Dispatch Group

在追加到Dispatch Queue中的多个处理全部结束后想执行结束处理

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_async(group, queue, ^{
        NSLog(@"0");
    });
    dispatch_group_async(group, queue, ^{
        NSLog(@"1");
    });
    dispatch_group_async(group, queue, ^{
        NSLog(@"2");
    });
    
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{
        NSLog(@"done");
    });