dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"1");
dispatch_async(concurrentQueue, ^(){
NSLog(@"2");
[NSThread sleepForTimeInterval:5];
NSLog(@"3");});
NSLog(@"4");
输出为：
2016-08-2511:50:51.601xxxx[1353:102804]12016-08-2511:50:51.601xxxx[1353:102804]22016-08-2511:50:56.603xxxx[1353:102804]32016-08-2511:50:56.603xxxx[1353:102804]4
再看：
dispatch_queue_t concurrentQueue = dispatch_queue_create("my.concurrent.queue", DISPATCH_QUEUE_CONCURRENT);
NSLog(@"1");
dispatch_async(concurrentQueue, ^(){
NSLog(@"2");
[NSThread sleepForTimeInterval:5];
NSLog(@"3");});
NSLog(@"4");
输出为：
2016-08-2511:52:29.022xxxx[1392:104246]12016-08-2511:52:29.023xxxx[1392:104246]42016-08-2511:52:29.023xxxx[1392:104284]22016-08-2511:52:34.029xxxx[1392:104284]3
通过上边的两个例子，我们可以总结出：
dispatch_sync(),同步添加操作。他是等待添加进队列里面的操作完成之后再继续执行。
dispatch_async ,异步添加进任务队列，它不会做任何等待
