iOS ViewController生命周期
【第一阶段】进入ViewController:
viewDidLoad
viewWillAppear
viewWillLayoutSubviews
viewDidlayoutSubviews
viewWillLayoutSubviews
viewDidlayoutSubviews
viewDidAppear
viewWillLayoutSubviews
viewDidlayoutSubviews
---------------------------------------------------
【第二阶段】退出ViewController:
viewWillDisappear
viewWillLayoutSubviews
viewDidlayoutSubviews
viewDidDisappear
dealloc