IOS View的生命周期

View确实加载

[ViewController viewDidLoad]

View将要出现

[ViewController viewWillAppear:]

View 将要布局子控件

[ViewController viewWillLayoutSubviews]

View 布局子控件完成

[ViewController viewDidLayoutSubviews]

View 确实消失

[ViewController viewDidAppear:]

View 将要消失

[ViewController viewWillDisappear:]