UIApplication window

[[[UIApplication sharedApplication] keyWindow].rootViewController viewWillAppear:YES];//可以通过这种fan方式来获得，某些操作。

[[[UIApplication sharedApplication].delegate window] setBackgroundColor:[UIColor greenColor]];//修改window的背景色.

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[[[UIApplication sharedApplication] keyWindow].rootViewController viewWillAppear:YES];//有很多妙用