概览

定义在group.h中，api较少，只有创建、提交、阻塞、通知组

• 创建组

dispatch_group_t
dispatch_group_create(void);

• 在组中异步提交block到队列

void
dispatch_group_async(dispatch_group_t group,
    dispatch_queue_t queue,
    dispatch_block_t block);

• 在组中异步提交c函数指针到队列

void
dispatch_group_async_f(dispatch_group_t group,
    dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

• 阻塞当前线程直到一组任务执行完毕或者超时

intptr_t
dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);

• 一组任务执行完毕通知回到block

void
dispatch_group_notify(dispatch_group_t group,
    dispatch_queue_t queue,
    dispatch_block_t block);

• 一组任务执行完毕通知回到c函数指针

void
dispatch_group_notify_f(dispatch_group_t group,
    dispatch_queue_t queue,
    void *_Nullable context,
    dispatch_function_t work);

• 进入组相当于dispatch_group_async，信号量+1

void
dispatch_group_enter(dispatch_group_t group);

离开组相当于group中的一个任务执行完毕，信号量-1

void
dispatch_group_leave(dispatch_group_t group);

注意：

group是基于信号量实现的，所以一组任务在任意队列执行，结果都是一样的

使用

NSLog(@"begin：%@",[NSThread currentThread]);
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_enter(group);
dispatch_async(queue, ^{
    NSLog(@"1：%@",[NSThread currentThread]);
    dispatch_group_leave(group);
});

dispatch_group_enter(group);
dispatch_async(queue, ^{
    NSLog(@"2：%@",[NSThread currentThread]);
    dispatch_group_leave(group);
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"end：%@",[NSThread currentThread]);
});
NSLog(@"任务提交完毕");

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
NSLog(@"等待任务全部完成后再往下执行");

NSLog(@"begin：%@",[NSThread currentThread]);
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

dispatch_group_async(group, queue, ^{
    NSLog(@"1：%@",[NSThread currentThread]);
});

dispatch_group_async(group, queue, ^{
    NSLog(@"2：%@",[NSThread currentThread]);
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"end：%@",[NSThread currentThread]);
});
NSLog(@"任务提交完毕");

dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
NSLog(@"等待任务全部完成后再往下执行");

上面这两种写法等效，输出结果如下：

2021-08-23 10:46:43.862446+0800 GCDDemo[4404:2489050] begin：<NSThread: 0x2838a08c0>{number = 1, name = main}
2021-08-23 10:46:43.862566+0800 GCDDemo[4404:2489050] 任务提交完毕
2021-08-23 10:46:43.862663+0800 GCDDemo[4404:2489062] 1：<NSThread: 0x2838e4ac0>{number = 6, name = (null)}
2021-08-23 10:46:43.862694+0800 GCDDemo[4404:2489064] 2：<NSThread: 0x2838e1100>{number = 7, name = (null)}
2021-08-23 10:46:43.862760+0800 GCDDemo[4404:2489050] 等待任务全部完成后再往下执行
2021-08-23 10:46:43.872680+0800 GCDDemo[4404:2489050] end：<NSThread: 0x2838a08c0>{number = 1, name = main}

总结

底层基于信号量实现，用于实现有依赖关系的并发操作
dispatch_group是一个初始值为LONG_MAX的信号量，根据信号量值是否恢复成初始值判断执行dispatch_group_notify，dispatch_group_enter执行信号量-1，dispatch_group_leave执行信号量+1, dispatch_group_wait会阻塞当前线程直到一组任务执行完毕或者超时