1、使用场景

1、异步执行多个耗时任务。
2、当多个耗时任务都执行完回到主线程执行任务。

2、dispatch_group_notify

监听 group 中任务的完成状态，当所有的任务都执行完成后，才执行dispatch_group_notify block 中的任务。

    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("cwwng-queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_async(group, queue, ^{
        sleep(4);
        NSLog(@"1");
    });
    dispatch_group_async(group, queue, ^{
        sleep(3);
        NSLog(@"2");
    });
    dispatch_group_async(group, queue, ^{
        sleep(2);
        NSLog(@"3");
    });
    dispatch_group_async(group, queue, ^{
        sleep(1);
        NSLog(@"4");
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"notify");
    });
2020-11-09 16:19:38.606736+0800 Demo[47641:2507387] 4
2020-11-09 16:19:39.605344+0800 Demo[47641:2507390] 3
2020-11-09 16:19:40.606060+0800 Demo[47641:2507389] 2
2020-11-09 16:19:41.605860+0800 Demo[47641:2507391] 1
2020-11-09 16:19:41.606227+0800 Demo[47641:2507391] notify

3、处理耗时操作 dispatch_group_enter、dispatch_group_leave

1、dispatch_group_enter 向 group追加一个任务，group 中未执行完任务数+1。
2、dispatch_group_leave 在group删去一个任务，group 中未执行完任务数-1。
3、当 group 中未执行完任务数为0时，会使dispatch_group_wait解除阻塞，然后执行dispatch_group_notify block中的任务。

    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("cwwng-queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"1");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"2");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"3");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"4");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_notify(group, queue, ^{
        NSLog(@"notify");
    });
2020-11-09 16:28:37.651037+0800 Demo[48060:2516999] 4
2020-11-09 16:28:38.650977+0800 Demo[48060:2517002] 3
2020-11-09 16:28:39.650961+0800 Demo[48060:2516999] 2
2020-11-09 16:28:40.651016+0800 Demo[48060:2517002] 1
2020-11-09 16:28:40.651360+0800 Demo[48060:2517002] notify

4、dispatch_group_wait

阻塞当前线程，等待指定的 group 中的任务执行完成后，才会往下继续执行。

    dispatch_group_t group = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("cwwng-queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(4.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"1");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(3.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"2");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(2.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"3");
            dispatch_group_leave(group);
        });
    });
    dispatch_group_enter(group);
    dispatch_group_async(group, queue, ^{
        // 模拟网络请求，耗时操作
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), queue, ^{
            NSLog(@"4");
            dispatch_group_leave(group);
        });
    });
    NSLog(@"wait 前");
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    NSLog(@"wait 后");
2020-11-09 16:33:04.240531+0800 Demo[48264:2521815] wait 前
2020-11-09 16:33:05.240633+0800 Demo[48264:2521916] 4
2020-11-09 16:33:06.240647+0800 Demo[48264:2521910] 3
2020-11-09 16:33:07.240654+0800 Demo[48264:2521910] 2
2020-11-09 16:33:08.243215+0800 Demo[48264:2521910] 1
2020-11-09 16:33:08.243479+0800 Demo[48264:2521815] wait 后