ViewController life cycle

at first it loads viewDidLoad

- (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

if you implement by the line NSLog(@"see what is that %s ", __func__); you will see the first 3 is coming one by one.