首先看一下官方解釋：
- (void)loadView; // This is where subclasses should create their custom view hierarchy if they aren't using a nib. Should never be called directly.
- (void)viewDidUnload NS_DEPRECATED_IOS(3_0,6_0); // Called after the view controller's view is released and set to nil. For example, a memory warning which causes the view to be purged. Not invoked as a result of -dealloc.
- (void)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.
- (void)viewWillAppear:(BOOL)animated; // Called when the view is about to made visible. Default does nothing
- (void)viewDidAppear:(BOOL)animated; // Called when the view has been fully transitioned onto the screen. Default does nothing
- (void)viewWillDisappear:(BOOL)animated; // Called when the view is dismissed, covered or otherwise hidden. Default does nothing
- (void)viewDidDisappear:(BOOL)animated; // Called after the view was dismissed, covered or otherwise hidden. Default does nothing
-viewDidLoad，這個方法在view被載入之後呼叫，如果沒有nib檔案，這個方法是在loadView方法被呼叫之後呼叫，如果有nib檔案，這個方法是在view從nib中載入後呼叫，總之當一個viewController被初始化後，-viewDidLoad方法僅被呼叫一次。
-viewWillAppear，在載入了view之後，在view出現在螢幕之前呼叫viewWillAppear
--viewDidAppear，view出現在螢幕之後開始呼叫
-viewWillDisappear，是指view將要消失的時候開始呼叫
-viewDidDisappear，是指view已經消失後開始呼叫
-viewDidUnload，是指一個view被release或被記憶體警告清除時呼叫，在dealloc後不會呼叫
我們會把一次執行的程式碼放在viewDidLoad裡面，把要多次執行的程式碼放在viewWillAppear裡面