Added FlutterActivity and FlutterFragment hook to cleanUpFlutterEngine() as symmetry for configureFlutterEngine(). (#41943)

[matthew-carroll]

Added FlutterActivity and FlutterFragment hook to cleanUpFlutterEngine() as symmetry for configureFlutterEngine(). #41943

[mklim]

The implementation itself LG but I want to make sure I understand the overall approach and problem here.

Is this from our discussion about how it's tedious as a plugin author to have to manually manage clean up method channels? Or is it solving something else? I'm trying to reason it out because the only thing that's happening in configureFlutterEngine that I know of at the moment is plugin registration. So this new hook would naturally be a place to unregister plugins, but I don't think we have a definition of what unregistering a plugin would mean on the FlutterPlugin interface itself. So it's not really clear to me what app or plugin authors should be doing with this hook exactly once it's added.

[matthew-carroll]

This is primarily for cases where an app developer is directly adding channels, plugins aside. @redbrogdon is working on a canonical example that shows communication from Flutter to the app and it became clear that if you set up channels in configureFlutterEngine(), there is no apparent place to tear them down. Technically you could do it in onDestroy() or onDetach(), but it's unclear if most developers would be able to discover that.

As for plugins, if we'd like to facilitate deregistration, then we probably want to add such a method to the GeneratedPluginRegistrant and call it from this new method, to maintain symmetry with registration. WDYT?

[mklim]

Ah okay, understood. LGTM.

I think for plugins this equivalent hook already exists in the form of onFlutterEngineDetached (may not be the exact name). So adding yet another hook for plugin authors to write their cleanup code in wouldn't be super useful. The case I was thinking of before would be if we could handle cleaning up the method channel references automatically somehow instead of requiring plugin authors to store references to the channels and then call setMethodHandler(null) on them manually when the engine is cleaned up.

This is off-topic for this PR, I wonder if it would be possible to borrow the RAII principle here so this kind of after the fact hook cleanup isn't necessary at all for the Channels. We don't have destructors in Java so I'm not really sure if we could handle this responsibility totally internally without a broad rewrite, but maybe it's worth looking into. It's not intuitive at all that setMethodHandler needs to be deliberately cleared after the fact, I expect there is and is going to be a lot of leaks based on that getting missed.

[redbrogdon]

LGTM.

[redbrogdon]

nit: "clean up" here and below