At first glance, it appears that the postSet method is where all of the eventing code would live.

初看上去，postSet方法似乎可以包含所有事件代码。

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The postSet() needs to fire an event to all of the registered listeners with the object being updated, the action of the update, the value before, and the value now.

postSet() 需要向注册的所有监听器触发一个事件，事件包含更新的对象、更新的操作、更新之前的值和目前的值。

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