ReactiveUI Google Maps - xamarin.android

Question

Question: I have a IReactiveDerivedList<SensorViewModel> in my ViewModel and want to subscribe to changes on it including what's already in the list before I observe it. This then gets fed into GoogleMaps for Xamarin Android.
This seems to work when I add something :
public void OnMapReady(GoogleMap map)
{
_itemsAdded = this.ViewModel.Sensors.ItemsAdded
.ObserveOn(RxApp.MainThreadScheduler)
.Select(s => new CircleOptions()
.InvokeCenter(new LatLng(s.Latitude, s.Longitude))
.InvokeRadius(1000)
.InvokeFillColor(Color.Blue.ToArgb())
.InvokeStrokeColor(Color.Red.ToArgb()))
.Subscribe(
Observer.Create<CircleOptions>(options => map.AddCircle(options)));
}
But I also need to keep track of the Circle returned from map.AddCircle so I can remove it from the map when the object goes away. What's the reactive way to handle a case like this?

Answer 1

I don't know if this is the most reactive way to do it but I figured something out that seems to work:
private IDisposable _itemsAdded;
private IDisposable _itemsRemoved;
private readonly Dictionary<string, Circle> _circleMap = new Dictionary<string, Circle>();
public void OnMapReady(GoogleMap map)
{
_circleMap.Clear();
_itemsAdded = this.ViewModel.Sensors.ItemsAdded
.StartWith(this.ViewModel.Sensors)
.Subscribe(s =>
{
var options = new CircleOptions()
.InvokeCenter(new LatLng(s.Latitude, s.Longitude))
.InvokeRadius(1000)
.InvokeFillColor(Color.Blue.ToArgb())
.InvokeStrokeColor(Color.Red.ToArgb());
var circle = map.AddCircle(options);
this._circleMap.Add(s.Id, circle);
});
_itemsRemoved = this.ViewModel.Sensors.ItemsRemoved
.Subscribe(s =>
{
Circle circle = null;
if (this._circleMap.TryGetValue(s.Id, out circle))
{
this._circleMap.Remove(s.Id);
circle.Remove();
}
});
}