一个UIViewController 的 - (void)viewDidLoad 中初始化了一个 UIScrollView 在它上面加一个 UIRefreshControl ，实现下拉刷新。下拉刷新的过程中ScrollView会出现跳跃。谁知道这是为什么？（请回答中不要告诉我可以采用其他方法如EGO，我就是想知道为什么，谢谢大神）
下面是 viewDidLoad 的代码。

 - (void)viewDidLoad
      {
       [super viewDidLoad];
       UIScrollView *scrollView = [[UIScrollView alloc]initWithFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.width, [UIScreen mainScreen].bounds.size.height)];
    [scrollView setBackgroundColor:[UIColor whiteColor]];
    [scrollView setContentSize:CGSizeMake([UIScreen mainScreen].bounds.size.width, [UIScreen mainScreen].bounds.size.height)];
    scrollView.alwaysBounceVertical = YES;
    [self.view addSubview:scrollView];
    UIView *topView = [[UIView alloc] initWithFrame:CGRectMake(0, -200, scrollView.bounds.size.width, 200)];
    [topView setBackgroundColor:[UIColor brownColor]];
    [scrollView addSubview:topView];
    UIRefreshControl *refreshControl = [[UIRefreshControl alloc] init];
    [refreshControl addTarget:self action:@selector(testRefresh:) forControlEvents:UIControlEventValueChanged];

    [scrollView addSubview:refreshControl];
    [self.view addSubview:scrollView];
    }