Great job

[wojciech-kulik]

Thank you for this code :) Saved me a lot of time!

I see it's also based on: https://thoughtbot.com/blog/how-to-handle-large-amounts-of-data-on-maps

I implemented also code for animation (from this article):

nativeMap.DidAddAnnotationViews += NativeMap_DidAddAnnotationViews;

private void AddBouncingTo(UIView view)
        {
            var bounceAnimation = new CAKeyFrameAnimation { KeyPath = "transform.scale" };
            bounceAnimation.Values = new NSObject[] { NSNumber.FromDouble(0.05), NSNumber.FromDouble(1.1), NSNumber.FromDouble(0.9), NSNumber.FromDouble(1) };
            bounceAnimation.Duration = 0.6;
            var timingFunctions = new List<CAMediaTimingFunction>();
            for (var i = 0; i < 4; i++)
            {
                timingFunctions.Add(CAMediaTimingFunction.FromName(CAMediaTimingFunction.EaseInEaseOut));
            }
            bounceAnimation.TimingFunctions = timingFunctions.ToArray();
            bounceAnimation.RemovedOnCompletion = false;
            view.Layer.AddAnimation(bounceAnimation, "bounce");
        }

        private void NativeMap_DidAddAnnotationViews(object sender, MKMapViewAnnotationEventArgs e)
        {
            foreach (var view in e.Views)
            {
                AddBouncingTo(view);
            }
        }

[wojciech-kulik]

Also you had issue with comparing sets and operations on them. Each time you create a new instance of FBAnnotationCluster, therefore a comparison of references always returns false.

To fix this you need to add to FBAnnotationCluster:

public override bool Equals(object obj)
{
    return obj.GetHashCode() == GetHashCode();
}

public override int GetHashCode()
{
    var formatted = $"{Coordinate.Latitude:F5} {Coordinate.Longitude:F5}";
    return formatted.GetHashCode();
}

It is easy to spot this bug when you add animations mentioned above :)

Here is missing code:
https://github.com/thoughtbot/TBAnnotationClustering/blob/master/TBAnnotationClustering/TBClusterAnnotation.m