A view tree observer is used to register listeners that canbe notified of global changes in the view tree. Such global eventsinclude, but are not limited to, layout of the whole tree,beginning of the drawing pass, touch mode change.... AViewTreeObserver should never be instantiated by applications as itis provided by the views hierarchy. Refer toA view tree observer is used to
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view变化监听器ViewTreeObserver介绍
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