- (void)viewDidLoad {      [super viewDidLoad];            //UIRefreshControl      //系统自带，继承自UIControl，UIControl继承自UIView      UIRefreshControl * refresh = [[UIRefreshControl alloc] init];      [self.tableView addSubview:refresh];            //设置标题      NSAttributedString * title = [[NSAttributedString alloc] initWithString:@"下拉刷新"];      refresh.attributedTitle = title;            //监听什么时候开始下拉刷新      [refresh addTarget:self action:@selector(refresh:) forControlEvents:UIControlEventValueChanged];      }      //监听方法  - (void)refresh:(UIRefreshControl *)refresh  {      NSLog(@"开始刷新");      dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{                   [NSThread sleepForTimeInterval:2];          //回主线程刷新           dispatch_sync(dispatch_get_main_queue(), ^{                              [refresh endRefreshing];           });      });  }