Sometime we need to know when an attached property change its value for a specific FrameworkElement. For example consider the following:
<Canvas>
<Button x:Name="move" Content="Move" Canvas.Left="10" Canvas.Top="10"/>
</Canvas>
If in a part of the code we have something like:
Canvas.SetLeft(this.move, 20);
And we need listen when the Left attached property changed for the button, we can use a DependencyPropertyDescriptor to do it. This class provides the FromProperty static method, which gives as an instance of itself receiving the DependencyProperty:
var descriptor = DependencyPropertyDescriptor.FromProperty(
Canvas.LeftProperty,
typeof(Button));
Once we have the instance of the DependencyPropertyDescriptor we can use it to get the notifications using AddValueChanged method:
descriptor.AddValueChanged(this.move, this.OnValueChanged);
Finally we need to implement the event handler:
private void OnValueChanged(object sender, EventArgs e)
{
var button = (Button)sender;
...
}