Data binding 将依赖项属性绑定为目标不'；行不通

我创建的依赖项属性如下所示：

public class MapShowViewModel : DependencyObject
{
    public static readonly DependencyProperty MapScaleProperty =
        DependencyProperty.Register("MapScale",
        typeof(double), typeof(MapShowViewModel),
        new FrameworkPropertyMetadata(Convert.ToDouble(1), OnMapScaleChanged));

    private static void OnMapScaleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var mapShowViewModel = d as MapShowViewModel;
        if (mapShowViewModel == null) return;

        mapShowViewModel.MapScale = Convert.ToDouble(e.NewValue);
    }

    public double MapScale
    {
        get { return (double)GetValue(MapScaleProperty); }
        set { SetValue(MapScaleProperty, value); }
    }
}

<baseVm:MapShowViewModel x:Key="MapShowViewModel" 
 MapScale="{Binding ElementName=MyMap, Path=Scale,
 UpdateSourceTrigger=PropertyChanged,     
 Mode=TwoWay, Converter={StaticResource MapScaleConverter}}" />

当在Resource中定义ViewModel时，我将XAML中的dp绑定到名为“MyMap”的元素，并有一个名为“Scale”的双类型属性，如下所示：

public class MapShowViewModel : DependencyObject
{
    public static readonly DependencyProperty MapScaleProperty =
        DependencyProperty.Register("MapScale",
        typeof(double), typeof(MapShowViewModel),
        new FrameworkPropertyMetadata(Convert.ToDouble(1), OnMapScaleChanged));

    private static void OnMapScaleChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
    {
        var mapShowViewModel = d as MapShowViewModel;
        if (mapShowViewModel == null) return;

        mapShowViewModel.MapScale = Convert.ToDouble(e.NewValue);
    }

    public double MapScale
    {
        get { return (double)GetValue(MapScaleProperty); }
        set { SetValue(MapScaleProperty, value); }
    }
}

<baseVm:MapShowViewModel x:Key="MapShowViewModel" 
 MapScale="{Binding ElementName=MyMap, Path=Scale,
 UpdateSourceTrigger=PropertyChanged,     
 Mode=TwoWay, Converter={StaticResource MapScaleConverter}}" />

但MapScale属性的值不会随地图元素的比例属性的更改而更改