GCD线程组

GCD

使用场景

模拟多个下载任务,当所有的下载任务完成后,返回下载结束.即将每个下载任务添加到一个线程组(调度组)中,至于是否在同一个队列中没有关系.最后对线程组添加监听,如果全部完成,则返回下载完成.

代码示例

//创建线程组和队列
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
//添加任务到线程组和相应队列
dispatch_group_async(group, queue, ^{
    NSLog(@"1");
});
dispatch_group_async(group, queue, ^{
    sleep(4);
    NSLog(@"2");
});
dispatch_group_async(group, queue, ^{
    NSLog(@"3");
});
//同步
dispatch_group_wait(group, DISPATCH_TIME_FOREVER);//结果是:3,1,2,0

    //异步
//    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
//        NSLog(@"OK");
//    });//结果是1,3,0,2,OK

NSLog(@"0");

另外一种添加到任务组的方法

//enter和leave必须对应.
dispatch_group_enter(group);
dispatch_async(queue, ^{
    NSLog(@"1");
    dispatch_group_leave(group);//必须在
});

dispatch_group_enter(group);
dispatch_async(queue, ^{
    sleep(3);
    NSLog(@"2");
    dispatch_group_leave(group);
});

dispatch_group_enter(group);
dispatch_async(queue, ^{
    NSLog(@"3");
    dispatch_group_leave(group);
});

dispatch_group_notify(group, dispatch_get_main_queue(), ^{
    NSLog(@"OK");
});
NSLog(@"0");//结果是1,0,3,2,OK