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？（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）

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。

dispatch_queue_tqueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0);
dispatch_group_tgroup = dispatch_group_create();
dispatch_group_async(group, queue, ^{
  /*加载图片1*/});
dispatch_group_async(group, queue, ^{
 /*加载图片2*/});
dispatch_group_async(group, queue, ^{
 /*加载图片3*/});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
  /*合并图片*/});