view的生命周期

UIViewController中声明如下方法：

//加载视图- (void)loadView; //将要卸载视图- (void)viewWillUnload NS_DEPRECATED_IOS(5_0,6_0);//已经卸载视图- (void)viewDidUnload NS_DEPRECATED_IOS(3_0,6_0); //已经加载- (void)viewDidLoad; //视图将要显示- (void)viewWillAppear:(BOOL)animated;    //视图已经显示- (void)viewDidAppear:(BOOL)animated;     //视图将要隐藏- (void)viewWillDisappear:(BOOL)animated; //视图已经隐藏- (void)viewDidDisappear:(BOOL)animated; //将要调用layoutSubviews- (void)viewWillLayoutSubviews NS_AVAILABLE_IOS(5_0);//已经调用layoutSubviews- (void)viewDidLayoutSubviews NS_AVAILABLE_IOS(5_0);