Called whenever the control's template changes.
Assembly: PresentationFramework (in PresentationFramework.dll)
The timing for changing the template is as follows:
The template is changed by a SetValue method call, a style trigger, or some other means.
The property value changes; the property-changed callback is called.
The old visual tree is removed.
An internal method is called which eventually calls the method.
Later, during a measure, OnApplyTemplate is called and a new visual tree is attached.