead of winit. See RUST.md's I2 for why this exists
+//! as a second backend rather than winit's own (unfinished, and blocked on
+//! `android-activity`'s backend-feature requirement) Android support, and
+//! for the pass condition this was built against.
+//!
+//! Structured to mirror `default/` module for module: `view.rs` is that
+//! module's `app.rs` + `state.rs` combined (android-view has one harness
+//! type, `ViewPeer`, where winit splits `ApplicationHandler` from the
+//! per-window state), `render.rs` is `render.rs`, `input.rs` is `input.rs`,
+//! `attr.rs` is `attr.rs`. `ime.rs` and `insets.rs` have no winit
+//! counterpart: winit cannot drive an IME beyond `Ime::Preedit`/`Commit`
+//! (RUST.md's E1) and has no concept of Android's window insets at all.
+
+mod attr;
+mod ime;
+mod input;
+mod insets;
+mod render;
+mod view;
+
+pub use insets::Insets;
+pub use render::AndroidRenderer;
+pub use view::{
+    AndroidAppState, AndroidRsc, AndroidUiState, HasAndroidUiState, IrisViewPeer, new_peer,
+};
+
+/// Registers the extra native methods this backend needs beyond what
+/// `android_view::register_view_class` covers (window insets -- see
+/// `insets.rs`'s doc comment for why that one could not ride along on an
+/// existing android-view callback the way the back gesture does). Call
+/// from `JNI_OnLoad` alongside `register_view_class`, on the same `View`
+/// subclass.
+pub fn register_native_methods<'local, 'other_local>(
+    env: &mut android_view::jni::JNIEnv<'local>,
+    class: impl android_view::jni::descriptors::Desc<
+        'local,
+        android_view::jni::objects::JClass<'other_local>,
+    >,
+) {
+    insets::register_native_methods(env, class);
+}

iris/src/android/render.rs

@@ -0,0 +1,194 @@
+use crate::task::RequestRedraw;
+use android_view::{
+    View,
+    jni::{JavaVM, objects::GlobalRef},
+    ndk::native_window::NativeWindow,
+};
+use iris_core::{UiData, UiRenderNode, UiRenderState};
+use pollster::FutureExt;
+use wgpu::{
+    rwh::{DisplayHandle, HandleError, HasDisplayHandle, HasWindowHandle, WindowHandle},
+    *,
+};
+
+pub const CLEAR_COLOR: Color = Color::BLACK;
+
+/// `NativeWindow` (from the surface android-view hands over in
+/// `surfaceChanged`) has a window handle but not a display one -- there is
+/// exactly one display on Android and `rwh` has a unit variant for it.
+/// Mirrors android-view's own demo (`demo/src/lib.rs`'s
+/// `AndroidWindowHandle`).
+struct AndroidWindowHandle {
+    window: NativeWindow,
+}
+
+impl HasDisplayHandle for AndroidWindowHandle {
+    fn display_handle(&self) -> Result<DisplayHandle<'_>, HandleError> {
+        Ok(DisplayHandle::android())
+    }
+}
+
+impl HasWindowHandle for AndroidWindowHandle {
+    fn window_handle(&self) -> Result<WindowHandle<'_>, HandleError> {
+        self.window.window_handle()
+    }
+}
+
+/// The android-view surface, unlike winit's window, does not outlive a
+/// backgrounding of the activity: `surfaceDestroyed`/`surfaceCreated` (via
+/// `SurfaceHolder.Callback`) recreate it, so this holds everything that
+/// depends on that surface rather than being built once at startup --
+/// `AndroidUiState` holds it as `Option<AndroidRenderer>`, `None` exactly
+/// when there is no surface to draw into.
+pub struct AndroidRenderer {
+    surface: Surface<'static>,
+    device: Device,
+    queue: Queue,
+    config: SurfaceConfiguration,
+    encoder: CommandEncoder,
+    pub ui: UiRenderNode,
+}
+
+impl AndroidRenderer {
+    pub fn new(window: NativeWindow, width: u32, height: u32) -> Self {
+        let instance = Instance::new(&InstanceDescriptor {
+            backends: Backends::PRIMARY,
+            ..Default::default()
+        });
+
+        // SAFETY: the `NativeWindow` outlives the surface built from it --
+        // android-view drops the old renderer (and this surface with it)
+        // before handing over a new window, in `surface_changed` below.
+        let surface = instance
+            .create_surface(SurfaceTarget::from(AndroidWindowHandle { window }))
+            .expect("Could not create android surface!");
+
+        let adapter = instance
+            .request_adapter(&RequestAdapterOptions {
+                power_preference: PowerPreference::default(),
+                compatible_surface: Some(&surface),
+                force_fallback_adapter: false,
+            })
+            .block_on()
+            .expect("Could not get adapter!");
+
+        // Same request as the winit backend's `UiRenderer::new` -- no
+        // binding-array features, see TEXTURES.md's "Recommended shape".
+        let (device, queue) = adapter
+            .request_device(&DeviceDescriptor {
+                required_limits: Limits {
+                    max_buffer_size: 1 << 30,
+                    ..Default::default()
+                },
+                ..Default::default()
+            })
+            .block_on()
+            .expect("Could not get device!");
+
+        let surface_caps = surface.get_capabilities(&adapter);
+        let surface_format = surface_caps
+            .formats
+            .iter()
+            .copied()
+            .find(|f| f.is_srgb())
+            .unwrap_or(surface_caps.formats[0]);
+
+        let config = SurfaceConfiguration {
+            usage: TextureUsages::RENDER_ATTACHMENT,
+            format: surface_format,
+            width,
+            height,
+            present_mode: PresentMode::AutoVsync,
+            alpha_mode: surface_caps.alpha_modes[0],
+            desired_maximum_frame_latency: 2,
+            view_formats: vec![],
+        };
+        surface.configure(&device, &config);
+
+        let encoder = Self::create_encoder(&device);
+        let ui = UiRenderNode::new(&device, &queue, &config);
+
+        Self {
+            surface,
+            device,
+            queue,
+            config,
+            encoder,
+            ui,
+        }
+    }
+
+    fn create_encoder(device: &Device) -> CommandEncoder {
+        device.create_command_encoder(&CommandEncoderDescriptor {
+            label: Some("Render Encoder"),
+        })
+    }
+
+    pub fn update(&mut self, ui: &mut UiData, render: &mut UiRenderState) {
+        self.ui.update(&self.device, &self.queue, ui, render);
+    }
+
+    pub fn draw(&mut self) {
+        let output = self.surface.get_current_texture().unwrap();
+        let view = output
+            .texture
+            .create_view(&TextureViewDescriptor::default());
+
+        let mut encoder = std::mem::replace(&mut self.encoder, Self::create_encoder(&self.device));
+        {
+            let render_pass = &mut encoder.begin_render_pass(&RenderPassDescriptor {
+                color_attachments: &[Some(RenderPassColorAttachment {
+                    view: &view,
+                    resolve_target: None,
+                    ops: Operations {
+                        load: LoadOp::Clear(CLEAR_COLOR),
+                        store: StoreOp::Store,
+                    },
+                    depth_slice: None,
+                })],
+                ..Default::default()
+            });
+            self.ui.draw(render_pass);
+        }
+
+        self.queue.submit(std::iter::once(encoder.finish()));
+        output.present();
+    }
+
+    pub fn size(&self) -> iris_core::util::Vec2 {
+        (self.config.width, self.config.height).into()
+    }
+
+    pub fn resize(&mut self, width: u32, height: u32) {
+        self.config.width = width;
+        self.config.height = height;
+        self.surface.configure(&self.device, &self.config);
+        self.ui.resize((width, height), &self.queue);
+    }
+}
+
+/// `Tasks`' redraw handle on Android: a background task finishes on the
+/// tokio thread `Tasks::init` spawned, which is not attached to the JVM, so
+/// asking for a frame means attaching first. `post_frame_callback` needs a
+/// live `View` reference; the global ref is what survives past the JNI call
+/// that handed it to us.
+pub struct AndroidRedrawHandle {
+    vm: JavaVM,
+    view: GlobalRef,
+}
+
+impl AndroidRedrawHandle {
+    pub fn new(vm: JavaVM, view: GlobalRef) -> Self {
+        Self { vm, view }
+    }
+}
+
+impl RequestRedraw for AndroidRedrawHandle {
+    fn request_redraw(&self) {
+        let Ok(mut env) = self.vm.attach_current_thread() else {
+            return;
+        };
+        let local = env.new_local_ref(&self.view).unwrap();
+        View(local).post_frame_callback(&mut env);
+    }
+}

iris/src/android/view.rs

@@ -0,0 +1,404 @@
+use crate::prelude::*;
+use crate::task::RequestRedraw;
+use android_view::{
+    CallbackCtx, Context, InputConnection, KeyEvent, MotionEvent, Rect, View, ViewPeer,
+    jni::JNIEnv,
+    ndk::event::{Keycode, MotionAction},
+};
+// `marker::Sized` explicitly: `crate::prelude::*` below also brings in the
+// `Sized` *widget* (`widget::position::sized::Sized`), and an unqualified
+// glob import shadows the language prelude -- `default/mod.rs` has the same
+// explicit import for the same reason.
+use std::{
+    cell::RefCell,
+    marker::{PhantomData, Sized},
+    rc::Rc,
+    sync::Arc,
+    time::Instant,
+};
+
+use super::{
+    insets::{Insets, Shared},
+    render::{AndroidRedrawHandle, AndroidRenderer},
+};
+
+/// The android-view analogue of `default::DefaultUiState`. `renderer` is an
+/// `Option` because a `SurfaceView`'s surface does not outlive backgrounding
+/// the way a winit `Window` does -- `surfaceDestroyed`/`surfaceCreated` can
+/// happen any number of times over the life of one `IrisViewPeer`.
+pub struct AndroidUiState {
+    pub root: Option<StrongWidget>,
+    pub renderer: Option<AndroidRenderer>,
+    pub focus: Option<WeakWidget<TextEdit>>,
+    pub cursor: CursorState,
+    pub last_click: Instant,
+    /// The IME preedit's previous length, in `char`s -- the same
+    /// re-send-the-whole-composition bookkeeping `default::DefaultUiState`
+    /// keeps for winit's `Ime::Preedit`, since android-view's
+    /// `setComposingText` has the identical shape (see `android/ime.rs`).
+    pub compose_len: usize,
+    /// Set by `attr::FocusHost::focus_gained` when a `TextEdit` is focused;
+    /// consumed by the touch handler after the sensor pass finishes, since
+    /// showing the keyboard is a JNI call and `focus_gained` runs deep
+    /// inside the platform-agnostic sensor dispatch with no `CallbackCtx`
+    /// in reach.
+    pub pending_show_keyboard: bool,
+    /// Window insets, filled in from outside the normal `ViewPeer` callback
+    /// path -- see `android/insets.rs` for why they need a registry of
+    /// their own.
+    shared: Rc<RefCell<Shared>>,
+}
+
+impl AndroidUiState {
+    fn new(shared: Rc<RefCell<Shared>>) -> Self {
+        Self {
+            root: None,
+            renderer: None,
+            focus: None,
+            cursor: Default::default(),
+            last_click: Instant::now(),
+            compose_len: 0,
+            pending_show_keyboard: false,
+            shared,
+        }
+    }
+
+    pub fn insets(&self) -> Insets {
+        self.shared.borrow().insets
+    }
+}
+
+impl HasRoot for AndroidUiState {
+    fn set_root(&mut self, root: StrongWidget) {
+        self.root = Some(root);
+    }
+}
+
+pub trait HasAndroidUiState: Sized + 'static {
+    fn android_state(&self) -> &AndroidUiState;
+    fn android_state_mut(&mut self) -> &mut AndroidUiState;
+}
+
+pub trait AndroidAppState: HasAndroidUiState {
+    fn new(ui_state: AndroidUiState, rsc: &mut AndroidRsc<Self>) -> Self;
+    /// The system back gesture/button. `true` means handled -- nothing
+    /// further happens; `false` lets the activity finish as it would with
+    /// no view at all. The default declines, since most screens have
+    /// nothing to intercept it for.
+    #[allow(unused_variables)]
+    fn back_pressed(&mut self, rsc: &mut AndroidRsc<Self>, render: &mut UiRenderState) -> bool {
+        false
+    }
+}
+
+/// The android-view analogue of `default::DefaultRsc` -- identical in
+/// substance, since none of `UiRsc`/`HasEvents`/`HasTasks`/`HasWidgetState`
+/// mention winit. Kept as a separate type rather than shared code because
+/// the two backends' `ViewPeer`/`ApplicationHandler` entry points hold
+/// their harness state differently (see RUST.md's I2).
+pub struct AndroidRsc<State: 'static> {
+    pub ui: UiData,
+    pub events: EventManager<Self>,
+    pub tasks: Tasks<Self>,
+    pub state: WidgetState,
+    _state: PhantomData<State>,
+}
+
+impl<State> AndroidRsc<State> {
+    pub fn create_state<T: 'static>(&mut self, id: impl IdLike, data: T) -> WeakState<T> {
+        self.state.add(id.id(), data)
+    }
+}
+
+impl<State> UiRsc for AndroidRsc<State> {
+    fn ui(&self) -> &UiData {
+        &self.ui
+    }
+    fn ui_mut(&mut self) -> &mut UiData {
+        &mut self.ui
+    }
+    fn on_draw(&mut self, active: &ActiveData) {
+        self.events.draw(active);
+    }
+    fn on_undraw(&mut self, active: &ActiveData) {
+        self.events.undraw(active);
+    }
+    fn on_remove(&mut self, id: WidgetId) {
+        self.events.remove(id);
+        self.state.remove(id);
+    }
+}
+
+impl<State: 'static> HasState for AndroidRsc<State> {
+    type State = State;
+}
+
+impl<State: 'static> HasEvents for AndroidRsc<State> {
+    fn events(&self) -> &EventManager<Self> {
+        &self.events
+    }
+    fn events_mut(&mut self) -> &mut EventManager<Self> {
+        &mut self.events
+    }
+}
+
+impl<State: 'static> HasTasks for AndroidRsc<State> {
+    fn tasks_mut(&mut self) -> &mut Tasks<Self> {
+        &mut self.tasks
+    }
+}
+
+impl<State: 'static> HasWidgetState for AndroidRsc<State> {
+    fn widget_state(&self) -> &WidgetState {
+        &self.state
+    }
+    fn widget_state_mut(&mut self) -> &mut WidgetState {
+        &mut self.state
+    }
+}
+
+/// The `ViewPeer` android-view dispatches every callback to. One per
+/// `RustView` instance; `new_peer` (below) builds it and hands the id to
+/// Java the same way android-view's own demo does.
+pub struct IrisViewPeer<State: AndroidAppState> {
+    pub(super) rsc: AndroidRsc<State>,
+    pub(super) render: UiRenderState,
+    pub(super) state: State,
+    task_recv: TaskMsgReceiver<AndroidRsc<State>>,
+}
+
+impl<State: 'static, I: RscIdx<AndroidRsc<State>>> std::ops::Index<I> for AndroidRsc<State> {
+    type Output = I::Output;
+
+    fn index(&self, index: I) -> &Self::Output {
+        index.get(self)
+    }
+}
+
+impl<State: 'static, I: RscIdx<AndroidRsc<State>>> std::ops::IndexMut<I> for AndroidRsc<State> {
+    fn index_mut(&mut self, index: I) -> &mut Self::Output {
+        index.get_mut(self)
+    }
+}
+
+impl<State: AndroidAppState> IrisViewPeer<State> {
+    fn drain_tasks(&mut self) {
+        while let Ok(update) = self.task_recv.try_recv() {
+            update(&mut self.state, &mut self.rsc);
+        }
+    }
+
+    /// Common tail for every callback that might have changed the cursor,
+    /// the text focus, or the widget tree: run the sensors that touch
+    /// input feeds, then ask for a frame if the result needs drawing.
+    /// Mirrors `default::DefaultApp::window_event`'s tail, split across
+    /// android-view's several entry points instead of winit's one.
+    pub(super) fn after_input(&mut self, ctx: &mut CallbackCtx) {
+        let window_size = self.window_size();
+        let ui_state = self.state.android_state_mut();
+        let cursor = ui_state.cursor.clone();
+        let old_focus = ui_state.focus;
+        self.render
+            .run_sensors(&mut self.rsc, &mut self.state, cursor, window_size);
+
+        let ui_state = self.state.android_state_mut();
+        if old_focus != ui_state.focus
+            && let Some(old) = old_focus
+        {
+            old.edit(&mut self.rsc).deselect();
+        }
+        if std::mem::take(&mut ui_state.pending_show_keyboard) {
+            show_soft_input(&mut ctx.env, &ctx.view);
+        }
+
+        let ui_state = self.state.android_state_mut();
+        ui_state.cursor.end_frame();
+        if self.render.needs_redraw(&ui_state.root, self.rsc.widgets()) {
+            ctx.view.post_frame_callback(&mut ctx.env);
+        }
+    }
+
+    fn window_size(&self) -> Vec2 {
+        let ui_state = self.state.android_state();
+        match &ui_state.renderer {
+            Some(r) => r.size(),
+            None => Vec2::ZERO,
+        }
+    }
+
+    fn render(&mut self) {
+        let ui_state = self.state.android_state();
+        if ui_state.renderer.is_none() {
+            return;
+        }
+        let ui_state = self.state.android_state_mut();
+        self.render.update(&ui_state.root, &mut self.rsc);
+        let ui_state = self.state.android_state_mut();
+        let Some(renderer) = &mut ui_state.renderer else {
+            return;
+        };
+        renderer.update(&mut self.rsc.ui, &mut self.render);
+        renderer.draw();
+    }
+}
+
+fn show_soft_input<'local>(env: &mut JNIEnv<'local>, view: &View<'local>) {
+    let imm = view.input_method_manager(env);
+    imm.show_soft_input(env, view, 0);
+}
+
+impl<State: AndroidAppState> ViewPeer for IrisViewPeer<State> {
+    fn on_key_down<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        key_code: Keycode,
+        event: &KeyEvent<'local>,
+    ) -> bool {
+        self.drain_tasks();
+        // With no `OnBackPressedCallback` registered on the Java side, the
+        // system still delivers the back gesture as a synthetic
+        // `KEYCODE_BACK` through this same path -- the legacy behaviour
+        // every view-based app gets by default, and enough for "the back
+        // gesture as an event" without a second JNI registry. See
+        // `android/insets.rs`'s doc comment for why insets could not take
+        // the same shortcut.
+        if key_code == Keycode::Back {
+            let handled = self.state.back_pressed(&mut self.rsc, &mut self.render);
+            if handled {
+                self.after_input(ctx);
+            }
+            return handled;
+        }
+        let handled = super::input::on_key(
+            &mut self.rsc,
+            &mut self.state,
+            &mut ctx.env,
+            key_code,
+            event,
+        );
+        if handled {
+            self.after_input(ctx);
+        }
+        handled
+    }
+
+    fn on_touch_event<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        event: &MotionEvent<'local>,
+    ) -> bool {
+        self.drain_tasks();
+        let action = event.action_masked(&mut ctx.env);
+        let x = event.x(&mut ctx.env);
+        let y = event.y(&mut ctx.env);
+        let ui_state = self.state.android_state_mut();
+        match action {
+            MotionAction::Down => {
+                ui_state.cursor.pos = vec2(x, y);
+                ui_state.cursor.exists = true;
+                ui_state.cursor.buttons.left.update(true);
+            }
+            MotionAction::Move => {
+                ui_state.cursor.pos = vec2(x, y);
+            }
+            MotionAction::Up | MotionAction::Cancel => {
+                ui_state.cursor.pos = vec2(x, y);
+                ui_state.cursor.buttons.left.update(false);
+            }
+            _ => return false,
+        }
+        self.after_input(ctx);
+        true
+    }
+
+    fn on_focus_changed<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        gain_focus: bool,
+        _direction: i32,
+        _previously_focused_rect: Option<&Rect<'local>>,
+    ) {
+        self.drain_tasks();
+        if !gain_focus {
+            let ui_state = self.state.android_state_mut();
+            if let Some(focus) = ui_state.focus.take() {
+                focus.edit(&mut self.rsc).deselect();
+            }
+        }
+        self.after_input(ctx);
+    }
+
+    fn on_attached_to_window(&mut self, _ctx: &mut CallbackCtx) {
+        self.drain_tasks();
+    }
+
+    fn surface_changed<'local>(
+        &mut self,
+        ctx: &mut CallbackCtx<'local>,
+        holder: &android_view::SurfaceHolder<'local>,
+        _format: i32,
+        width: i32,
+        height: i32,
+    ) {
+        self.drain_tasks();
+        let window = holder.surface(&mut ctx.env).to_native_window(&mut ctx.env);
+        // Drop the old renderer (and the surface it owns) before building
+        // one from the new window -- see `AndroidRenderer`'s doc comment.
+        let ui_state = self.state.android_state_mut();
+        ui_state.renderer = None;
+        ui_state.renderer = Some(AndroidRenderer::new(window, width as u32, height as u32));
+        self.render();
+    }
+
+    fn surface_destroyed<'local>(
+        &mut self,
+        _ctx: &mut CallbackCtx<'local>,
+        _holder: &android_view::SurfaceHolder<'local>,
+    ) {
+        self.state.android_state_mut().renderer = None;
+    }
+
+    fn do_frame(&mut self, _ctx: &mut CallbackCtx, _frame_time_nanos: i64) {
+        self.drain_tasks();
+        self.render();
+    }
+
+    fn as_input_connection(&mut self) -> Option<&mut dyn InputConnection> {
+        Some(self)
+    }
+}
+
+/// Registers `IrisViewPeer<State>`'s native methods and builds one on every
+/// `newViewPeer` call from Java. `State`'s app crate wraps this in a
+/// concrete `extern "system" fn` (a generic function cannot be handed to
+/// `register_view_class`, which wants a plain function pointer) -- see
+/// `iris/android-app/src/lib.rs`.
+pub fn new_peer<'local, State: AndroidAppState>(
+    env: JNIEnv<'local>,
+    view: View<'local>,
+    _context: Context<'local>,
+) -> android_view::jni::sys::jlong {
+    let vm = env.get_java_vm().unwrap();
+    let global_view = env.new_global_ref(&view.0).unwrap();
+    let redraw: Arc<dyn RequestRedraw> = Arc::new(AndroidRedrawHandle::new(vm, global_view));
+    let (tasks, task_recv) = Tasks::init(redraw);
+    let mut rsc = AndroidRsc {
+        ui: Default::default(),
+        events: Default::default(),
+        tasks,
+        state: Default::default(),
+        _state: PhantomData,
+    };
+    let shared = Rc::new(RefCell::new(Shared::default()));
+    let ui_state = AndroidUiState::new(shared.clone());
+    let state = State::new(ui_state, &mut rsc);
+    let peer = IrisViewPeer {
+        rsc,
+        render: UiRenderState::new(),
+        state,
+        task_recv,
+    };
+    let id = android_view::register_view_peer(peer);
+    super::insets::register(id, shared);
+    id
+}

iris/src/attr.rs

@@ -0,0 +1,105 @@
+use crate::prelude::*;
+use std::time::{Duration, Instant};
+
+/// What focusing a text field takes from whichever backend is running --
+/// tracked here rather than duplicated per backend, since `Selector` and
+/// `Selectable` (below) are the *only* thing that decides which `TextEdit`
+/// is the IME's target, and both platforms need the same double-click
+/// timing and the same "remember which one" bookkeeping. What differs is
+/// what happens *after* the focus record is set: winit tells the
+/// compositor an IME area (`focus_gained`, in `default/attr.rs`); on
+/// android-view a keyboard has to be asked for explicitly, and only from a
+/// JNI call this crate cannot make outside a view callback -- so
+/// `focus_gained` there (`android/attr.rs`) just raises a flag the next
+/// touch callback consumes. See RUST.md's I2.
+pub trait FocusHost {
+    /// True on a click close enough in time to the previous one to grow a
+    /// selection instead of starting a new one, updating the clock as a
+    /// side effect the way a real double-click timer does.
+    fn recent_click(&mut self) -> bool;
+    fn set_focus(&mut self, id: Option<WeakWidget<TextEdit>>);
+    /// Called after a `TextEdit` becomes the focus target, with the region
+    /// it was hit in (`None` when the widget could not be located, which
+    /// happens for one it was just deselected from).
+    fn focus_gained(&mut self, region: Option<PixelRegion>);
+}
+
+/// Helper shared by every `FocusHost` impl, so the double-click window is
+/// one constant rather than one per backend.
+pub fn recent_click(last_click: &mut Instant) -> bool {
+    let now = Instant::now();
+    let recent = (now - *last_click) < Duration::from_millis(300);
+    *last_click = now;
+    recent
+}
+
+pub struct Selector;
+
+impl<Rsc: HasEvents, W: Widget + 'static> WidgetAttr<Rsc, W> for Selector
+where
+    Rsc::State: FocusHost,
+{
+    type Input = WeakWidget<TextEdit>;
+
+    fn run(rsc: &mut Rsc, container: WeakWidget<W>, id: Self::Input) {
+        rsc.register_event(container, CursorSense::click_or_drag(), move |ctx, rsc| {
+            let region = ctx.data.render.window_region(&id, &*rsc).unwrap();
+            let id_pos = region.top_left;
+            let container_pos = ctx
+                .data
+                .render
+                .window_region(&container, &*rsc)
+                .unwrap()
+                .top_left;
+            let pos = ctx.data.pos + container_pos - id_pos;
+            let size = region.size();
+            select(
+                rsc,
+                ctx.data.render,
+                ctx.state,
+                id,
+                pos,
+                size,
+                ctx.data.sense.is_dragging(),
+            );
+        });
+    }
+}
+
+pub struct Selectable;
+
+impl<Rsc: HasEvents> WidgetAttr<Rsc, TextEdit> for Selectable
+where
+    Rsc::State: FocusHost,
+{
+    type Input = ();
+
+    fn run(rsc: &mut Rsc, id: WeakWidget<TextEdit>, _: Self::Input) {
+        rsc.register_event(id, CursorSense::click_or_drag(), move |ctx, rsc| {
+            select(
+                rsc,
+                ctx.data.render,
+                ctx.state,
+                id,
+                ctx.data.pos,
+                ctx.data.size,
+                ctx.data.sense.is_dragging(),
+            );
+        });
+    }
+}
+
+fn select(
+    rsc: &mut impl UiRsc,
+    render: &UiRenderState,
+    state: &mut impl FocusHost,
+    id: WeakWidget<TextEdit>,
+    pos: Vec2,
+    size: Vec2,
+    dragging: bool,
+) {
+    let recent = state.recent_click();
+    id.edit(rsc).select(pos, size, dragging, recent);
+    state.set_focus(Some(id));
+    state.focus_gained(render.window_region(&id, &*rsc));
+}

iris/src/default/attr.rs

@@ -1,83 +1,22 @@
 use crate::prelude::*;
-use std::time::{Duration, Instant};
 use winit::dpi::{LogicalPosition, LogicalSize};
 
-pub struct Selector;
+impl<T: HasDefaultUiState> FocusHost for T {
-
+    fn recent_click(&mut self) -> bool {
-impl<Rsc: HasEvents, W: Widget + 'static> WidgetAttr<Rsc, W> for Selector
+        crate::attr::recent_click(&mut self.default_state_mut().last_click)
-where
-    Rsc::State: HasDefaultUiState,
-{
-    type Input = WeakWidget<TextEdit>;
-
-    fn run(rsc: &mut Rsc, container: WeakWidget<W>, id: Self::Input) {
-        rsc.register_event(container, CursorSense::click_or_drag(), move |ctx, rsc| {
-            let region = ctx.data.render.window_region(&id, &*rsc).unwrap();
-            let id_pos = region.top_left;
-            let container_pos = ctx
-                .data
-                .render
-                .window_region(&container, &*rsc)
-                .unwrap()
-                .top_left;
-            let pos = ctx.data.pos + container_pos - id_pos;
-            let size = region.size();
-            select(
-                rsc,
-                ctx.data.render,
-                ctx.state,
-                id,
-                pos,
-                size,
-                ctx.data.sense.is_dragging(),
-            );
-        });
-    }
     }
 
-pub struct Selectable;
+    fn set_focus(&mut self, id: Option<WeakWidget<TextEdit>>) {
-
+        self.default_state_mut().focus = id;
-impl<Rsc: HasEvents> WidgetAttr<Rsc, TextEdit> for Selectable
-where
-    Rsc::State: HasDefaultUiState,
-{
-    type Input = ();
-
-    fn run(rsc: &mut Rsc, id: WeakWidget<TextEdit>, _: Self::Input) {
-        rsc.register_event(id, CursorSense::click_or_drag(), move |ctx, rsc| {
-            select(
-                rsc,
-                ctx.data.render,
-                ctx.state,
-                id,
-                ctx.data.pos,
-                ctx.data.size,
-                ctx.data.sense.is_dragging(),
-            );
-        });
-    }
     }
 
-fn select(
+    fn focus_gained(&mut self, region: Option<PixelRegion>) {
-    rsc: &mut impl UiRsc,
+        let state = self.default_state_mut();
-    render: &UiRenderState,
+        let Some(region) = region else { return };
-    state: &mut impl HasDefaultUiState,
-    id: WeakWidget<TextEdit>,
-    pos: Vec2,
-    size: Vec2,
-    dragging: bool,
-) {
-    let state = state.default_state_mut();
-    let now = Instant::now();
-    let recent = (now - state.last_click) < Duration::from_millis(300);
-    state.last_click = now;
-    id.edit(rsc).select(pos, size, dragging, recent);
-    if let Some(region) = render.window_region(&id, &*rsc) {
         state.window.set_ime_allowed(true);
         state.window.set_ime_cursor_area(
             LogicalPosition::<f32>::from(region.top_left.tuple()),
             LogicalSize::<f32>::from(region.size().tuple()),
         );
     }
-    state.focus = Some(id);
 }

iris/src/default/event.rs

@@ -1,9 +0,0 @@
-use iris_core::Event;
-
-#[derive(Eq, PartialEq, Hash, Clone)]
-pub struct Submit;
-impl Event for Submit {}
-
-#[derive(Eq, PartialEq, Hash, Clone)]
-pub struct Edited;
-impl Event for Edited {}

iris/src/default/mod.rs

@@ -13,21 +13,12 @@ use winit::{
 
 mod app;
 mod attr;
-mod event;
 mod input;
 mod render;
-mod sense;
-mod state;
-mod task;
 
 pub use app::*;
-pub use attr::*;
-pub use event::*;
 pub use input::*;
 pub use render::*;
-pub use sense::*;
-pub use state::*;
-pub use task::*;
 
 pub type Proxy<Event> = EventLoopProxy<Event>;
 
@@ -309,12 +300,6 @@ impl<State: DefaultAppState> AppState for DefaultApp<State> {
     }
 }
 
-pub trait RscIdx<Rsc> {
-    type Output;
-    fn get(self, rsc: &Rsc) -> &Self::Output;
-    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output;
-}
-
 impl<State: 'static, I: RscIdx<DefaultRsc<State>>> std::ops::Index<I> for DefaultRsc<State> {
     type Output = I::Output;
 
@@ -328,27 +313,3 @@ impl<State: 'static, I: RscIdx<DefaultRsc<State>>> std::ops::IndexMut<I> for Def
         index.get_mut(self)
     }
 }
-
-impl<W: Widget, Rsc: UiRsc> RscIdx<Rsc> for WeakWidget<W> {
-    type Output = W;
-
-    fn get(self, rsc: &Rsc) -> &Self::Output {
-        &rsc.ui().widgets[self]
-    }
-
-    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output {
-        &mut rsc.ui_mut().widgets[self]
-    }
-}
-
-impl<T: 'static, Rsc: HasWidgetState> RscIdx<Rsc> for WeakState<T> {
-    type Output = T;
-
-    fn get(self, rsc: &Rsc) -> &Self::Output {
-        rsc.widget_state().get(self)
-    }
-
-    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output {
-        rsc.widget_state_mut().get_mut(self)
-    }
-}

iris/src/default/render.rs

@@ -1,3 +1,4 @@
+use crate::task::RequestRedraw;
 use iris_core::{UiData, UiRenderNode, UiRenderState};
 use pollster::FutureExt;
 use std::sync::Arc;
@@ -6,6 +7,12 @@ use winit::{dpi::PhysicalSize, window::Window};
 
 pub const CLEAR_COLOR: Color = Color::BLACK;
 
+impl RequestRedraw for Window {
+    fn request_redraw(&self) {
+        Window::request_redraw(self);
+    }
+}
+
 pub struct UiRenderer {
     window: Arc<Window>,
     surface: Surface<'static>,

iris/src/event.rs

@@ -2,7 +2,21 @@ use iris_core::*;
 use iris_macro::*;
 use std::sync::Arc;
 
-use crate::default::{TaskCtx, TaskUpdate, Tasks};
+use crate::task::{TaskCtx, TaskUpdate, Tasks};
+
+/// A field's Enter key (without a shift, in a multi-line field). Backend
+/// input handling raises it directly rather than through `on`, since a
+/// field does not know ahead of time whether anything is listening.
+#[derive(Eq, PartialEq, Hash, Clone)]
+pub struct Submit;
+impl Event for Submit {}
+
+/// A field's content changed as a result of input the backend applied
+/// directly to it (a keystroke, an IME commit) rather than through a
+/// widget event handler.
+#[derive(Eq, PartialEq, Hash, Clone)]
+pub struct Edited;
+impl Event for Edited {}
 
 pub trait Eventable<Rsc: HasEvents, Tag>: WidgetLike<Rsc, Tag> {
     fn on<E: EventLike>(

iris/src/lib.rs

@@ -1,12 +1,29 @@
 #![feature(unboxed_closures)]
 #![feature(fn_traits)]
-#![feature(associated_type_defaults)]
+// Only `default::DefaultAppState::Event`'s default uses this; unused (and
+// warned about) on the android target, which has no such default.
+#![cfg_attr(not(target_os = "android"), feature(associated_type_defaults))]
 #![feature(unsize)]
 #![feature(option_into_flat_iter)]
 #![feature(async_fn_traits)]
 
+// Two windowing backends live side by side, chosen by target rather than by
+// feature flag: winit everywhere but Android, android-view on it. They are
+// mutually exclusive rather than both-compiled-in because winit's own
+// Android support pulls in `android-activity`, which needs one of its
+// `game-activity`/`native-activity` features selected -- exactly what
+// `iris-core` was kept free of, and android-view is the framework's own
+// answer to the same surface on that platform. See RUST.md's I2.
+#[cfg(target_os = "android")]
+pub mod android;
+#[cfg(not(target_os = "android"))]
 pub mod default;
+
+pub mod attr;
 pub mod event;
+pub mod sense;
+pub mod state;
+pub mod task;
 pub mod widget;
 
 #[cfg(test)]
@@ -19,10 +36,18 @@ pub use iris_macro as macros;
 
 pub mod prelude {
     use super::*;
+    #[cfg(target_os = "android")]
+    pub use android::*;
+    #[cfg(not(target_os = "android"))]
     pub use default::*;
+
+    pub use attr::*;
     pub use event::*;
     pub use iris_core::*;
     pub use iris_macro::*;
+    pub use sense::*;
+    pub use state::*;
+    pub use task::*;
     pub use widget::*;
 
     pub use iris_core::util::Vec2;

iris/src/state.rs

@@ -1,7 +1,8 @@
 use iris_core::{
-    WidgetId,
+    UiRsc, WidgetId,
     util::{HashMap, HashSet},
 };
+use iris_core::{WeakWidget, Widget};
 use std::{
     any::{Any, TypeId},
     marker::PhantomData,
@@ -73,3 +74,39 @@ impl<'a, T: 'static> FnOnce<(&'a mut WidgetState,)> for WeakState<T> {
         state.get_mut(self)
     }
 }
+
+/// What `Rsc[weak_handle]` indexes through -- one impl per kind of handle
+/// (a widget, a piece of per-widget state), shared by both backends' `Rsc`
+/// types since indexing a widget tree has nothing to do with windowing.
+/// Each backend still needs its own `Index`/`IndexMut for ItsRsc<State>`
+/// (`default/mod.rs`, `android/view.rs`), because a blanket impl over every
+/// `I: RscIdx<Rsc>` for every possible `Rsc` would conflict between crates.
+pub trait RscIdx<Rsc> {
+    type Output;
+    fn get(self, rsc: &Rsc) -> &Self::Output;
+    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output;
+}
+
+impl<W: Widget, Rsc: UiRsc> RscIdx<Rsc> for WeakWidget<W> {
+    type Output = W;
+
+    fn get(self, rsc: &Rsc) -> &Self::Output {
+        &rsc.ui().widgets[self]
+    }
+
+    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output {
+        &mut rsc.ui_mut().widgets[self]
+    }
+}
+
+impl<T: 'static, Rsc: HasWidgetState> RscIdx<Rsc> for WeakState<T> {
+    type Output = T;
+
+    fn get(self, rsc: &Rsc) -> &Self::Output {
+        rsc.widget_state().get(self)
+    }
+
+    fn get_mut(self, rsc: &mut Rsc) -> &mut Self::Output {
+        rsc.widget_state_mut().get_mut(self)
+    }
+}

iris/src/task.rs

@@ -13,7 +13,17 @@ use tokio::{
         unbounded_channel as async_channel,
     },
 };
-use winit::window::Window;
+
+/// What a completed task nudges when it wants its result drawn. Shared
+/// between backends rather than typed as `winit::window::Window` directly:
+/// android-view has no `Window` at all, and the redraw request there is a
+/// JNI call (`View::post_frame_callback`) rather than a method call on a
+/// value this crate owns. Each backend supplies its own implementation --
+/// `default/render.rs` for winit, `android/render.rs` for android-view --
+/// and this module never needs to know which one it is holding.
+pub trait RequestRedraw: Send + Sync + 'static {
+    fn request_redraw(&self);
+}
 
 pub type TaskMsgSender<Rsc> = SyncSender<Box<dyn TaskUpdate<Rsc>>>;
 pub type TaskMsgReceiver<Rsc> = SyncReceiver<Box<dyn TaskUpdate<Rsc>>>;
@@ -23,7 +33,7 @@ impl<F: FnOnce(&mut Rsc::State, &mut Rsc) + Send, Rsc: HasState> TaskUpdate<Rsc>
 
 pub struct Tasks<Rsc: HasState> {
     start: AsyncSender<BoxTask>,
-    window: Arc<Window>,
+    redraw: Arc<dyn RequestRedraw>,
     msg_send: SyncSender<Box<dyn TaskUpdate<Rsc>>>,
 }
 
@@ -45,7 +55,7 @@ impl<Rsc: HasState + 'static> TaskCtx<Rsc> {
 type BoxTask = Pin<Box<dyn Future<Output = ()> + Send>>;
 
 impl<Rsc: HasState> Tasks<Rsc> {
-    pub fn init(window: Arc<Window>) -> (Self, TaskMsgReceiver<Rsc>) {
+    pub fn init(redraw: Arc<dyn RequestRedraw>) -> (Self, TaskMsgReceiver<Rsc>) {
         let (start, start_recv) = async_channel();
         let (msgs, msgs_recv) = sync_channel();
         std::thread::spawn(|| {
@@ -56,7 +66,7 @@ impl<Rsc: HasState> Tasks<Rsc> {
             Self {
                 start,
                 msg_send: msgs,
-                window,
+                redraw,
             },
             msgs_recv,
         )
@@ -67,10 +77,10 @@ impl<Rsc: HasState> Tasks<Rsc> {
         F::CallOnceFuture: Send,
     {
         let send = self.msg_send.clone();
-        let window = self.window.clone();
+        let redraw = self.redraw.clone();
         let _ = self.start.send(Box::pin(async move {
             task(TaskCtx::new(send)).await;
-            window.request_redraw();
+            redraw.request_redraw();
         }));
     }
 }

iris/src/widget/text/edit.rs

@@ -2,6 +2,7 @@ use crate::prelude::*;
 use iris_core::{TextData, UiColor};
 use parley::{Affinity, Layout, Selection};
 use std::ops::{Deref, DerefMut};
+#[cfg(not(target_os = "android"))]
 use winit::{
     event::KeyEvent,
     keyboard::{Key, NamedKey},
@@ -28,6 +29,7 @@ pub struct TextEdit {
     /// way to say, since it always denotes some position in the text. A
     /// collapsed selection is a caret; an uncollapsed one is a span.
     selection: Option<Selection>,
+    #[cfg_attr(target_os = "android", allow(dead_code))]
     history: Vec<(String, Option<Selection>)>,
     double_hit: Option<usize>,
     pub mode: EditMode,
@@ -57,6 +59,26 @@ impl TextEdit {
         }
         Some(self.buf.text()[sel.text_range()].to_string())
     }
+
+    /// The field's content. Byte-indexed, like everything else here since
+    /// I1 moved to parley -- an IME bridge (`android/ime.rs`) converts to
+    /// and from UTF-16 code units at its own edge rather than this type
+    /// knowing about that encoding.
+    pub fn text(&self) -> &str {
+        self.view.buf.text()
+    }
+
+    /// The selection as a byte range, collapsed to `caret..caret` when
+    /// there is no span. `None` when the field is not focused.
+    pub fn selection_range(&self) -> Option<std::ops::Range<usize>> {
+        Some(self.selection?.text_range())
+    }
+
+    /// The caret's byte offset -- the focus end of the selection, which is
+    /// where typing lands regardless of which end of a span it is.
+    pub fn caret(&self) -> Option<usize> {
+        Some(self.selection?.focus().index())
+    }
 }
 
 impl Widget for TextEdit {
@@ -115,6 +137,7 @@ impl<'a> TextEditCtx<'a> {
     }
 
     /// Keep the selection valid after the text underneath it changed.
+    #[cfg_attr(target_os = "android", allow(dead_code))]
     fn refresh(&mut self) {
         if let Some(sel) = self.text.selection {
             let layout = self.layout();
@@ -279,6 +302,20 @@ impl<'a> TextEditCtx<'a> {
         self.set_caret(start);
     }
 
+    /// The same range delete, exposed for callers that already have byte
+    /// offsets in hand rather than a `Motion` -- the IME's
+    /// `deleteSurroundingText`, which android-view hands over in UTF-16
+    /// code units that `android/ime.rs` converts before calling this.
+    pub fn delete_byte_range(&mut self, start: usize, end: usize) {
+        self.delete_range(start, end);
+    }
+
+    /// Move the caret to a byte offset, collapsing any selection -- the
+    /// IME's `setSelection`.
+    pub fn set_cursor_byte(&mut self, index: usize) {
+        self.set_caret(index);
+    }
+
     pub fn select_all(&mut self) {
         let len = self.text.view.buf.text().len();
         if len == 0 {
@@ -336,6 +373,7 @@ impl<'a> TextEditCtx<'a> {
         self.text.double_hit = None;
     }
 
+    #[cfg(not(target_os = "android"))]
     pub fn apply_event(&mut self, event: &KeyEvent, modifiers: &Modifiers) -> TextInputResult {
         let old = (self.text.view.buf.text().to_string(), self.text.selection);
         let mut undo = false;
@@ -352,6 +390,7 @@ impl<'a> TextEditCtx<'a> {
         res
     }
 
+    #[cfg(not(target_os = "android"))]
     fn apply_event_inner(
         &mut self,
         event: &KeyEvent,