UWP附加属性未触发属性更改了回调

Question

I have a custom attached property defined:

 public static IList<Object> GetBindings(DependencyObject obj)
 {
    return (IList<Object>)obj.GetValue(BindingsProperty);
 }

 public static void SetBindings(DependencyObject obj, IList<Object> value)
 {
    obj.SetValue(BindingsProperty, value);
 }

 // Using a DependencyProperty as the backing store for Bindings.  This enables animation, styling, binding, etc...
 public static readonly DependencyProperty BindingsProperty =
    DependencyProperty.RegisterAttached("Bindings", typeof(IList<Object>), typeof(KeyboardInput), new PropertyMetadata(new List<Object>(), OnBindingsChanged));


 private static void OnBindingsChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
 {
     Debugger.Break();
 }

Xaml:

<TextBox x:Name="textbox" 
         PlaceholderText="Add a comment...">

    <app:KeyboardInput.Bindings> 
      <app:KeyBinding />
    </app:KeyboardInput.Bindings>

</TextBox>

And method, which listens to the property changes ( OnBindingsChanged ). Why it isn't firing when the default value ( new List<object>() ) is assigned to a property? What is the way to access a target object after it was already contructed in xaml?

Answer 1

The event won't fire because you have given the property a default value of new List<object> so in XAML when you set the binding like:

<app:KeyboardInput.Bindings> 
      <app:KeyBinding />
</app:KeyboardInput.Bindings>

you are simply just adding an item to the existing list which doesn't change the actual value of the property. I recommend replacing it with an ObservableCollection and then listening to the CollectionChanged event which will fire every time an item is added or removed.