图中没有提到viewWillLayoutSubviews和viewDidLayoutSubviews方法，但是在viewWillAppear和viewDidAppear之间调用这些方法。它们可以被多次调用。

海德尔的答案适用于ios 6之前的版本。然而，在iOS 6中，viewDidUnload和viewWillUnload从未被调用。文档声明:“视图在低内存条件下不再被清除，因此该方法永远不会被调用。”

让我们集中在方法上，它们负责UIViewController的生命周期:

Creation: - (void)init - (void)initWithNibName: View creation: - (BOOL)isViewLoaded - (void)loadView - (void)viewDidLoad - (UIView *)initWithFrame:(CGRect)frame - (UIView *)initWithCoder:(NSCoder *)coder Handling of view state changing: - (void)viewDidLoad - (void)viewWillAppear:(BOOL)animated - (void)viewDidAppear:(BOOL)animated - (void)viewWillDisappear:(BOOL)animated - (void)viewDidDisappear:(BOOL)animated - (void)viewDidUnload Memory warning handling: - (void)didReceiveMemoryWarning Deallocation - (void)viewDidUnload - (void)dealloc

更多信息请查看UIViewController类参考。

这里有很多过时和不完整的信息。仅适用于iOS 6及更新版本:

(一)未经 viewDidLoad[一] 那些有点 viewWillLayoutSubviews是边界第一次被最终确定 viewDidLayoutSubviews 在viewDidAppear * viewWillLayoutSubviews [b] * viewDidLayoutSubviews [b]

脚注:

(a) -如果在didReceiveMemoryWarning期间手动空出视图，loadView和viewDidLoad将再次被调用。也就是说，默认情况下，loadView和viewDidLoad每个视图控制器实例只被调用一次。

(b)可称为额外0次或更多次。

在官方文档中解释状态转换:https://developer.apple.com/library/ios/documentation/uikit/reference/UIViewController_Class/index.html

这张图片显示了各种视图' will '和' did '回调方法之间的有效状态转换

有效的状态转换:

取自:https://developer.apple.com/library/ios/documentation/uikit/reference/UIViewController_Class/Art/UIViewController类Reference_2x.png

当你加载/显示/隐藏视图控制器时，iOS会在适当的时间自动调用所有这些命令。需要注意的是这些方法附加到UIViewController而不是uiview本身。仅仅使用UIView你不会得到这些特性。

苹果网站上有很好的文档。简单地说:

ViewDidLoad - Called when you create the class and load from xib. Great for initial setup and one-time-only work. ViewWillAppear - Called right before your view appears, good for hiding/showing fields or any operations that you want to happen every time before the view is visible. Because you might be going back and forth between views, this will be called every time your view is about to appear on the screen. ViewDidAppear - Called after the view appears - great place to start an animations or the loading of external data from an API. ViewWillDisappear/DidDisappear - Same idea as ViewWillAppear/ViewDidAppear. ViewDidUnload/ViewDidDispose - In Objective-C, this is where you do your clean-up and release of stuff, but this is handled automatically so not much you really need to do here.