视图生命周期

先执行viewDidLoad()

 1 -(void) viewWillAppear:(BOOL)animated
 2 {
 3     NSLog(@"视图即将出现");
 4 }
 5 -(void) viewDidAppear:(BOOL)animated
 6 {
 7     NSLog(@"视图已经出现");
 8 }
 9 -(void) viewWillDisappear:(BOOL)animated
10 {
11     NSLog(@"视图即将消失");
12 }
13 -(void) viewDidDisAppear:(BOOL)animated
14 {
15     NSLog(@"视图已经消失");
16 }