Question

I’m having an issue with a binding that I’m trying to implement. It will update the DP once, but after that, it’s never updated again.

In XAML I have two controls binding to a listview.selected item.

<controls:MapControl DataContext="{Binding ElementName=availableMapsListView, Path=SelectedItem}" MapData="{Binding .}">

and

<TextBlock DataContext="{Binding ElementName=availableMapsListView, Path=SelectedItem}" Text="{Binding Name}" />

The textblock update as expected with each change of the listview’s selected item.

My custom control creates the dependency property like so:

public class MapControl : UserControl
{
  public MapData MapData
    {
        get { return (MapData)GetValue(MapDataProperty); }
        set { SetValue(MapDataProperty, value); }
    }

    public static readonly DependencyProperty MapDataProperty =
        DependencyProperty.Register("MapData", typeof(MapData), typeof(MapControl),
            new FrameworkPropertyMetadata(
                null,
                FrameworkPropertyMetadataOptions.AffectsRender,
                new PropertyChangedCallback(OnMapDataPropertyChanged),
                new CoerceValueCallback(OnMapCoerceValue)
            )
        );

    private static void OnMapDataPropertyChanged(DependencyObject source, DependencyPropertyChangedEventArgs e)
    {
        if (e.NewValue != null)
        {
            ((MapControl)source).MapData = (MapData)e.NewValue;                
        }
    }

    private static object OnMapCoerceValue(DependencyObject dpo, Object obj)
    {           
        return obj;
    }
    ...
}

I’m pretty much at my wits end and not sure what I should do from here. Any help is greatly appreciated.

Answer 1

Not sure exactly what you’re trying to achieve or why your code appears so convoluted. If you explain more someone may be able to provide you with a much simpler solution.

That said, by the sounds of it the problem is simply that you’re overwriting the binding with a local value. This looks like the culprit:

((MapControl)source).MapData = (MapData)e.NewValue;

When you do this, the MapControl.MapData property will no longer be bound to ‘.’ Instead, it will take on whatever value you’ve assigned. So your MapControl.DataContext property is likely perfectly correct, but it’s not being transferred to the MapData property because you’ve destroyed the binding.