问题描述
我把 NSLog（@％@ ::％@，[[self class] description]，NSStringFromSelector（_cmd））; 放在<$ c $中c> viewDidLoad 和视图控制器的 viewDidUnload 。
I put NSLog(@"%@::%@", [[self class] description], NSStringFromSelector(_cmd)); in both viewDidLoad and viewDidUnload of a view controller.
在日志中，我当应用程序移入和移出不同的.nib时，发现 viewDidLoad 被调用远远超过 viewDidUnload 。
In the log, I found viewDidLoad is called a lot more than viewDidUnload when the app moves to and from different .nibs.
为什么？
推荐答案
viewDidLoad和viewDidUnload彼此不对应。
The viewDidLoad and viewDidUnload is not corresponding to each other.
只有在收到内存警告时才会调用viewDidUnload。然后系统将自动调用你的viewDidUnload。
The viewDidUnload will only be called when you receive a memory warning. The system then will automatically call your viewDidUnload.
在正常情况下，当你按下MyViewController并将其弹出时。生命周期将如下所示：
In the normal case, when you push a MyViewController and pop it out. The life cycle will happens like this:
init
viewDidLoad
release
这意味着，无论何时初始化和推送/呈现视图，都会调用viewDidLoad。但是当您弹出视图时，将在正常情况下调用该版本，并且将在内存警告情况下调用viewDidUnload。
That means, whenever you init and push/present a view, a viewDidLoad will be called. But when you pop the view, the release will be called in normal case and viewDidUnload will be called in memory warning case.
这是非常含蓄的，Apple没有声明它明确地在指南上。以下是一些参考：
This is quite implicit and Apple doesn't state it clearly on the Guide. Here is some reference: Load and Unload cycle
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