如果想在dispatch_queue中所有的任务执行完成后在做某种操作，在串行队列中，可以把该操作放到最后一个任务执行完成后继续，但是在并行队列中怎么做呢。这就有dispatch_group 成组操作。
dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next", DISPATCH_QUEUE_CONCURRENT);
dispatch_group_t dispatchGroup=dispatch_group_create();
dispatch_group_async(dispatchGroup, dispatchQueue,^(){
NSLog(@"dispatch-1");
});
dispatch_group_async(dispatchGroup, dispatchQueue,^(){
NSLog(@"dspatch-2");
});
dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(),^(){
NSLog(@"end");
});
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