[Bug] Polygon.Points doesn't respond to CollectionChanged events

[Inrego]

Description

When modifying the points collection of Polygon, the shape doesn't change, even though PointsCollection implements INotifyCollectionChanged.

Steps to Reproduce

1. Make polygon
2. Add tap gesture recognizer, which will add, replace or remove points from the polygon

Expected Behavior

The polygon will change to reflect the current collection of points.

Actual Behavior

Polygon doesn't change.

Basic Information

• Version with issue: 4.7.0.1142
• Platform Target Frameworks:
  Only tested on Android, I expect it's a general problem.

Screenshots

Reproduction Link

Workaround

Replace the whole Points collection with a new collection with the modified points. It's quite ineffective for animations for example.

[Inrego]

I managed a better workaround. Custom renderers:
iOS

public class CustomPolygonRenderer : PolygonRenderer
{
    private PointCollection _points;
    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs args)
    {
        base.OnElementPropertyChanged(sender, args);
        if (args.PropertyName == Polygon.PointsProperty.PropertyName)
        {
            var polygon = (Polygon)sender;
            if (_points != null)
                _points.CollectionChanged -= PointsOnCollectionChanged;
            _points = polygon.Points;
            _points.CollectionChanged += PointsOnCollectionChanged;
        }
    }

    protected override void OnElementChanged(ElementChangedEventArgs<Polygon> args)
    {
        base.OnElementChanged(args);
        if (args.NewElement == null && _points != null)
        {
            _points.CollectionChanged -= PointsOnCollectionChanged;
        }
    }

    private void PointsOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        Control.UpdatePoints(_points.ToCGPoints());
    }
}

Android

public class CustomPolygonRenderer : PolygonRenderer
{
    public CustomPolygonRenderer(Context context) : base(context)
    {
    }

    private PointCollection _points;
    protected override void OnElementPropertyChanged(object sender, PropertyChangedEventArgs args)
    {
        base.OnElementPropertyChanged(sender, args);
        if (args.PropertyName == Polygon.PointsProperty.PropertyName)
        {
            var polygon = (Polygon) sender;
            if (_points != null)
                _points.CollectionChanged -= PointsOnCollectionChanged;
            _points = polygon.Points;
            _points.CollectionChanged += PointsOnCollectionChanged;
        }
    }

    protected override void OnElementChanged(ElementChangedEventArgs<Polygon> args)
    {
        base.OnElementChanged(args);
        if (args.NewElement == null && _points != null)
        {
            _points.CollectionChanged -= PointsOnCollectionChanged;
        }
    }


    private void PointsOnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
    {
        Control.UpdatePoints(Element.Points);
    }
}