iOS小知识点（GCD）

GCD延时加载

double delayInSeconds = 1;
dispatch_time_t delay = dispatch_time(DISPATCH_TIME_NOW, (int64_t)(delayInSeconds * NSEC_PER_SEC));
       dispatch_after(delay, dispatch_get_main_queue(), ^(void){
               //执行的方法
       });

主线程更新UI

__weak typeof(self) weakSelf = self;
dispatch_async(dispatch_get_main_queue(), ^{
     weakSelf.tableView.tableHeaderView = weakSelf.headView;
     [weakSelf.tableView reloadData];
});