主线程，执行UI刷新

dispatch_async(dispatch_get_global_queue(0, 0), ^{
      //执行耗时的异步操作..
      //................
      //................
      dispatch_async(dispatch_get_main_queue(), ^{
          //回到主线程，执行UI刷新操作
          //................
          //................
      });
  });

延时操作

 dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(10 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
        //code to be executed after a specified delay code将在10后执行
 });

dispatch_group_async队列组实现多线程异步操作

1.调用队列组的 dispatch_group_async先把任务放到队列中，然后将队列放入队列组中。
[或者使用队列组的 dispatch_group_enter、dispatch_group_leave组合 来实现dispatch_group_async]
2.当所有任务都执行完成之后，才执行dispatch_group_notify block 中的任务。

#@property(nonatomic, strong) dispatch_group_t group_t;
#@property(nonatomic, strong) dispatch_queue_t queue_t;

#pragma mark dispatch_group_async
- (void)gcd_dispatch_group {
    // 打印当前线程
    NSLog(@"当前线程---%@",[NSThread currentThread]);
    NSLog(@"队列组---begin");
    dispatch_group_t group =  dispatch_group_create();
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

    dispatch_group_async(group, queue, ^{
        // 追加任务1111
        for (int i = 0; i < 5; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:4];
            //打印当前线程
            NSLog(@"1111---%@",[NSThread currentThread]);
        }
    });
    
    dispatch_group_async(group, queue, ^{
        // 追加任务2222
        for (int i = 0; i < 6; ++i) {
            //模拟耗时操作
            [NSThread sleepForTimeInterval:4];
            //打印当前线程
            NSLog(@"2222---%@",[NSThread currentThread]);
        }
    });

    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        //等前面的异步任务1、任务2都执行完毕后，回到主线程执行下边任务
        NSLog(@"任务1、任务2都执行完毕");
        for (int i = 0; i < 4; ++i) {
            // 模拟耗时操作
            [NSThread sleepForTimeInterval:2];
            // 打印当前线程
            NSLog(@"3333---%@",[NSThread currentThread]);
        }
        NSLog(@"队列组---end");
    });
    NSLog(@"代码---end");
}