[google_maps_flutter] Support Google Maps onMapLoaded

[hearsilent]

Implements the GoogleMap.OnMapLoadedCallback interface and triggers the onMapLoaded event via platform channels on Android and iOS, with corresponding handling on Web. Updates the generated Pigeon message files and platform interface to support the new event, enabling Flutter apps to listen for when the map has fully loaded across all supported platforms.

Fixes flutter/flutter#99610

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[gemini-code-assist]

Code Review

This pull request introduces support for the onMapLoaded callback in the google_maps_flutter plugin for both Android and iOS. The changes are well-implemented across the platform interface, Android, and iOS packages. The platform interface is correctly extended with the new MapLoadedEvent and onMapLoaded stream. The native implementations use the appropriate platform-specific callbacks (OnMapLoadedCallback for Android and mapViewDidFinishTileRendering: for iOS) and communicate with the Dart side via updated Pigeon message definitions. New tests have been added to verify the functionality on both platforms, and all relevant package versions and changelogs have been updated accordingly. The code is clean, consistent, and follows existing patterns.

[stuartmorgan-g]

Thanks for the contribution!

Two initial high-level notes while this waits for review:

• Please follow the instructions at https://github.com/flutter/flutter/blob/master/docs/ecosystem/contributing/README.md#changing-federated-plugins so that CI can run successfully.
• The PR needs to address web in some way, either by adding web support, or by explaining in the PR why it can't be, and adding a support query. Currently clients who put important code in this callback would be silently broken on web.

[hearsilent]

Thanks for the notes!

Both points have been fully addressed:

• The federated plugin change instructions have been followed, and CI should now run successfully.
• Web is now handled appropriately, so clients relying on this callback will not be silently broken.

Please let me know if there’s anything else to adjust while this is under review.

[stuartmorgan-g]

The CI failures all look like real failures now, which will need to be resolved (other than the expected federated safety check).

[hearsilent]

Thanks for the heads up. All CI failures have now been resolved, except for the expected federated safety check.

[vashworth]

iOS side LGTM

[LongCatIsLooong]

The iOS implementation lgtm (modulo a potentially broken link).

[LongCatIsLooong]

I'm not familiar with the API conventions of this package, but each event in the Stream basically carries no information so it might as well be Void. It looks to me that the MapLoadedEvent type only exists so that this implementation can use it to filter the event stream and extract the relevant events, and MapLoadedEvent is not useful to the caller of this API at all. If that's indeed the case, would it be possible to hide the MapLoadedEvent type? Or is it made public for subclassing?

[LongCatIsLooong]

I assume with the current interface, to provide a "start loading" event stream in the future would require adding a new onMapLoading method to this interface. I would expect a lot of use cases of this API would also involve listening to the "Map started loading" event stream and combining the two streams (e.g., I want to perform asynchronous work on map load, and cancel that work if the map starts loading again).

IMO it might be a bit more ergonomic to return a Stream that gives you both start and finish events (so something like Stream<bool>, or the boolean flag can be added to MapLoadEvent), or even a ValueNotifier<bool> (but now the caller would have to manage the lifespan of the ValueNotifier so I'm not sure about that).

[LongCatIsLooong]

Ah it looks like this is a low-level interface and most app developers don't have to use this directly. Feel free to ignore the comments then.