viewWillAppear:
Called when the view is about to made visible. Default doesnothing
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.Default does nothing
视图被驳回时调用，覆盖或以其他方式隐藏。默认情况下不执行任何操作
viewWillDisappear
在屏幕变黑锁屏后，并不会触发这个事件