*-mainloop: add dummy render to unblock input

Newer versions of Android will stop delivering input to applications
that don't appear to be responsive because they aren't rendering
anything.

This updates na-mainloop and agdk-mainloop so that they at least
do a NOOP post of an untouched framebuffer each frame, so that they
continue to be delivered touch input.

Author: rib

agdk-mainloop/src/lib.rs

@@ -7,7 +7,7 @@ fn android_main(app: AndroidApp) {
 
     let mut quit = false;
     let mut redraw_pending = true;
-    let mut render_state: Option<()> = Default::default();
+    let mut native_window: Option<ndk::native_window::NativeWindow> = None;
 
     while !quit {
         app.poll_events(
@@ -37,11 +37,11 @@ fn android_main(app: AndroidApp) {
                                 }
                             }
                             MainEvent::InitWindow { .. } => {
-                                render_state = Some(());
+                                native_window = app.native_window();
                                 redraw_pending = true;
                             }
                             MainEvent::TerminateWindow { .. } => {
-                                render_state = None;
+                                native_window = None;
                             }
                             MainEvent::WindowResized { .. } => {
                                 redraw_pending = true;
@@ -65,7 +65,7 @@ fn android_main(app: AndroidApp) {
                 }
 
                 if redraw_pending {
-                    if let Some(_rs) = render_state {
+                    if let Some(native_window) = &native_window {
                         redraw_pending = false;
 
                         // Handle input
@@ -75,9 +75,28 @@ fn android_main(app: AndroidApp) {
                         });
 
                         info!("Render...");
+                        dummy_render(native_window);
                     }
                 }
             },
         );
     }
 }
+
+/// Post a NOP frame to the window
+///
+/// Since this is a bare minimum test app we don't depend
+/// on any GPU graphics APIs but we do need to at least
+/// convince Android that we're drawing something and are
+/// responsive, otherwise it will stop delivering input
+/// events to us.
+fn dummy_render(native_window: &ndk::native_window::NativeWindow) {
+    unsafe {
+        let mut buf: ndk_sys::ANativeWindow_Buffer = std::mem::zeroed();
+        let mut rect: ndk_sys::ARect = std::mem::zeroed();
+        ndk_sys::ANativeWindow_lock(native_window.ptr().as_ptr() as _, &mut buf as _, &mut rect as _);
+        // Note: we don't try and touch the buffer since that
+        // also requires us to handle various buffer formats
+        ndk_sys::ANativeWindow_unlockAndPost(native_window.ptr().as_ptr() as _);
+    }
+}

na-mainloop/src/lib.rs

@@ -7,7 +7,7 @@ fn android_main(app: AndroidApp) {
 
     let mut quit = false;
     let mut redraw_pending = true;
-    let mut render_state: Option<()> = Default::default();
+    let mut native_window: Option<ndk::native_window::NativeWindow> = None;
 
     while !quit {
         app.poll_events(
@@ -37,11 +37,11 @@ fn android_main(app: AndroidApp) {
                                 }
                             }
                             MainEvent::InitWindow { .. } => {
-                                render_state = Some(());
+                                native_window = app.native_window();
                                 redraw_pending = true;
                             }
                             MainEvent::TerminateWindow { .. } => {
-                                render_state = None;
+                                native_window = None;
                             }
                             MainEvent::WindowResized { .. } => {
                                 redraw_pending = true;
@@ -65,7 +65,7 @@ fn android_main(app: AndroidApp) {
                 }
 
                 if redraw_pending {
-                    if let Some(_rs) = render_state {
+                    if let Some(native_window) = &native_window {
                         redraw_pending = false;
 
                         // Handle input
@@ -75,9 +75,28 @@ fn android_main(app: AndroidApp) {
                         });
 
                         info!("Render...");
+                        dummy_render(native_window);
                     }
                 }
             },
         );
     }
 }
+
+/// Post a NOP frame to the window
+///
+/// Since this is a bare minimum test app we don't depend
+/// on any GPU graphics APIs but we do need to at least
+/// convince Android that we're drawing something and are
+/// responsive, otherwise it will stop delivering input
+/// events to us.
+fn dummy_render(native_window: &ndk::native_window::NativeWindow) {
+    unsafe {
+        let mut buf: ndk_sys::ANativeWindow_Buffer = std::mem::zeroed();
+        let mut rect: ndk_sys::ARect = std::mem::zeroed();
+        ndk_sys::ANativeWindow_lock(native_window.ptr().as_ptr() as _, &mut buf as _, &mut rect as _);
+        // Note: we don't try and touch the buffer since that
+        // also requires us to handle various buffer formats
+        ndk_sys::ANativeWindow_unlockAndPost(native_window.ptr().as_ptr() as _);
+    }
+}