摘要：
viewWillAppear:Called when the view is about to made visible. Default does nothing视图即将可见时调用。默认情况下不执行任何操作viewDidAppear:Called when the view has been fully transitioned onto the screen. Default does nothing视图已完全过渡到屏幕上时调用viewWillDisappear:Called when the view is dismissed, covered or otherwise hidden. 阅读全文
