The Handler created through new Handler(Looper.getMainLooper()) is subject to synchronization barriers like vsync. This means that posting tasks to it can result in scheduling delays of up to 16ms. Empirically I have noticed this happening only before the first frame has been drawn, but not sure if it might happen on other occasions.

This is a common pattern for platform message handlers on the main thread, and the delay can stack up and affect startup latency when apps do something like:

void main() async {
  final a = await sendPlatformMessage();
  final b = await sendPlatformMessageWithA(a);
  final c = await sendPlatformMessageWithB(b);

  runApp(MyApp(c));
}

Handler.createAsync helps to avoid this, but this API is only available on SDK >=28. Ideally we would use androidx.core.os.HandlerCompat but we don't have a dependency on it so I opted for a small reimplementation. It differs slightly in that we don't do the reflection fallback to keep things simple.

Tested: Internal global presubmit. I did not add new tests since this is an implementation change and should be covered by existing tests.

This also improves the startup latency for customer:money by 3%. See b/237035671 for more details.

Related Issues

b/237035671
Might be related to flutter/flutter#81559 (comment)

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