iOS开发-如何用GCD同步若干个异步调用？

如何用GCD同步若干个异步调用？（比如根据若干个url异步加载多张图片，然后在都下载完成后合成一张整图展示给用户）

使用Dispatch Group追加block到Global Group Queue,这些block如果全部执行完毕，就会执行Main Dispatch Queue中的结束处理的block。
代码如下

-(void)test{    //获取并发队列    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);    //获取组    dispatch_group_t group = dispatch_group_create();    dispatch_group_async(group, queue, ^{        NSLog(@"线程：%@--加载图片1", [NSThread currentThread]);        });    dispatch_group_async(group, queue, ^{      NSLog(@"线程：%@--加载图片2", [NSThread currentThread]);;    });    dispatch_group_async(group, queue, ^{        NSLog(@"线程：%@--加载图片3", [NSThread currentThread]);    });    dispatch_group_notify(group, dispatch_get_main_queue(), ^{      NSLog(@"线程：%@--合并图片", [NSThread currentThread]);    });}