由于特殊原因，需要在一个重复动作之后对同一个键使用addObserver绑定监听，这样做会不会有什么问题？
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No problem, you will receive addObserver messages as many times as you call observeValueForKeyPath.
addObserver
observeValueForKeyPath
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No problem, you will receive
addObservermessages as many times as you callobserveValueForKeyPath.