Ensure awaitFrame() only awaits a single frame when used from the main thread

Author: pablobaxter

ui/kotlinx-coroutines-android/src/HandlerDispatcher.kt

@@ -190,11 +190,14 @@ public suspend fun awaitFrame(): Long {
             postFrameCallback(choreographer, cont)
         }
     }
-    // post into looper thread to figure it out
     return suspendCancellableCoroutine { cont ->
-        Dispatchers.Main.dispatch(EmptyCoroutineContext, Runnable {
+        if (Looper.myLooper() === Looper.getMainLooper()) { // Check if we are already in the main looper thread
             updateChoreographerAndPostFrameCallback(cont)
-        })
+        } else { // post into looper thread to figure it out
+            Dispatchers.Main.dispatch(EmptyCoroutineContext, Runnable {
+                updateChoreographerAndPostFrameCallback(cont)
+            })
+        }
     }
 }
 

ui/kotlinx-coroutines-android/test/HandlerDispatcherTest.kt

@@ -109,14 +109,14 @@ class HandlerDispatcherTest : TestBase() {
         launch(Dispatchers.Main, start = CoroutineStart.UNDISPATCHED) {
             expect(1)
             awaitFrame()
-            expect(5)
+            expect(3)
         }
         expect(2)
         // Run choreographer detection
         mainLooper.runOneTask()
-        expect(3)
-        mainLooper.scheduler.advanceBy(50, TimeUnit.MILLISECONDS)
         expect(4)
+        mainLooper.scheduler.advanceBy(50, TimeUnit.MILLISECONDS)
+        expect(5)
         mainLooper.scheduler.advanceBy(51, TimeUnit.MILLISECONDS)
         finish(6)
     }