首先看一下官方解释：
- (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，这个方法在view被加载之后调用，如果没有nib文件，这个方法是在loadView方法被调用之后调用，如果有nib文件，这个方法是在view从nib中加载后调用，总之当一个viewController被初始化后，-viewDidLoad方法仅被调用一次。-viewWillAppear，在加载了view之后，在view出现在屏幕之前调用viewWillAppear
--viewDidAppear，view出现在屏幕之后开始调用
-viewWillDisappear，是指view将要消失的时候开始调用
-viewDidDisappear，是指view已经消失后开始调用
-viewDidUnload，是指一个view被release或被内存警告清除时调用，在dealloc后不会调用
我们会把一次执行的代码放在viewDidLoad里面，把要多次执行的代码放在viewWillAppear里面