iOS开发之GCD（6）API Dispatch Group

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首先看一段代码：

        dispatch_queue_t globalDispatchQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);                dispatch_async(globalDispatchQueue, ^{            NSLog(@"1");        });        dispatch_async(globalDispatchQueue, ^{            NSLog(@"2");        });        dispatch_async(globalDispatchQueue, ^{            NSLog(@"3");        });

平时当我这么追加的的时候，因为是concurrent dispatch queue，所以不能确定执行顺序，如果需要在全部执行结束之后进行一些操作的话，实现起来比较麻烦。这时候就用到了Dispatch Group。

 dispatch_queue_t globalDispatchQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);                dispatch_group_t group = dispatch_group_create();                dispatch_group_async(group, globalDispatchQueue, ^{            NSLog(@"1");        });        dispatch_group_async(group, globalDispatchQueue, ^{            NSLog(@"2");        });        dispatch_group_async(group, globalDispatchQueue, ^{            NSLog(@"3");                    });                dispatch_group_notify(group, globalDispatchQueue, ^{            NSLog(@"执行完毕");        });

执行结果：

2015-03-17 15:12:19.698 GCD_Demo[872:1203] 12015-03-17 15:12:19.698 GCD_Demo[872:1403] 32015-03-17 15:12:19.698 GCD_Demo[872:1303] 22015-03-17 15:12:19.701 GCD_Demo[872:1303] 执行完毕

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