当配置发生改变时允许你的Activity重启，但让其携带一个有状态的对象到你的新activity实例中。

Allow your activity to restart when a configuration changes, but carry a stateful Object to the new instance of your activity.

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Command对象可能是有状态的，也可能是无状态的。

Command objects can be either stateful or stateless.

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在Windows中，事件对象是那些需要使用SetEvent函数显式地将其状态设置为有信号状态的同步对象。

In Windows, event objects are one of the synchronization objects whose state needs to be explicitly set to signaled using the SetEvent function. Event objects come in two types.

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