WIP: attempt to run a slint example on Android

I just wanted to know if it worked out of the box,  but it doesn't. We
get a panic:

Cannot get the native window, it's null and will always be null before Event::Resumed and after Event::Suspended. Make sure you only call this function between those events.

api/rs/slint/lib.rs

@@ -253,6 +253,9 @@ pub use i_slint_core::sharedvector::SharedVector;
 pub use i_slint_core::timers::{Timer, TimerMode};
 pub use i_slint_core::{format, string::SharedString};
 
+#[cfg(target_os = "android")]
+pub use i_slint_backend_selector::android_init;
+
 pub mod private_unstable_api;
 
 /// Enters the main event loop. This is necessary in order to receive

examples/printerdemo/rust/Cargo.toml

@@ -15,7 +15,7 @@ path = "main.rs"
 name = "printerdemo"
 
 [dependencies]
-slint = { path = "../../../api/rs/slint" }
+slint = { path = "../../../api/rs/slint", default-features=false, features=["std", "backend-winit", "renderer-winit-femtovg", "compat-1-0"] }
 
 chrono = { version = "0.4", default-features = false, features = ["clock", "std"]}
 
@@ -34,3 +34,10 @@ slint-build = { path = "../../../api/rs/build" }
 #wasm# wasm-bindgen = { version = "0.2" }
 #wasm# web-sys = { version = "0.3", features=["console"] }
 #wasm# console_error_panic_hook = "0.1.5"
+
+
+ [lib]
+ path = "main.rs"
+ crate-type = ["cdylib"]
+[target.'cfg(target_os = "android")'.dependencies]
+android-activity = "0.4"

examples/printerdemo/rust/main.rs

@@ -101,3 +101,10 @@ pub fn main() {
 
     main_window.run().unwrap();
 }
+
+#[cfg(target_os = "android")]
+#[no_mangle]
+fn android_main(app: android_activity::AndroidApp) {
+    slint::android_init(app).unwrap();
+    main();
+}

internal/backends/selector/lib.rs

@@ -92,3 +92,6 @@ pub fn use_modules() {
     #[cfg(feature = "i-slint-backend-winit")]
     i_slint_backend_winit::use_modules();
 }
+
+#[cfg(all(feature = "i-slint-backend-winit", target_os = "android"))]
+pub use i_slint_backend_winit::android_init;

internal/backends/winit/Cargo.toml

@@ -44,7 +44,7 @@ once_cell = "1.5"
 pin-weak = "1"
 scoped-tls-hkt = "0.1"
 # Pinned due to https://github.com/slint-ui/slint/issues/2424
-winit = { version = "=0.28.3", default-features = false }
+winit = { version = "=0.28.3", default-features = false, features = ["android-native-activity"] }
 instant = "0.1"
 raw-window-handle = { version = "0.5", features = ["alloc"] }
 scopeguard =  { version = "1.1.0", default-features = false }

internal/backends/winit/event_loop.rs

@@ -64,9 +64,23 @@ struct NotRunningEventLoop {
     event_loop_proxy: winit::event_loop::EventLoopProxy<CustomEvent>,
 }
 
+#[cfg(target_os = "android")]
+pub(crate) static ANDROID_APP: once_cell::sync::OnceCell<
+    winit::platform::android::activity::AndroidApp,
+> = once_cell::sync::OnceCell::new();
+
 impl NotRunningEventLoop {
     fn new() -> Self {
-        let instance = winit::event_loop::EventLoopBuilder::with_user_event().build();
+        #[allow(unused_mut)]
+        let mut builder = winit::event_loop::EventLoopBuilder::with_user_event();
+        #[cfg(target_os = "android")]
+        {
+            use winit::platform::android::EventLoopBuilderExtAndroid;
+            builder.with_android_app(
+                ANDROID_APP.get().expect("android must be initialized with android_init").clone(),
+            );
+        }
+        let instance = builder.build();
         let event_loop_proxy = instance.create_proxy();
         let clipboard = RefCell::new(create_clipboard(&instance));
         Self { clipboard, instance, event_loop_proxy }
@@ -468,7 +482,7 @@ pub fn run() -> Result<(), corelib::platform::PlatformError> {
     use winit::event_loop::{ControlFlow, EventLoopWindowTarget};
 
     let not_running_loop_instance = MAYBE_LOOP_INSTANCE.with(|loop_instance| {
-        loop_instance.borrow_mut().take().unwrap_or_else(NotRunningEventLoop::new)
+        loop_instance.borrow_mut().take().unwrap_or_else(|| NotRunningEventLoop::new())
     });
 
     let event_loop_proxy = not_running_loop_instance.event_loop_proxy;

internal/backends/winit/lib.rs

@@ -200,3 +200,10 @@ impl i_slint_core::platform::Platform for Backend {
         })
     }
 }
+
+#[cfg(target_os = "android")]
+pub fn android_init(
+    android_app: winit::platform::android::activity::AndroidApp,
+) -> Result<(), String> {
+    event_loop::ANDROID_APP.set(android_app).map_err(|_| "android_init called twice".into())
+}

internal/renderers/femtovg/lib.rs

@@ -373,6 +373,10 @@ impl Renderer for FemtoVGRenderer {
         &self,
         data: &'static [u8],
     ) -> Result<(), Box<dyn std::error::Error>> {
+        // FIXME: this panics
+        #[cfg(target_os = "android")]
+        return Ok(());
+
         fonts::register_font_from_memory(data)
     }