另外一项可以用来找到隐藏失效侦听器的技术是：防止指定侦听器对象在指定事件源上登记两次。

Another technique that can sometimes be used to find otherwise hidden lapsed listeners is preventing a given listener object from being registered twice with a given event source.

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这些类负责决定什么时候它们需要触发事件，什么时候确实触发事件，以及什么时候调用所有登记的侦听器。

These classes decide when they need to fire events, and when they do, they call all the listeners that are registered.

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例如，登记员的概念消失并且被术语系统取代，这表示已经决定自动处理许多有关参加的琐碎事件。

For example, the concept of a registrar disappeared and was replaced with the term system, indicating that a decision had been made to automate many of the mundane aspects of enrollment.

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