要使之对客户而言更为简便，此服务首先要作为IWindowListener注册自身以便当新窗口打开时被通知到。

To make it simple for the consumer, the service first registers itself as an IWindowListener to be notified when a new window opens.

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当用户在当前窗口之上再打开一个新窗口时，可能会对焦点变化感到疑惑。

Confusing changes in focus can happen when the user opens a new window on the top of the current one.

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