[google_maps_flutter] Fixes exception when dispose is called while asynchronous update from didUpdateWidget is executed

[JamesMcIntosh]

This PR fixes an exception in Google Maps Flutter which occurs when dispose is called while asynchronous code is executed from didUpdateWidget.

flutter/flutter#43785

I've done the work in 2 commits, the first one just used mount checks on after each awaited retrieval of the controller.
The second reuses the controller to limit the number of awaited async calls.
I am a little unsure of the expected execution order of all the unawaited futures which update the controller in regards to whether dispose could still be called in between their executions.
Testing locally I haven't been able to reproduce the exception with a single awaited controller.

I bumped the version 2.12.2 as no feature behaviour has changed.
No new tests in this PR, I'm not sure that you can test this change using widget tests.
Existing test coverage ensures that the values are still being updated correctly.

Pre-Review Checklist

• [/] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [/] I read the Tree Hygiene page, which explains my responsibilities.
• [/] I read and followed the relevant style guides and ran the auto-formatter.
• [/] I signed the CLA.
• [/] The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• [/] I linked to at least one issue that this PR fixes in the description above.
• [/] I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or I have commented below to indicate which version change exemption this PR falls under¹.
• [/] I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style, or I have commented below to indicate which CHANGELOG exemption this PR falls under¹.
• [/] I updated/added any relevant documentation (doc comments with ///).
• [/] I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• [/] All existing and new tests are passing.

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Code Sample

This sample app can be used to trigger the error, just repeatedly tapping the button at the bottom of the screen to reload the map. I did the testing in Chrome using the google_maps_flutter_web example project and the javascript key in web/index.html

import 'package:flutter/material.dart';
import 'package:google_maps_flutter/google_maps_flutter.dart';

void main() {
  runApp(const MapToggleWidget());
}

class MapToggleWidget extends StatefulWidget {
  const MapToggleWidget({Key? key}) : super(key: key);

  @override
  _MapToggleWidgetState createState() => _MapToggleWidgetState();
}

class _MapToggleWidgetState extends State<MapToggleWidget> {
  bool _showMap = true;

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(title: const Text('Map Toggle Example')),
        body: Column(
          children: [
            Expanded(child: _showMap ? buildGoogleMap() : buildNoMapContainer()),
            Padding(
              padding: const EdgeInsets.all(8.0),
              child: ElevatedButton(
                onPressed: () => setState(() => _showMap = !_showMap),
                child: Text(_showMap ? 'Hide Map' : 'Show Map'),
              ),
            ),
          ],
        ),
      ),
    );
  }

  Container buildNoMapContainer() {
    return Container(
      color: Colors.grey[300],
      child: const Center(
        child: Text(
          'Map is hidden',
          style: TextStyle(fontSize: 18),
        ),
      ),
    );
  }

  GoogleMap buildGoogleMap() {
    return const GoogleMap(
      mapType: MapType.satellite,
      initialCameraPosition: CameraPosition(
        target: LatLng(-38.9000, 175.8000),
        zoom: 10.0,
      ),
    );
  }

}

Footnotes

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[JamesMcIntosh]

I'm not sure how I would go about writing a test to make sure that the call from async code is not executed if the widget state has been disposed.
If it's possible would the execution of the async code being non-deterministic probably result in a flaky test.

[stuartmorgan-g]

If it's possible would the execution of the async code being non-deterministic probably result in a flaky test.

Why would it be non-deterministic in a test? onPlatformViewCreated is public, so you should be able to control when it is called (and thus when awaiting _controller resolves) relative to other methods, by calling it explicitly.

[JamesMcIntosh]

@stuartmorgan-g I've added tests covering both the didUpdateWidget and onPlatformViewCreated calls.

I have also expanded the mount check around the call to onMapCreated inside onPlatformViewCreated as it seems strange to me that you'd want to call it if the widget has been disposed. Do you agree with this or just back it off to only include the call to _updateTileOverlays?

If the mount check around onMapCreated is fine then I'll add a test for it also.

I was also wondering if there is a technical reason the tile overlays are updated only after the controller is available and not set in the initState like the rest of the state?