Xamarin.Forms.Maps自定义折线渲染器在视图中不渲染

Question

I am trying to follow this guide in order to draw a polyline on a map in a Xamarin.Forms app. It should track the user's position in real-time and update the polyline when new position data comes in.

I wrote a custom map renderer that will render the polyline, but for some reason it does not update when the map is in view. I have to navigate back to the main launch page and navigate to the mapping page again for it to update.

I extracted the minimum code to reproduce the problem, but it is still too much to paste here, so I hosted it on GitHub:

https://github.com/Steztric/MapWithWaylineSample

Please could somebody let me know what I am doing wrong. You can demonstrate the problem by cloning the repo and running it.

Answer 1

You need to create renderer every time, so remove class variable polylineRenderer and use local one.

MKOverlayRenderer GetOverlayRenderer(MKMapView mapView, IMKOverlay overlayWrapper)
        {
            IMKOverlay overlay = Runtime.GetNSObject(overlayWrapper.Handle) as IMKOverlay;

            if (overlay is MKPolyline)
            {
                var polylineRenderer = new MKPolylineRenderer(overlay as MKPolyline);
                polylineRenderer.FillColor = UIColor.Blue;
                polylineRenderer.StrokeColor = UIColor.Red;
                polylineRenderer.LineWidth = 3;
                polylineRenderer.Alpha = 0.4f;
                return polylineRenderer;
            }
            else
            {
                return null;
            }
        }

Also you can simplify things a little

Define MKPolyline currentWayline; then

    var wayline = MKPolyline.FromCoordinates(coords.ToArray());
    //IMKOverlay overlay = Runtime.GetNSObject(wayline.Handle) as IMKOverlay;
    nativeMap.AddOverlay(wayline);
    currentWayline = wayline;