[google_maps_flutter] Add support for custom cluster icon

[Mohit8G]

Use case

So Clustering in Google map is just landed but we don't have any support for custom cluster icon. There are some package on pub.dev but they are not proper solution to the problem.

Proposal

Need some properties that can override default cluster marker.
Like this
https://developers.google.com/maps/documentation/android-sdk/utility/marker-clustering

The ClusterManager constructor creates a DefaultClusterRenderer and a NonHierarchicalDistanceBasedAlgorithm. You can change the ClusterRenderer and the Algorithm using the setAlgorithm(Algorithm algorithm) and setRenderer(ClusterRenderer view) methods of ClusterManager.

You can implement ClusterRenderer to customize the rendering of the clusters. DefaultClusterRenderer provides a good base to start from. By subclassing DefaultClusterRenderer, you can override the defaults.

[stuartmorgan-g]

It looks like this is not going to be implementable without https://issuetracker.google.com/issues/140415468 (or, alternately, #150525), since the cluster renderer protocol requires a synchronous response. Usually when we have something that requires a synchronous response we pre-generated the necessary information, send it asynchronously, and store it on the platform thread for immediate response, but there's no way to pre-generate all possible images.

Even if clients didn't want the ability to change the icon dynamically (which they well might), they will almost certainly want to add a number, and that can be arbitrary as well.

[stuartmorgan-g]

Please use a thumbs-up reaction on this comment specifically (in addition to the original feature request) if the following limited version of this feature would address your use case:

We could potentially implement an API that allowed pre-providing a map of {cluster count, image bitmap} cluster options, and have the cluster icon picked by finding the image in that map whose cluster count is the largest value that is <= the actual count. For instance, if someone provided {1: imageA, 10: imageB, 50: imageC, 100: imageD}, then any clusters of size 1-9 would use imageA, 10-49 would use imageB, 50-99 would use imageC, and anything larger would use imageD.

[Rishyash]

@stuartmorgan Needed one suggestion for this as I am not a native developer, what if I fork this package and add custom cluster logic for my usecase on native side code will this reflect on the flutter side map view?