View + Tree + Observer 리스너 등록을 통하여 View Tree 안에서의 변화를 감지할 수 있습니다. A view tree observer is used to register listeners that can be notified of global changes in the view tree. Such global events include, but are not limited to, layout of the whole tree, beginning of the drawing pass, touch mode change.... A ViewTreeObserver should never be instantiated by applications as it is provided by the view..