dispatch_group_t----dispatch_group_enter----dispatch_group_leave----di

  dispatch_group_t group = dispatch_group_create();
   for ( int i = 0; i < 10; i++) {
       //    把一个异步操作加入到group中
       dispatch_group_enter(group);
       dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
           NSURLSessionDataTask * dataTask = [[NSURLSession sharedSession] dataTaskWithURL:[NSURL URLWithString:@"https://www.baidu.com"] completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
               NSString * str = [NSString stringWithFormat:@"%d  asdfa",i];
               NSLog(@"--==-str : %@---",str);
               dispatch_group_leave(group);
           }];
           [dataTask resume];
       });
   }

   //    等待group完成所有异步操作
   dispatch_group_notify(group, dispatch_get_main_queue(), ^{
       NSLog(@"--==- finish ---");
   });