dispatch_group嵌套异步函数的正确使用

最新推荐文章于 2024-05-14 15:23:47 发布

原创最新推荐文章于 2024-05-14 15:23:47 发布 · 1.8k 阅读

0 ·

CC 4.0 BY-SA版权

iOS 专栏收录该内容

37 篇文章

订阅专栏

本文通过一个具体的示例，介绍了如何使用GCD的组通知机制来确保多个异步任务全部完成后再进行后续操作。文章对比了错误的实现方式与正确的实现方式，并展示了正确的打印顺序。

举个栗子：异步请求A图片和B图片和C图片，在最后合成ABC图

-(void)group{
    dispatch_group_t group = dispatch_group_create();
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
       //这里走了个异步函数，该函数不等里边请求完毕就返回
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
             sleep(3);
             NSLog(@"A图片");
        });
       
        
    });
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        NSLog(@"B图片");
        
    });
    
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
        NSLog(@"C图片");
        
    });
    
    
    dispatch_notify(group, dispatch_get_main_queue(), ^{
        
        NSLog(@"合成图");
        
    });
}

//打印顺序：

2018-04-06 16:01:16.960640+0800 COCOCOCO[13546:578928] B

2018-04-06 16:01:16.960675+0800 COCOCOCO[13546:578926] C

2018-04-06 16:01:16.961241+0800 COCOCOCO[13546:578877] D

2018-04-06 16:01:19.964354+0800 COCOCOCO[13546:578930] A

正确写法：

-(void)rightGroup{
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);
//这里不一定用dispatch_group_async，也可以dispatch_async
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        
       
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
             sleep(3);
             NSLog(@"拿到A图片");
             dispatch_group_leave(group);
        });
       
        
    });
    
     dispatch_group_enter(group);
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         sleep(2);
        NSLog(@"拿到B图片");
         dispatch_group_leave(group);
    });
    
     dispatch_group_enter(group);
    dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
         sleep(1);
        NSLog(@"拿到C图片");
         dispatch_group_leave(group);
    });
    
    
    dispatch_notify(group, dispatch_get_main_queue(), ^{
        
        NSLog(@"合成D图片");
        
    });
}
打印：
2018-04-06 16:11:40.070776+0800 COCOCOCO[13688:585282] 拿到C图片
2018-04-06 16:11:41.072284+0800 COCOCOCO[13688:585280] 拿到B图片
2018-04-06 16:11:42.071223+0800 COCOCOCO[13688:585279] 拿到A图片
2018-04-06 16:11:42.071636+0800 COCOCOCO[13688:585221] 合成D图片