Silverlight检测Dependency属性何时具有Xaml绑定

Question

Is it possible to detect when a Xaml binding is set on a DependencyProperty in Silverlight?

Eg if I had a custom user-control with single dependency property and a binding declared like this:

public class MyControl : UserControl
{ 
   public static readonly DependencyProperty TestProperty = 
           DependencyProperty.Register("Test", 
              typeof(object), typeof(MyControl), 
              new PropertyMetadata(null));

   public object Test
   { 
       get { return GetValue(TestProperty); } 
       set { SetValue(TestProperty, value); } 
   }
}

<MyControl Test="{Binding APropInViewModel}>
</MyControl>

Can I in the MyControl code to something like this?

// Ctor
public MyControl()
{ 
    TestProperty.BindingChanged += new EventHandler(...)
} 

eg can I get a notification of binding?

NOTE:

This is to solve a tricky order of precedence problem, described here so just checking for new values in the DependencyPropertyChanged handler won't work - because the property changed handler doesn't fire!!

Answer 1

It is possible for value changes in this binding. You can detect changes using propertychanged callback method which is static for Dependency properties.

public static readonly DependencyProperty TestProperty =
       DependencyProperty.Register("Test",
          typeof(object), typeof(MyControl),
          new PropertyMetadata(null, TestChangedCallbackHandler));

    private static void TestChangedCallbackHandler(DependencyObject sender, DependencyPropertyChangedEventArgs args)
    {
        MyControl obj = sender as MyControl;

        Test = args.NewValue; 
    }

However, this might cause following event listening cases. If you want to listen changes on this dependency property is explained in this link : Listen DepencenyProperty value changes

public void RegisterForNotification(string propertyName, FrameworkElement element, PropertyChangedCallback callback)
    {
        Binding b = new Binding(propertyName) { Source = element };
        var prop = System.Windows.DependencyProperty.RegisterAttached(
            "ListenAttached" + propertyName,
            typeof(object),
            typeof(UserControl),
            new System.Windows.PropertyMetadata(callback));

        element.SetBinding(prop, b);
    }

and call like this

this.RegisterForNotification("Test", this, TestChangedCallback);