问当绑定到ViewModel属性时，WPF用户控件依赖项属性无法工作

EN

public partial class LocationTreeView: UserControl
{
    public EventHandler LocationChanged;
    ...

    public static readonly DependencyProperty SelectedLocationProperty = 
         DependencyProperty.Register("SelectedLocation",typeof(Location), typeof(LocationTreeView),
         new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault, SelectedLocationChanged));
    ...

    public static void SelectedLocationChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
         LocationTreeView sender = (d as LocationTreeView);
         Location loc = e.NewValue as Location;
         //Navigate the treeview to the selected location
         sender.LoadLTreeViewPathToLocation(loc);
    }

    public Location SelectedLocation
    {
         get { return (Location)GetValue(SelectedLocationProperty); }
         set
         {
              if (SelectedLocation != value)
              {
                 SetValue(SelectedLocationProperty, value);
                 if (LocationChanged != null)
                 {
                     LocationChanged(this, EventArgs.Empty);
                 }
              }
         }
    }
    ...
}

public partial class LocationPicker: UserControl
{
    public static readonly DependencyProperty SelectedLocationProperty = 
         DependencyProperty.Register("SelectedLocation",typeof(Location), typeof(LocationPicker),
         new FrameworkPropertyMetadata(null, FrameworkPropertyMetadataOptions.BindsTwoWayByDefault));
    ...

    public Location SelectedLocation
    {
         get { return (Location)GetValue(SelectedLocationProperty); }
         set { SetValue(SelectedLocationProperty, value); }
    }
    ...

    private void Button_Click(object sender, RoutedEventArgs e)
    {
         // create a window with a locationtreeview on it. Set the treeview's
         // selectedlocation property, open the window, wait for the window to close,
         //  set this.SelectedLoctation to the treeview's selected location.
    }
}

<UserControl.DataContext>
    <VM:LocationTreeViewViewModel />
</UserControl.DataContext>

<Widow.DataContext>
    <VM:TestWindowViewModel />
</Window.DataContext>
<Grid>
...
<UC:LocationPicker x:Name="picker" SelectedLocation="{Binding Location}" /> 

<!-- this does not work -->
<UC:LocationTreeView SelectedLocaiton="{Binding Location}" />

<!-- but this works --->
<UC:LocationTreeView SelectedLocaiton="{Binding SelectedLocation, ElementName=picker}" />
...
</Grid>