Support running `calc` on Android (#474)

This works without any other code modifications.

![Xilem Calc on
Android.](https://github.com/user-attachments/assets/b6648c04-bc9e-464c-9a19-0f03e958ade9)

.github/workflows/ci.yml

@@ -21,6 +21,9 @@ env:
   # This is required because `cargo hack` does not support -p {x} --exclude {x}
   RUST_MIN_VER_WASM_PKGS: "-p xilem_core"
 
+  # Only some of our examples support Android (primarily due to extra required boilerplate).
+  ANDROID_TARGETS: -p xilem --example mason_android --example calc_android
+
   # We do not run the masonry snapshot tests, because those currently require a specific font stack
   # See https://github.com/linebender/xilem/pull/233
   SKIP_RENDER_SNAPSHOTS: 1
@@ -230,7 +233,7 @@ jobs:
         run: cargo install cargo-apk
 
       - name: cargo apk check (android)
-        run: cargo apk check -p xilem --example mason_android
+        run: cargo apk check ${{ env.ANDROID_TARGETS }}
         env:
           # This is a bit of a hack, but cargo apk doesn't seem to allow customising this otherwise
           RUSTFLAGS: '-D warnings'

xilem/Cargo.toml

@@ -18,6 +18,7 @@ cargo-args = ["-Zunstable-options", "-Zrustdoc-scrape-examples"]
 [[example]]
 name = "mason"
 
+# Also add to ANDROID_TARGETS in .github/ci.yml if adding a new Android example
 [[example]]
 # A custom example target which uses the same `mason.rs` file but for android
 name = "mason_android"
@@ -25,6 +26,15 @@ path = "examples/mason.rs"
 # cdylib is required for cargo-apk
 crate-type = ["cdylib"]
 
+[[example]]
+name = "calc"
+
+[[example]]
+name = "calc_android"
+path = "examples/calc.rs"
+# cdylib is required for cargo-apk
+crate-type = ["cdylib"]
+
 [lints]
 workspace = true
 

xilem/examples/calc.rs

@@ -6,6 +6,7 @@ use winit::dpi::LogicalSize;
 use winit::error::EventLoopError;
 use winit::window::Window;
 use xilem::view::Flex;
+use xilem::EventLoopBuilder;
 use xilem::{
     view::{button, flex, label, sized_box, Axis, FlexExt as _, FlexSpacer},
     EventLoop, WidgetView, Xilem,
@@ -300,7 +301,7 @@ fn digit_button(digit: &'static str) -> impl WidgetView<Calculator> {
     })
 }
 
-fn main() -> Result<(), EventLoopError> {
+fn run(event_loop: EventLoopBuilder) -> Result<(), EventLoopError> {
     let data = Calculator {
         current_num_index: 0,
         clear_current_entry_on_input: false,
@@ -317,6 +318,39 @@ fn main() -> Result<(), EventLoopError> {
         .with_resizable(true)
         .with_min_inner_size(min_window_size)
         .with_inner_size(window_size);
-    app.run_windowed_in(EventLoop::with_user_event(), window_attributes)?;
+    app.run_windowed_in(event_loop, window_attributes)?;
     Ok(())
 }
+
+#[cfg(not(target_os = "android"))]
+#[allow(dead_code)]
+// This is treated as dead code by the Android version of the example, but is actually live
+// This hackery is required because Cargo doesn't care to support this use case, of one
+// example which works across Android and desktop
+fn main() -> Result<(), EventLoopError> {
+    run(EventLoop::with_user_event())
+}
+
+// Boilerplate code for android: Identical across all applications
+
+#[cfg(target_os = "android")]
+// Safety: We are following `android_activity`'s docs here
+// We believe that there are no other declarations using this name in the compiled objects here
+#[allow(unsafe_code)]
+#[no_mangle]
+fn android_main(app: winit::platform::android::activity::AndroidApp) {
+    use winit::platform::android::EventLoopBuilderExtAndroid;
+
+    let mut event_loop = EventLoop::with_user_event();
+    event_loop.with_android_app(app);
+
+    run(event_loop).expect("Can create app");
+}
+
+// TODO: This is a hack because of how we handle our examples in Cargo.toml
+// Ideally, we change Cargo to be more sensible here?
+#[cfg(target_os = "android")]
+#[allow(dead_code)]
+fn main() {
+    unreachable!()
+}