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. 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
viewDidDisappear:
Called after the view was dismissed, covered or otherwise hidden. Default does nothing
视图被驳回后调用，覆盖或以其他方式隐藏。默认情况下不执行任何操作
loadView;
This is where subclasses should create their custom view hierarchy if they aren't using a nib. Should never be called directly.
这是当他们没有正在使用nib视图页面，子类将会创建自己的自定义视图层。绝不能直接调用。
viewDidLoad;
Called after the view has been loaded. For view controllers created in code, this is after -loadView. For view controllers unarchived from a nib, this is after the view is set.
在视图加载后被调用，如果是在代码中创建的视图加载器，他将会在loadView方法后被调用，如果是从nib视图页面输出，他将会在视图设置好后后被调用。
