Expose main thread's looper for runOnUiThread compatibility

android-activity/src/lib.rs

@@ -549,6 +549,23 @@ impl AndroidApp {
         self.inner.read().unwrap().native_window()
     }
 
+    /// Returns a pointer to the current looper associated with the main thread
+    ///
+    /// This is appropriate for posting callbacks that need to run on the UI thread
+    /// Wrap in an [`ndk::looper::ForeignLooper `]
+    /// ```ignore
+    /// # use ndk;
+    /// # let app: AndroidApp = todo!();
+    /// let looper = unsafe {
+    ///     let non_null_looper = ptr::NonNull::new(_app.main_looper_as_ptr()).unwrap();
+    ///     ndk::looper::ForeignLooper::from_ptr(non_null_looper)
+    /// };
+    /// looper.add_fd_with_callback(todo!(), ndk::looper::FdEvent::INPUT, todo!()).unwrap();
+    /// ```
+    pub fn main_looper_as_ptr(&self) -> *mut ndk_sys::ALooper {
+        self.inner.read().unwrap().main_looper()
+    }
+
     /// Returns a pointer to the Java Virtual Machine, for making JNI calls
     ///
     /// This returns a pointer to the Java Virtual Machine which can be used

android-activity/src/native_activity/glue.rs

@@ -848,6 +848,7 @@ extern "C" fn ANativeActivity_onCreate(
         let rust_glue = jvm_glue.clone();
         // Let us Send the NativeActivity pointer to the Rust main() thread without a wrapper type
         let activity_ptr: libc::intptr_t = activity as _;
+        let main_looper_ptr: libc::intptr_t = unsafe { ndk_sys::ALooper_forThread() } as _;
 
         // Note: we drop the thread handle which will detach the thread
         std::thread::spawn(move || {
@@ -867,7 +868,7 @@ extern "C" fn ANativeActivity_onCreate(
                 jvm
             });
 
-            let app = AndroidApp::new(rust_glue.clone(), jvm.clone());
+            let app = AndroidApp::new(rust_glue.clone(), jvm.clone(), main_looper_ptr as *mut _);
 
             rust_glue.notify_main_thread_running();
 

android-activity/src/native_activity/mod.rs

@@ -86,7 +86,11 @@ impl AndroidAppWaker {
 }
 
 impl AndroidApp {
-    pub(crate) fn new(native_activity: NativeActivityGlue, jvm: CloneJavaVM) -> Self {
+    pub(crate) fn new(
+        native_activity: NativeActivityGlue,
+        jvm: CloneJavaVM,
+        main_looper_ptr: *mut ndk_sys::ALooper
+    ) -> Self {
         let mut env = jvm.get_env().unwrap(); // We attach to the thread before creating the AndroidApp
 
         let key_map_binding = match KeyCharacterMapBinding::new(&mut env) {
@@ -103,6 +107,9 @@ impl AndroidApp {
                 looper: Looper {
                     ptr: ptr::null_mut(),
                 },
+                main_looper: Looper {
+                    ptr: main_looper_ptr,
+                },
                 key_map_binding: Arc::new(key_map_binding),
                 key_maps: Mutex::new(HashMap::new()),
                 input_receiver: Mutex::new(None),
@@ -147,6 +154,9 @@ pub(crate) struct AndroidAppInner {
     pub(crate) native_activity: NativeActivityGlue,
     looper: Looper,
 
+    /// Looper associated with the activy's main thread, sometimes called the UI thread.
+    main_looper: Looper,
+
     /// Shared JNI bindings for the `KeyCharacterMap` class
     key_map_binding: Arc<KeyCharacterMapBinding>,
 
@@ -179,6 +189,10 @@ impl AndroidAppInner {
         self.looper.ptr
     }
 
+    pub fn main_looper(&self) -> *mut ndk_sys::ALooper {
+        self.main_looper.ptr
+    }
+
     pub fn native_window(&self) -> Option<NativeWindow> {
         self.native_activity.mutex.lock().unwrap().window.clone()
     }