WatchKey poll returns the next WatchKey that has had some of its events occur, or null if no registered events have happened.
如果有相关事件出现，WatchKey poll会返回下一个WatchKey，或者没有注册的事件发生，会返回null。
As I said, last year there was a shade under 23,000 people dancing, 271 registered events, as far as I know, around the world this year.
正如我所说的，去年共有23万人跳舞。今年据我所知，世界各地共进行271场经过登记的活动。
Browsers pass events by calling functions that were registered in advance.
浏览器通过调用预先注册的函数来传递事件。
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