viewDidLoad and viewDidUnload的關係

參考

http://www.cnblogs.com/wwwkhd/archive/2011/03/14/1983192.html

由init、loadView、viewDidLoad、viewDidUnload、dealloc的關係說起

init方法

在init方法中實例化必要的對象（遵從LazyLoad思想）

init方法中初始化ViewController本身

loadView方法

當view需要被展示而它卻是nil時，viewController會調用該方法。 不要直接調用該方法。

如果手工維護views，必須重載重寫該方法

如果使用IB維護views，必須不能重載重寫該方法

loadView和IB構建view

你在控制器中實現了loadView方法，那麼你可能會在應用運行的某個時候被內存管理控制調用。 如果設備內存不足的時候， view 控制器會收到didReceiveMemoryWarning的消息。 默認的實現是檢查當前控制器的view是否在使用。 如果它的view不在當前正在使用的view hierarchy裡面，且你的控制器實現了loadView方法，那麼這個view將被release, loadView方法將被再次調用來創建一個新的view。

viewDidLoad方法

viewDidLoad 此方法只有當view從nib文件初始化的時候才被調用。

重載重寫該方法以進一步定制view

在iPhone OS 3.0及之後的版本中，還應該重載重寫viewDidUnload來釋放對view的任何索引

viewDidLoad後調用數據Model

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viewDidUnload方法

當系統內存吃緊的時候會調用該方法（注：viewController沒有被dealloc）

內存吃緊時，在iPhone OS 3.0之前didReceiveMemoryWarning是釋放無用內存的唯一方式，但是OS 3.0及以後viewDidUnload方法是更好的方式

在該方法中將所有IBOutlet（無論是property還是實例變量）置為nil（系統release view時已經將其release掉了）

在 該方法中釋放其他與view有關的對象、其他在運行時創建（但非系統必須）的對象、在viewDidLoad中被創建的對象、緩存數據等release對 像後，將對象置為nil（IBOutlet只需要將其置為nil，系統release view時已經將其release掉了）

一般認為viewDidUnload是viewDidLoad的鏡像，因為當view被重新請求時，viewDidLoad還會重新被執行

viewDidUnload中被release的對象必須是很容易被重新創建的對象（比如在viewDidLoad或其他方法中創建的對象），不要release用戶數據或其他很難被重新創建的對象

dealloc方法

viewDidUnload和dealloc方法沒有關聯，dealloc還是繼續做它該做的事情

參考圖從APPLE文件上截出

 原文資料

Task	Methods	Discussion
Allocating critical data structures required by your view controller	Initialization methods	Your custom initialization method (whether it is named init or something else) is always responsible for putting your view controller object in a known good state. This includes allocating whatever data structures are needed to ensure proper operation.
Creating your view objects	loadView	Overriding the loadView method is required only if you intend to create your views programmatically. If you are using storyboards, the views are loaded automatically from the storyboard file.
Allocating or loading data to be displayed in your view	viewDidLoad	Typically, data objects are provided by configuring your view controller’s properties. Any additional data objects your view controller wants to create should be done by overriding the viewDidLoad method. By the time this method is called, your view objects are guaranteed to exist and to be in a known good state.
Releasing references to view objects	viewDidUnload dealloc	If you maintain strong references to any view objects in your view hierarchy using outlets or other instance variables in your class, you must always release those references when the views are no longer needed. You release a view object by setting your outlet or variable to nil. For more information about when views get released, see “Understanding How Views Are Loaded and Unloaded” (page 57).

eleasing data that is not needed when your view is not displayed	viewWillUnload viewDidUnload	You can use the viewDidUnload method to deallocate any data that is view specific and that can be re-created easily enough if the view is loaded into memory again. If re-creating the data might be too time-consuming, though, you do not have to release the corresponding data objects here. Instead, you should consider releasing those objects in your didReceiveMemoryWarning method.
Responding to low-memory notifications	didReceiveMemory- Warning	Use this method to deallocate all noncritical custom data structures associated with your view controller. Although you would not use this method to release references to view objects, you might use it to release any view-related data structures that you did not already release in your viewDidUnload method. ( The view objects themselves should always be released in the viewDidUnload method.)
Releasing critical data structures required by your view controller	dealloc	Override this method only to perform any last-minute cleanup of your view controller class. Objects stored in instance variables and properties are automatically released; you do not need to release them explicitly.