官方解释
UIView implements this method by returning the UIViewController object that manages it (if it has one) or its superview (if it doesn’t); UIViewController implements the method by returning its view’s superview; UIWindow returns the application object, and UIApplication returns nil.
4、如何隐藏tab bar controller 的tab bar
在Iphone编程中,很多人喜欢把Tab bar controller和Nav 一起混合使用,在点击了Tab bar上面的一个按钮切换到另一个view的时候,界面上的Tab Bar 没有消失.