Setting the Viewable Area of the Map

,

An alternative way of setting the viewable area of the map is to use the Map control’s SetView method. SetView accepts a LocationRectangle object, which specifies a rectangular region of the map.

The MapViewModel contains a LocationRectangle property that is applied to the map within the viewmodel’s PropertyChanged event handler, as shown in the following excerpt:

Click here to view code image

public MapView()
{
    var viewModel = new MapViewModel(new RouteCalculator());

    viewModel.PropertyChanged += (sender, args) =>
        {
            switch (args.PropertyName)
            {
                ...
                case "LocationRectangle":
                    LocationRectangle rectangle = ViewModel.LocationRectangle;
                    if (rectangle != null)
                    {
                        map.SetView(ViewModel.LocationRectangle);
                    }
                    break;
            }
        };

    ...
}

Later in this chapter, you see how the Map control’s SetView method is used to pan and zoom to an area of the map to display the shortest path between two points.

Caution

When the lifespan of the viewmodel exceeds that of the view, subscribing to a viewmodel’s PropertyChanged event from the view can cause a memory leak. This is because the target of an event subscription (in this case the view) is kept alive by the garbage collector while the event source (the viewmodel) exists in memory. To prevent this, you may forgo the use of the PropertyChanged event in favor of an alternative approach, such as weak events, or a messaging system to communicate with the view, both of which are to be found in various MVVM frameworks including the Calcium SDK project.

The Map control’s ZoomLevel property allows you to control the altitude of the viewpoint. This property ranges between 0 and 20, with 0 being completely zoomed out so that the entire map is visible, and 20 being zoomed in as close as possible. Unlike the old Bing Maps control, 20 is a hard limit. If you attempt to assign the zoom level to a higher value, an ArgumentOutOfRangeException is thrown. To demonstrate the ZoomLevel property of the Map control, the sample view uses a data-binding to the viewmodel’s ZoomLevel property, as shown:

Click here to view code image

<m:Map x:Name="map"
    ...
    ZoomLevel="{Binding ZoomLevel, Mode=TwoWay}"
    ...
</m:Map>

As the user performs pinch and spread gestures, the viewmodel’s ZoomLevel property is updated.

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