CGD调度组

封装任务并把任务添加到队列中

     参数1: 调度组

     参数2: 队列

     参数3: 封装任务

notify和wait都可以  就是wait会阻塞线程 notify 不会阻塞

dispatch_queue_t queue = dispatch_queue_create("queue", DISPATCH_QUEUE_CONCURRENT);

    dispatch_group_t group = dispatch_group_create();

    dispatch_group_enter(group);

    dispatch_group_async(group, queue, ^{

        NSLog(@"--1");

        dispatch_group_leave(group);

    });

    dispatch_group_enter(group);

    dispatch_group_async(group, queue, ^{

        NSLog(@"--2");

         dispatch_group_leave(group);

    });

   dispatch_group_enter(group);

    dispatch_group_async(group, queue, ^{

        NSLog(@"--3");

        dispatch_group_leave(group);

    });

    dispatch_group_enter(group);

    dispatch_group_async(group, queue, ^{

        NSLog(@"--4");

        dispatch_group_leave(group);

    });

    /* 当队列组中所有的任务都执行完毕的时候调用下面的函数  函数本身是不阻塞的 */

//    dispatch_group_notify(group, queue, ^{

//        NSLog(@"完了");

//    });

    /*函数本身是阻塞的*/

    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);

    NSLog(@"执行完毕了是吗");

//    必须等到1,2,3,4三个任务完成了以后才会执行dispatch_group_notify(,,_)中block代码块中的代码.

转自：https://www.jianshu.com/p/e93fd15d93d3