[google_maps_flutter] Improved perfomance of clusterization

[OlehSv]

Google Maps implementation for flutter web is invoking rendering after each added marker to cluster. It is extremely non optimal, as result app is freezing for some time in case of significant number of markers (200+, but it is more obvious for 1k+) especially if they are using images instead of default icon.

As fix I am proposing to utilize method addItems of JS implementation instead of addMarker and render only as batch of markers are added.

I have tested new implementation with 3k markers and before it was freezed for 15-30 seconds, now it works without any freeze at all.

Fixes #179456

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.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

Note: The Flutter team is currently trialing the use of Gemini Code Assist for GitHub. Comments from the gemini-code-assist bot should not be taken as authoritative feedback from the Flutter team. If you find its comments useful you can update your code accordingly, but if you are unsure or disagree with the feedback, please feel free to wait for a Flutter team member's review for guidance on which automated comments should be addressed.

Footnotes

1. Regular contributors who have demonstrated familiarity with the repository guidelines only need to comment if the PR is not auto-exempted by repo tooling. ↩ ↩² ↩³

[flutter-dashboard]

It looks like this pull request may not have tests. Please make sure to add tests or get an explicit test exemption before merging.

If you are not sure if you need tests, consider this rule of thumb: the purpose of a test is to make sure someone doesn't accidentally revert the fix. Ask yourself, is there anything in your PR that you feel it is important we not accidentally revert back to how it was before your fix?

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing. If you believe this PR qualifies for a test exemption, contact "@test-exemption-reviewer" in the #hackers channel in Discord (don't just cc them here, they won't see it!). The test exemption team is a small volunteer group, so all reviewers should feel empowered to ask for tests, without delegating that responsibility entirely to the test exemption group.

[gemini-code-assist]

Code Review

This pull request introduces a significant performance improvement for marker clustering on the web by batching add and remove operations. Instead of re-rendering after each individual marker change, the changes are now grouped by cluster manager and applied in a single batch, followed by a single render call. This is achieved by introducing addItems and removeItems methods in ClusterManagersController and updating MarkersController to use them. The code in MarkersController has been refactored to support this, with _addMarker being split into a new _createMarker method for better code reuse. The changes look solid and directly address the performance issue described.