Within this routine, first a check for non-null context handle is made.

在这个例程中，首先检查是否有非空的上下文句柄。

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Each of these methods will return null if its opposite component or window is in a different context.

如果其对立组件或窗口处于一个不同的上下文，这些方法中每一个都会返回空。

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This method will return null if the focus owner is not in the same context as the calling thread.

如果该焦点所有者未和调用线程处于同一个上下文中，那么该方法会返回空。

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