学步园

代码：

- (void)viewDidLoad {
    [super viewDidLoad];
    
    NSLog(@"View Did Load");
}

- (void)viewDidAppear:(BOOL)animated {
    NSLog(@"View Did Appear");
}

- (void)viewDidDisappear:(BOOL)animated {
    NSLog(@"View Did Disappear");
}

运行时控制台输出：

2014-02-16 22:14:03.631 ViewDidAppear[4306:70b] View Did Load
2014-02-16 22:14:03.683 ViewDidAppear[4306:70b] View Did Appear
2014-02-16 22:14:04.793 ViewDidAppear[4306:70b] View Did Disappear
2014-02-16 22:14:05.829 ViewDidAppear[4306:70b] View Did Appear
2014-02-16 22:14:11.247 ViewDidAppear[4306:70b] View Did Disappear
2014-02-16 22:14:12.692 ViewDidAppear[4306:70b] View Did Appear

可见ViewDidLoad方法只执行一次，而ViewDidAppear和ViewDidDisappear方法每次都会随着视图的出现和消失而执行，另外ViewDidLoad方法执行先于ViewDidAppear方法。

对于一些需要每次加载视图都要执行的动作，应该放在ViewDidAppear方法中，而不是ViewDidLoad方法中。