GCD学习之DispatchGroup 和信号量

问题引出

最近在看GCD多线程方面的东西，偶然看到一个问题如下：有A B C D E五个任务，D需要 A 和B 都执行完了才执行，E需要B 和C执行完才开始执行，给出一个GCD的实现方案

解决方案

• 第一反应是用dispatch_group，等A B C 都执行完再执行 D E，这种解决方案如果C消耗的时间比较长，会阻塞任务D，代码如下

 dispatch_group_t groupTask = dispatch_group_create();
    dispatch_queue_t groupQueue = dispatch_queue_create("com.task.group", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_async(groupTask, groupQueue, ^{
        NSLog(@"Doing task A");
        sleep(5);
        NSLog(@"A complete");
    });
    dispatch_group_async(groupTask, groupQueue, ^{
        NSLog(@"Doing task B");
        sleep(5);
        NSLog(@"B complete");
    });
    dispatch_group_async(groupTask, groupQueue, ^{
        NSLog(@"Doing task C");
        sleep(5);
        NSLog(@"C complete");
    });
    
    dispatch_group_notify(groupTask, groupQueue, ^{
        NSLog(@"Doing task D");
        NSLog(@"Doing task E");
    });

执行结果：

2019-07-15 17:25:11.299511+0800 TestBlock[95871:4803740] Doing task A
2019-07-15 17:25:11.299513+0800 TestBlock[95871:4803742] Doing task B
2019-07-15 17:25:11.299517+0800 TestBlock[95871:4803739] Doing task C
2019-07-15 17:25:16.303104+0800 TestBlock[95871:4803740] A complete
2019-07-15 17:25:16.303104+0800 TestBlock[95871:4803739] C complete
2019-07-15 17:25:16.303104+0800 TestBlock[95871:4803742] B complete
2019-07-15 17:25:16.303581+0800 TestBlock[95871:4803742] Doing task D
2019-07-15 17:25:16.303714+0800 TestBlock[95871:4803742] Doing task E

• 这个方案好像不太符合需求，再思考一遍题目，D 对A和B有依赖，这个用dispatch_group 来解决，E 对B 和 C有依赖，E在执行时，需要B的执行状态，由此联想到可以用信号量控制，因此引出以下的解决方案，

dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t groupQueue = dispatch_queue_create("com.group.queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    
    // run task A B in group
    dispatch_group_async(group, groupQueue, ^{
        NSLog(@"Doing task A");
        sleep(5);
        NSLog(@"A complete");
    });
    dispatch_group_async(group, groupQueue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"Doing task B");
        sleep(10);
         NSLog(@"B complete");
        dispatch_semaphore_signal(semaphore);
    });

    // run task C in another group
    dispatch_group_t group2 = dispatch_group_create();
    dispatch_group_async(group2, groupQueue, ^{
        NSLog(@"Doing task C");
        sleep(5);
        NSLog(@"C complete");
    });

    // After A and B completed, run task D
    dispatch_group_notify(group, groupQueue, ^{
        NSLog(@"Doing task D");
    });
    
    // After B and C completed, run task E
    dispatch_group_notify(group2, groupQueue, ^{
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        NSLog(@"Doing task E");
        dispatch_semaphore_signal(semaphore);
    });

C执行完要执行E时，查看B任务的状态，B完成了就可以开始，执行结果如下：

2019-07-15 17:37:31.511863+0800 TestBlock[96007:4818708] Doing task A
2019-07-15 17:37:31.511867+0800 TestBlock[96007:4818707] Doing task C
2019-07-15 17:37:31.511863+0800 TestBlock[96007:4818710] Doing task B
2019-07-15 17:37:36.515497+0800 TestBlock[96007:4818708] A complete
2019-07-15 17:37:41.513354+0800 TestBlock[96007:4818710] B complete
2019-07-15 17:37:41.513640+0800 TestBlock[96007:4818710] Doing task D
2019-07-15 17:37:46.513255+0800 TestBlock[96007:4818707] C complete
2019-07-15 17:37:46.513455+0800 TestBlock[96007:4818707] Doing task E

由以上的执行结果可以看出，C如果是比较耗时的任务，不会阻塞任务D

总结

如果用GCD实现一些依赖性的任务，可以考虑GCD + semaphore 结合的方式。另外，如有更好的解决方案，可以告知。