Android "back" quits the app completely

[gnprice]

Repro steps:

• On Android, launch the app. Wait for data to load.
• Swipe back several times, until you're back at the system home screen.
• Launch the app again.
• Actual: The app has to load the data again from scratch.
• Expected: The app was still running in the background, so that the data is all ready.

This is an upstream Flutter issue:

• Back gesture on Android 12+ kills app rather than backgrounding it flutter/flutter#117061

The expected behavior is new in Android 12 (so in 2021); before that, our actual behavior was expected. See Android docs:
https://developer.android.com/about/versions/12/behavior-changes-all#back-press

I don't think there's a particularly straightforward workaround for us within the app, but hopefully it'll get fixed upstream. (So filing this issue mainly to track that this is a known issue and what the root cause is, in case people notice it again in the future.) Here's my summary of the state of the issue: flutter/flutter#117061 (comment)

[simon-the-shark]

Hello, have you tried this: flutter/flutter#117061 (comment)?

[gnprice]

Interesting, thanks @simon-the-shark!

I'd encourage you to try fixing this bug in Flutter upstream. That would be especially valuable because it means that everyone using Flutter would get the benefit of your fix.

Another benefit of an upstream fix is that I think it can ultimately be a lot less code. Fundamentally Flutter already has a sensible system for delegating from one piece of code to another the decision of what should happen on a back gesture: falling back from individual routes with custom logic, to the navigator's generic logic, to platform logic for when at the root of the navigation. The trouble is just that at the very end of that fallback chain, it makes the wrong decision (wrong since Android 12), at this line of code:

  private void popSystemNavigator() {
    // …
      activity.finish();

So by fixing that line to instead call Activity#onBackPressed, it becomes unnecessary to intercept Flutter's normal flow for deciding what to do with a back gesture, or to introduce a new MethodChannel.

If you do send a PR upstream, please feel free to ping me there and/or on this thread, and I'd be glad to do a review (I'm a Flutter committer). For questions leading up to a PR, you may find the Flutter contributor Discord helpful; try #hackers or #hackers-android, and feel free to ping me in a channel on that Discord too.