如何使用Xamarin Maps缩放到边界框？

<maps:Map x:Name="myMap" 
           MapType="Street" 
           IsShowingUser="true"
           WidthRequest="300" 
           HeightRequest="300"
           VerticalOptions="FillAndExpand" 
           HorizontalOptions="FillAndExpand"
           ZoomLevel="10"
           Center="37,-122">
</maps:Map>

var southwest = new Position(37.7749, -122.4194);
var northeast = new Position(37.8199, -122.3748);
var bounds = new Bounds(southwest, northeast);
myMap.MoveToRegion(MapSpan.FromBounds(bounds));