GCD 异步分组执行

//分组执行
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
//异步执行
dispatch_async(queue, ^{
    dispatch_group_t group = dispatch_group_create();
    //异步
    dispatch_group_async(group, queue, ^{
        NSLog(@"哇");
        [NSThread sleepForTimeInterval:2];
    });

    dispatch_group_async(group, queue, ^{
        NSLog(@"哈哈");
        [NSThread sleepForTimeInterval:2];
    });
    //当group组里面的东西运行完成后，才会运行
    dispatch_group_notify(group, queue, ^{
        NSLog(@"完成");
    });

});