When is data binding in WPF evaluated?

I am binding the CustomerID property in my custom control to the same property in an ancestor. The ancesor is a TopLevelControl.

I set up the binding in the child control's constructor and access the property in OnApplyTemplate() where I also do some other initialization. But it seems to me that the binding is not evaluated when OnApplyTemplate() is called. Why and when is it updated with the binding?

My CustomChildControl:

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public String CustomerID {
  get{ return (bool) base.GetValue(CustomerIDProperty);}
  set{ base.SetValue(CustomerIDProperty, value);}
}

public CustomChildControl()
{
     binding = new Binding("CustomerID")
     { 
         RelativeSource = new RelativeSource(RelativeSourceMode.FindAncestor, typeof(TopLevelControl),1)
     };
     SetBinding(CustomerIDProperty, binding);
}

override OnApplyTemplate(){

   base.OnApplyTemplate();
   // CustomerID is null here... why? 

   Initialize(CustomerID);
}

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The reason is that controls are not added to the visual tree until after they are initialized. Since you are binding the control to an ancestor, the data source (the ancestor) doesn't exist (from the control's perspective) until the control has been added to the visual tree (after ApplyTemplate is done).

I'd recommend moving that code to the Load event.