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*/});
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