首先说明一下任务队列和线程是两码事

下面的例子均为在控制器的viewDidLoad函数中执行

同步

dispatch_async(dispatch_get_main_queue(), ^{xxxx
});

貌似很多人都觉得是在主线程中异步执行xxxx那样你就too yang了

那句代码是指在异步加入要执行的xxxx到主任务队列中但是什么时候执行派发机制说了算(你说了不算我说了也不算)

那有人要问了我能不能这样写

dispatch_sync(dispatch_get_main_queue(), ^{xxxx
});

恭喜你中招了 这样写会死锁这这里 让我们解释一下这个为毛会死锁 我们都知道主队列是串行队列 串行队列的特点是一个任务执行完毕在分发下一个任务去执行 但是主队列里面的任务是无穷无尽的(根runloop有关下面说)主队列的任务某一个时刻在整执行但是我们有这样写 就是导致正在执行的主队列任务阻塞起来运行我们的同步任务但是主线程不知道啊接着运行已经被阻塞的任务但是它已经被阻塞在那了 但主线头很贴被阻塞的任务不完成我就不处理下一个任务 一个劲的执行被阻塞的任务 CPU空转长时间间下去硬件就GG了这是硬件工程师不愿看到的 碰到这种情况 由硬件发生中断 被系统捕获上报异常 也就会看到

Thread 1: EXC_BAD_INSTRUCTION (code=EXC_I386_INVOP, subcode=0x0)

下面说一下主队列任务为毛无穷无尽 说起来原因就是为了让app一直运行下去的一种机制 iOS中叫Runloop就可以理解为一直跑 死循环的意思很形象也很贴切 否则的程序代码执行完毕就return了这种情况下程序就推出了还记得

int main(int argc, char * argv[]) {@autoreleasepool {return UIApplicationMain(argc, argv, nil, NSStringFromClass([AppDelegate class]));}
}

还记得main.mm中的函数吗内部就是让主线程一直跑下去 (重点注意)线程跑下去的充要条件是有任务执行 其他的都没用 包括什么强引用都没卵用 所以为了主线程保活UIApplicationMain函数内部开启了一个Runloop Cocoa层接口NSRunloop CoreFoundation层接口CFRunLoopRef

所有说主任务队列中有无穷无尽的任务 这也就是不能载主队列中进行同步操作的根本原因

看到这里有人就可能会想 串行任务对列 不敢用同步操作了 小伙子不要怕 只要理清楚你的队列中是否有任务一直执行 如果有是万不能的 没有大胆放心的用吧

dispatch_queue_t queue = dispatch_queue_create("com.qmtv.queue", DISPATCH_QUEUE_SERIAL);
dispatch_sync(queue, ^{for (NSUInteger i = 0; i < 5; i ++) {NSLog(@"A---%@---%lu", [NSThread currentThread], (unsigned long)i);}
});dispatch_sync(queue, ^{for (NSUInteger i = 0; i < 5; i ++) {NSLog(@"B---%@---%lu", [NSThread currentThread], (unsigned long)i);}
});

让我们运行一下

2018-08-24 17:46:34.312175+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---0
2018-08-24 17:46:34.312406+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---1
2018-08-24 17:46:34.312597+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---2
2018-08-24 17:46:34.312861+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---3
2018-08-24 17:46:34.313324+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---4
2018-08-24 17:46:34.313876+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---5
2018-08-24 17:46:34.314048+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---6
2018-08-24 17:46:34.314203+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---7
2018-08-24 17:46:34.314456+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---8
2018-08-24 17:46:34.314857+0800 GCD[97071:926186] A---<NSThread: 0x60000007cc40>{number = 1, name = main}---9
2018-08-24 17:46:34.315200+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---0
2018-08-24 17:46:34.315880+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---1
2018-08-24 17:46:34.316495+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---2
2018-08-24 17:46:34.317007+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---3
2018-08-24 17:46:34.317237+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---4
2018-08-24 17:46:34.317522+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---5
2018-08-24 17:46:34.317816+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---6
2018-08-24 17:46:34.318219+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---7
2018-08-24 17:46:34.318464+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---8
2018-08-24 17:46:34.318931+0800 GCD[97071:926186] B---<NSThread: 0x60000007cc40>{number = 1, name = main}---9

没有任何错误

异步

我知道就算我说这么多 还是有人害怕 还是会有人用 异步的 好吧 让我们再来说说异步

异步也是对任务队列而言的一样的开场白

dispatch_async(dispatch_get_main_queue(), ^{xxxx
});

任务会被在某条线程中执行完毕基本没什么好说的 但是需要注意的是我们要执行的任务到底会在那条线程上呢？这就要先说一个结论

异步不能推导出开辟了线程 开辟了线程一定能推导出异步 异步是开辟线程的必要条件

同步操作是不会开辟新的线程的

因此不能得出任务会在主线程中执行 再来一个列子

for (NSUInteger i = 0; i < 50; i ++) {dispatch_queue_t queue = dispatch_queue_create([NSString stringWithFormat:@"%lu", (unsigned long)i].UTF8String, DISPATCH_QUEUE_SERIAL);dispatch_async(queue, ^{NSLog(@"%@", [NSThread currentThread]);});
}

打印结果

2018-08-25 08:52:08.508073+0800 GrandCentralDispatch[62139:11970403] <NSThread: 0x60000046fdc0>{number = 3, name = (null)}
2018-08-25 08:52:08.508080+0800 GrandCentralDispatch[62139:11970404] <NSThread: 0x604000275380>{number = 6, name = (null)}
2018-08-25 08:52:08.508102+0800 GrandCentralDispatch[62139:11970405] <NSThread: 0x600000470b40>{number = 5, name = (null)}
2018-08-25 08:52:08.508145+0800 GrandCentralDispatch[62139:11970410] <NSThread: 0x600000470bc0>{number = 7, name = (null)}
2018-08-25 08:52:08.508158+0800 GrandCentralDispatch[62139:11970406] <NSThread: 0x604000274080>{number = 4, name = (null)}
2018-08-25 08:52:08.508207+0800 GrandCentralDispatch[62139:11970413] <NSThread: 0x600000470d00>{number = 9, name = (null)}
2018-08-25 08:52:08.508231+0800 GrandCentralDispatch[62139:11970412] <NSThread: 0x600000470d40>{number = 10, name = (null)}
2018-08-25 08:52:08.508242+0800 GrandCentralDispatch[62139:11970411] <NSThread: 0x600000470c80>{number = 8, name = (null)}
2018-08-25 08:52:08.508244+0800 GrandCentralDispatch[62139:11970414] <NSThread: 0x604000275100>{number = 11, name = (null)}
2018-08-25 08:52:08.508305+0800 GrandCentralDispatch[62139:11970415] <NSThread: 0x600000470dc0>{number = 12, name = (null)}
2018-08-25 08:52:08.508349+0800 GrandCentralDispatch[62139:11970416] <NSThread: 0x600000470e40>{number = 13, name = (null)}
2018-08-25 08:52:08.508432+0800 GrandCentralDispatch[62139:11970417] <NSThread: 0x6040002746c0>{number = 14, name = (null)}
2018-08-25 08:52:08.508516+0800 GrandCentralDispatch[62139:11970418] <NSThread: 0x604000275bc0>{number = 15, name = (null)}
2018-08-25 08:52:08.508544+0800 GrandCentralDispatch[62139:11970419] <NSThread: 0x600000471180>{number = 16, name = (null)}
2018-08-25 08:52:08.508595+0800 GrandCentralDispatch[62139:11970403] <NSThread: 0x60000046fdc0>{number = 3, name = (null)}
2018-08-25 08:52:08.508615+0800 GrandCentralDispatch[62139:11970420] <NSThread: 0x604000275d80>{number = 17, name = (null)}
2018-08-25 08:52:08.508623+0800 GrandCentralDispatch[62139:11970421] <NSThread: 0x604000276040>{number = 18, name = (null)}
2018-08-25 08:52:08.508698+0800 GrandCentralDispatch[62139:11970422] <NSThread: 0x600000470b00>{number = 19, name = (null)}
2018-08-25 08:52:08.508734+0800 GrandCentralDispatch[62139:11970423] <NSThread: 0x6040002744c0>{number = 20, name = (null)}
2018-08-25 08:52:08.508775+0800 GrandCentralDispatch[62139:11970404] <NSThread: 0x604000275380>{number = 6, name = (null)}
2018-08-25 08:52:08.508789+0800 GrandCentralDispatch[62139:11970424] <NSThread: 0x6040002743c0>{number = 21, name = (null)}
2018-08-25 08:52:08.508822+0800 GrandCentralDispatch[62139:11970425] <NSThread: 0x604000276180>{number = 22, name = (null)}
2018-08-25 08:52:08.508833+0800 GrandCentralDispatch[62139:11970426] <NSThread: 0x600000471280>{number = 23, name = (null)}
2018-08-25 08:52:08.508885+0800 GrandCentralDispatch[62139:11970427] <NSThread: 0x604000276200>{number = 24, name = (null)}
2018-08-25 08:52:08.508902+0800 GrandCentralDispatch[62139:11970428] <NSThread: 0x604000276280>{number = 25, name = (null)}
2018-08-25 08:52:08.508987+0800 GrandCentralDispatch[62139:11970430] <NSThread: 0x6000004711c0>{number = 26, name = (null)}
2018-08-25 08:52:08.509040+0800 GrandCentralDispatch[62139:11970405] <NSThread: 0x600000470b40>{number = 5, name = (null)}
2018-08-25 08:52:08.509113+0800 GrandCentralDispatch[62139:11970431] <NSThread: 0x604000276480>{number = 27, name = (null)}
2018-08-25 08:52:08.509216+0800 GrandCentralDispatch[62139:11970429] <NSThread: 0x604000276600>{number = 28, name = (null)}
2018-08-25 08:52:08.509283+0800 GrandCentralDispatch[62139:11970434] <NSThread: 0x600000471140>{number = 30, name = (null)}
2018-08-25 08:52:08.509286+0800 GrandCentralDispatch[62139:11970433] <NSThread: 0x604000276680>{number = 29, name = (null)}
2018-08-25 08:52:08.509409+0800 GrandCentralDispatch[62139:11970436] <NSThread: 0x604000276700>{number = 32, name = (null)}
2018-08-25 08:52:08.509431+0800 GrandCentralDispatch[62139:11970435] <NSThread: 0x600000471080>{number = 31, name = (null)}
2018-08-25 08:52:08.509522+0800 GrandCentralDispatch[62139:11970438] <NSThread: 0x6040002767c0>{number = 33, name = (null)}
2018-08-25 08:52:08.509584+0800 GrandCentralDispatch[62139:11970440] <NSThread: 0x6040002763c0>{number = 34, name = (null)}
2018-08-25 08:52:08.509697+0800 GrandCentralDispatch[62139:11970439] <NSThread: 0x604000276300>{number = 35, name = (null)}
2018-08-25 08:52:08.509826+0800 GrandCentralDispatch[62139:11970441] <NSThread: 0x6000004714c0>{number = 36, name = (null)}
2018-08-25 08:52:08.509933+0800 GrandCentralDispatch[62139:11970442] <NSThread: 0x6040002769c0>{number = 37, name = (null)}
2018-08-25 08:52:08.509959+0800 GrandCentralDispatch[62139:11970437] <NSThread: 0x604000276a40>{number = 38, name = (null)}
2018-08-25 08:52:08.509989+0800 GrandCentralDispatch[62139:11970443] <NSThread: 0x604000276a80>{number = 39, name = (null)}
2018-08-25 08:52:08.510077+0800 GrandCentralDispatch[62139:11970444] <NSThread: 0x604000276b00>{number = 40, name = (null)}
2018-08-25 08:52:08.510077+0800 GrandCentralDispatch[62139:11970445] <NSThread: 0x604000276b80>{number = 41, name = (null)}
2018-08-25 08:52:08.510156+0800 GrandCentralDispatch[62139:11970447] <NSThread: 0x604000276c80>{number = 43, name = (null)}
2018-08-25 08:52:08.510164+0800 GrandCentralDispatch[62139:11970446] <NSThread: 0x604000276c00>{number = 42, name = (null)}
2018-08-25 08:52:08.510214+0800 GrandCentralDispatch[62139:11970448] <NSThread: 0x604000276d00>{number = 44, name = (null)}
2018-08-25 08:52:08.510247+0800 GrandCentralDispatch[62139:11970449] <NSThread: 0x600000471800>{number = 45, name = (null)}
2018-08-25 08:52:08.510272+0800 GrandCentralDispatch[62139:11970432] <NSThread: 0x604000276d40>{number = 46, name = (null)}
2018-08-25 08:52:08.510285+0800 GrandCentralDispatch[62139:11970450] <NSThread: 0x604000276dc0>{number = 47, name = (null)}
2018-08-25 08:52:08.510330+0800 GrandCentralDispatch[62139:11970451] <NSThread: 0x6000004719c0>{number = 48, name = (null)}
2018-08-25 08:52:08.510335+0800 GrandCentralDispatch[62139:11970452] <NSThread: 0x604000276e80>{number = 49, name = (null)}

可以看出number=3的那条线程出现两次 说明某些任务使用了之前的线程 或者说异步任务有时并不会开辟新的线程

for (NSUInteger i = 0; i < 50; i ++) {dispatch_queue_t queue = dispatch_queue_create([NSString stringWithFormat:@"%lu", (unsigned long)i].UTF8String, DISPATCH_QUEUE_SERIAL);dispatch_sync(queue, ^{NSLog(@"%@", [NSThread currentThread]);});
}

打印结果

2018-08-25 08:57:25.714230+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.714470+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.714631+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.714909+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715040+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715154+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715275+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715391+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715557+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715695+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715837+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.715979+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.716165+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.716570+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.716869+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.717137+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.717343+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.717542+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.717763+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.717936+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718231+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718386+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718531+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718682+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718834+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.718976+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.719124+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.719301+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.720072+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.720243+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.720388+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.720719+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.720861+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.721203+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.721374+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.721530+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722184+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722353+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722501+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722635+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722778+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.722915+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.723262+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.723438+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.723730+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.723984+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.724225+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.724553+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.724722+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}
2018-08-25 08:57:25.724959+0800 GrandCentralDispatch[62236:11978540] <NSThread: 0x600000262780>{number = 1, name = main}

清一色的在主线程中执行

上面两个例子可以总结出 串行任务队列的任务执行是否开辟新的线程 与同步异步有关 同步不会开辟 异步可能会开辟

并行队列呢 接着看

for (NSUInteger i = 0; i < 50; i ++) {dispatch_queue_t queue = dispatch_queue_create([NSString stringWithFormat:@"%lu", (unsigned long)i].UTF8String, DISPATCH_QUEUE_CONCURRENT);dispatch_async(queue, ^{NSLog(@"%@", [NSThread currentThread]);});
}

2018-08-25 09:01:59.951218+0800 GrandCentralDispatch[62323:11985644] <NSThread: 0x60000026e940>{number = 3, name = (null)}
2018-08-25 09:01:59.951223+0800 GrandCentralDispatch[62323:11985642] <NSThread: 0x60000026f6c0>{number = 4, name = (null)}
2018-08-25 09:01:59.951230+0800 GrandCentralDispatch[62323:11985645] <NSThread: 0x60400027c1c0>{number = 5, name = (null)}
2018-08-25 09:01:59.951250+0800 GrandCentralDispatch[62323:11985639] <NSThread: 0x60400027b340>{number = 6, name = (null)}
2018-08-25 09:01:59.951340+0800 GrandCentralDispatch[62323:11985656] <NSThread: 0x60000026f7c0>{number = 7, name = (null)}
2018-08-25 09:01:59.951360+0800 GrandCentralDispatch[62323:11985657] <NSThread: 0x60000026f700>{number = 8, name = (null)}
2018-08-25 09:01:59.951439+0800 GrandCentralDispatch[62323:11985644] <NSThread: 0x60000026e940>{number = 3, name = (null)}
2018-08-25 09:01:59.951471+0800 GrandCentralDispatch[62323:11985642] <NSThread: 0x60000026f6c0>{number = 4, name = (null)}
2018-08-25 09:01:59.951557+0800 GrandCentralDispatch[62323:11985639] <NSThread: 0x60400027b340>{number = 6, name = (null)}
2018-08-25 09:01:59.951555+0800 GrandCentralDispatch[62323:11985645] <NSThread: 0x60400027c1c0>{number = 5, name = (null)}
2018-08-25 09:01:59.951588+0800 GrandCentralDispatch[62323:11985656] <NSThread: 0x60000026f7c0>{number = 7, name = (null)}
2018-08-25 09:01:59.951661+0800 GrandCentralDispatch[62323:11985657] <NSThread: 0x60000026f700>{number = 8, name = (null)}
2018-08-25 09:01:59.951685+0800 GrandCentralDispatch[62323:11985644] <NSThread: 0x60000026e940>{number = 3, name = (null)}
2018-08-25 09:01:59.951786+0800 GrandCentralDispatch[62323:11985658] <NSThread: 0x60000026fa80>{number = 9, name = (null)}
2018-08-25 09:01:59.951823+0800 GrandCentralDispatch[62323:11985659] <NSThread: 0x60000026f680>{number = 10, name = (null)}
2018-08-25 09:01:59.951858+0800 GrandCentralDispatch[62323:11985660] <NSThread: 0x60400027c300>{number = 11, name = (null)}
2018-08-25 09:01:59.951885+0800 GrandCentralDispatch[62323:11985661] <NSThread: 0x60000026f9c0>{number = 12, name = (null)}
2018-08-25 09:01:59.951917+0800 GrandCentralDispatch[62323:11985662] <NSThread: 0x60000026fac0>{number = 13, name = (null)}
2018-08-25 09:01:59.952001+0800 GrandCentralDispatch[62323:11985642] <NSThread: 0x60000026f6c0>{number = 4, name = (null)}
2018-08-25 09:01:59.952014+0800 GrandCentralDispatch[62323:11985665] <NSThread: 0x60000026fc00>{number = 16, name = (null)}
2018-08-25 09:01:59.952032+0800 GrandCentralDispatch[62323:11985663] <NSThread: 0x60400027b480>{number = 14, name = (null)}
2018-08-25 09:01:59.952039+0800 GrandCentralDispatch[62323:11985664] <NSThread: 0x60000026f940>{number = 15, name = (null)}
2018-08-25 09:01:59.952157+0800 GrandCentralDispatch[62323:11985666] <NSThread: 0x60000026fc80>{number = 17, name = (null)}
2018-08-25 09:01:59.952392+0800 GrandCentralDispatch[62323:11985668] <NSThread: 0x60400027d240>{number = 18, name = (null)}
2018-08-25 09:01:59.952423+0800 GrandCentralDispatch[62323:11985667] <NSThread: 0x60000026fdc0>{number = 19, name = (null)}
2018-08-25 09:01:59.952517+0800 GrandCentralDispatch[62323:11985669] <NSThread: 0x60400027d340>{number = 20, name = (null)}
2018-08-25 09:01:59.952576+0800 GrandCentralDispatch[62323:11985670] <NSThread: 0x60000026fa40>{number = 21, name = (null)}
2018-08-25 09:01:59.952749+0800 GrandCentralDispatch[62323:11985672] <NSThread: 0x60000026f840>{number = 22, name = (null)}
2018-08-25 09:01:59.953056+0800 GrandCentralDispatch[62323:11985671] <NSThread: 0x60400027d580>{number = 25, name = (null)}
2018-08-25 09:01:59.952878+0800 GrandCentralDispatch[62323:11985673] <NSThread: 0x600000270100>{number = 23, name = (null)}
2018-08-25 09:01:59.953070+0800 GrandCentralDispatch[62323:11985674] <NSThread: 0x60000026fd00>{number = 24, name = (null)}
2018-08-25 09:01:59.953115+0800 GrandCentralDispatch[62323:11985675] <NSThread: 0x60400027d400>{number = 26, name = (null)}
2018-08-25 09:01:59.953169+0800 GrandCentralDispatch[62323:11985677] <NSThread: 0x600000270380>{number = 27, name = (null)}
2018-08-25 09:01:59.953171+0800 GrandCentralDispatch[62323:11985676] <NSThread: 0x60400027d3c0>{number = 28, name = (null)}
2018-08-25 09:01:59.953462+0800 GrandCentralDispatch[62323:11985678] <NSThread: 0x6000002705c0>{number = 29, name = (null)}
2018-08-25 09:01:59.953472+0800 GrandCentralDispatch[62323:119852018-08-25 09:01:59.954227+0800 GrandCentralDispatch[62323:11985681] <NSThread: 0x60400027fc00>{number = 32, name = (null)}
639] <NSThread: 0x60400027b340>{number = 6, name = (null)}
2018-08-25 09:01:59.953897+0800 GrandCentralDispatch[62323:11985679] <NSThread: 0x600000272940>{number = 30, name = (null)}
2018-08-25 09:01:59.954212+0800 GrandCentralDispatch[62323:11985680] <NSThread: 0x600000272d00>{number = 31, name = (null)}
2018-08-25 09:01:59.954249+0800 GrandCentralDispatch[62323:11985682] <NSThread: 0x60400027fc80>{number = 33, name = (null)}
2018-08-25 09:01:59.954562+0800 GrandCentralDispatch[62323:11985683] <NSThread: 0x604000460880>{number = 34, name = (null)}
2018-08-25 09:01:59.954591+0800 GrandCentralDispatch[62323:11985684] <NSThread: 0x600000273580>{number = 35, name = (null)}
2018-08-25 09:01:59.954636+0800 GrandCentralDispatch[62323:11985685] <NSThread: 0x604000460900>{number = 36, name = (null)}
2018-08-25 09:01:59.954656+0800 GrandCentralDispatch[62323:11985686] <NSThread: 0x604000460980>{number = 37, name = (null)}
2018-08-25 09:01:59.954698+0800 GrandCentralDispatch[62323:11985687] <NSThread: 0x600000273600>{number = 38, name = (null)}
2018-08-25 09:01:59.954817+0800 GrandCentralDispatch[62323:11985688] <NSThread: 0x604000460a00>{number = 39, name = (null)}
2018-08-25 09:01:59.954820+0800 GrandCentralDispatch[62323:11985689] <NSThread: 0x600000273640>{number = 40, name = (null)}
2018-08-25 09:01:59.954834+0800 GrandCentralDispatch[62323:11985690] <NSThread: 0x604000460ac0>{number = 41, name = (null)}
2018-08-25 09:01:59.954881+0800 GrandCentralDispatch[62323:11985645] <NSThread: 0x60400027c1c0>{number = 5, name = (null)}
2018-08-25 09:01:59.955141+0800 GrandCentralDispatch[62323:11985691] <NSThread: 0x604000461d80>{number = 42, name = (null)}

也可以看到与串行线程的异步任务差不多 都有使用number=3的线程出现两次 但是不清楚为毛都是3为啥不是其他值呢待研究

for (NSUInteger i = 0; i < 50; i ++) {dispatch_queue_t queue = dispatch_queue_create([NSString stringWithFormat:@"%lu", (unsigned long)i].UTF8String, DISPATCH_QUEUE_CONCURRENT);dispatch_sync(queue, ^{NSLog(@"%@", [NSThread currentThread]);});
}

2018-08-25 09:05:59.637262+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.637501+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.637643+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.637785+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.637898+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.638081+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.638208+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.638354+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.638635+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.638936+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.639143+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.639271+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.639638+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.640119+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.640323+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.640739+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.641097+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.641347+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.641742+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.642001+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.642204+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.642547+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.642893+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.643178+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.643405+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.643632+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.643818+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.644153+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.644449+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.644685+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.644917+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.645238+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.645522+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.645710+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.646441+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.646640+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.646849+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.647478+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.647907+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.648226+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.648540+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.648929+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.649108+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.649293+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.649800+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.650010+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.650569+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.650945+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.651388+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}
2018-08-25 09:05:59.651552+0800 GrandCentralDispatch[62392:11991830] <NSThread: 0x604000078b40>{number = 1, name = main}

并行队列的同步任务也是清一色的在主线程中执行

到现在我们可以的出 那句话是否开新的线程与队列类型无关 与队列任务的同步异步有关即 同步不会开新线程 异步可能开新线程

由细心的小伙伴应该可以发现当我的串行队列和并行队列都在主线程中生产他们的任务执行也在主线程中 要是在子线程中呢没错下一个例子来了

[[[NSThread alloc] initWithBlock:^{NSLog(@"%@", [NSThread currentThread]);dispatch_queue_t queue = dispatch_queue_create("com.qmtv", DISPATCH_QUEUE_SERIAL);for (NSUInteger i = 0; i < 50; i ++) {dispatch_sync(queue, ^{NSLog(@"%@", [NSThread currentThread]);});}
}] start];

瞅一瞅结果

2018-08-25 09:14:19.619140+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.639529+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.640350+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.641648+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.642835+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.644052+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.644779+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.645137+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.645834+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.646290+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.646693+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.661954+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.662288+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.662439+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.662815+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.663044+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.663289+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.663801+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.665270+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.665645+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.679102+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.679785+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.680697+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.683770+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.684134+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.684386+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.685857+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.686884+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.688009+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.689648+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.691383+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.691674+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.691939+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.692216+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.692452+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.692798+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.693229+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.693498+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.693715+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.694436+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.694696+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.694974+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.695137+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.695383+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.695758+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.696352+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.696617+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.697134+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.697602+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.697797+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}
2018-08-25 09:14:23.698271+0800 GrandCentralDispatch[62528:12004403] <NSThread: 0x604000275b00>{number = 3, name = (null)}

全都在number=3的这条线程上执行

[[[NSThread alloc] initWithBlock:^{NSLog(@"%@", [NSThread currentThread]);dispatch_queue_t queue = dispatch_queue_create("com.qmtv", DISPATCH_QUEUE_SERIAL);for (NSUInteger i = 0; i < 50; i ++) {dispatch_async(queue, ^{NSLog(@"%@", [NSThread currentThread]);});}
}] start];

看下结果

2018-08-25 09:17:40.441840+0800 GrandCentralDispatch[62611:12010351] <NSThread: 0x60400027db00>{number = 3, name = (null)}
2018-08-25 09:17:40.449019+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.449381+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.449513+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.449928+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.450150+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.450296+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.451069+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.451582+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.451808+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.469481+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.469845+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.470146+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.470530+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.471045+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.471546+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.472218+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.473453+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.474232+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.476067+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.478674+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.479913+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.482061+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.483298+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.483722+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.483920+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.484699+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.485266+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.485778+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.486274+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.486898+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.487276+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.487635+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.487854+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.488409+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.490661+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.490931+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.491494+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.491663+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.491994+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.492378+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.492870+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.493076+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.493324+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.493673+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.493880+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.494351+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.494596+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.494973+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.495609+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}
2018-08-25 09:17:40.495880+0800 GrandCentralDispatch[62611:12010330] <NSThread: 0x60400027db00>{number = 4, name = (null)}

可以发现串行队列任务的同步任务在一条线程上执行 异步任务也在同一条线程上执行 还会有就是 同步任务所在线程与任务队列所处线程相同

看看并行队列如何

[[[NSThread alloc] initWithBlock:^{NSLog(@"%@", [NSThread currentThread]);dispatch_queue_t queue = dispatch_queue_create("com.qmtv", DISPATCH_QUEUE_CONCURRENT);for (NSUInteger i = 0; i < 50; i ++) {dispatch_sync(queue, ^{NSLog(@"%@", [NSThread currentThread]);});}
}] start];

打印顺序

2018-08-25 09:22:23.903902+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.904351+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.904602+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.906247+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.907025+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.907405+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.907626+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.907879+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.908303+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.908592+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.930231+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.931307+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.931508+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.933881+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.934998+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.938615+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.940645+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.941507+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.942235+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.942913+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.943488+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.944171+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.944346+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.944973+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.945535+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.946036+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.948377+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.948505+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.948742+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.948930+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.949222+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.949469+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.949933+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.950346+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.950568+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.950886+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.951082+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.951254+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.951613+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.951976+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.952467+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.953024+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.953639+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.953973+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.954561+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.955312+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.958078+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.958439+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.958627+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.958984+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}
2018-08-25 09:22:23.959267+0800 GrandCentralDispatch[62698:12017763] <NSThread: 0x604000463ac0>{number = 3, name = (null)}

结果与串行线程的同步任务是一样的

[[[NSThread alloc] initWithBlock:^{NSLog(@"%@", [NSThread currentThread]);dispatch_queue_t queue = dispatch_queue_create("com.qmtv", DISPATCH_QUEUE_CONCURRENT);for (NSUInteger i = 0; i < 50; i ++) {dispatch_async(queue, ^{NSLog(@"%@", [NSThread currentThread]);});}
}] start];

2018-08-25 09:24:47.769120+0800 GrandCentralDispatch[62753:12021956] <NSThread: 0x60000046f680>{number = 3, name = (null)}
2018-08-25 09:24:47.770748+0800 GrandCentralDispatch[62753:12021945] <NSThread: 0x60000046f680>{number = 4, name = (null)}
2018-08-25 09:24:47.770850+0800 GrandCentralDispatch[62753:12021951] <NSThread: 0x600000470b80>{number = 5, name = (null)}
2018-08-25 09:24:47.770899+0800 GrandCentralDispatch[62753:12021947] <NSThread: 0x600000470c40>{number = 6, name = (null)}
2018-08-25 09:24:47.770954+0800 GrandCentralDispatch[62753:12021949] <NSThread: 0x600000470c80>{number = 7, name = (null)}
2018-08-25 09:24:47.771064+0800 GrandCentralDispatch[62753:12022071] <NSThread: 0x600000470d00>{number = 8, name = (null)}
2018-08-25 09:24:47.771724+0800 GrandCentralDispatch[62753:12021945] <NSThread: 0x60000046f680>{number = 4, name = (null)}
2018-08-25 09:24:47.772724+0800 GrandCentralDispatch[62753:12022072] <NSThread: 0x604000468b00>{number = 9, name = (null)}
2018-08-25 09:24:47.773219+0800 GrandCentralDispatch[62753:12021951] <NSThread: 0x600000470b80>{number = 5, name = (null)}
2018-08-25 09:24:47.773756+0800 GrandCentralDispatch[62753:12021947] <NSThread: 0x600000470c40>{number = 6, name = (null)}
2018-08-25 09:24:47.775057+0800 GrandCentralDispatch[62753:12021949] <NSThread: 0x600000470c80>{number = 7, name = (null)}
2018-08-25 09:24:47.777410+0800 GrandCentralDispatch[62753:12022071] <NSThread: 0x600000470d00>{number = 8, name = (null)}
2018-08-25 09:24:47.780470+0800 GrandCentralDispatch[62753:12021945] <NSThread: 0x60000046f680>{number = 4, name = (null)}
2018-08-25 09:24:47.780581+0800 GrandCentralDispatch[62753:12022073] <NSThread: 0x6000004644c0>{number = 10, name = (null)}
2018-08-25 09:24:47.780949+0800 GrandCentralDispatch[62753:12022072] <NSThread: 0x604000468b00>{number = 9, name = (null)}
2018-08-25 09:24:47.781129+0800 GrandCentralDispatch[62753:12021951] <NSThread: 0x600000470b80>{number = 5, name = (null)}
2018-08-25 09:24:47.781332+0800 GrandCentralDispatch[62753:12021947] <NSThread: 0x600000470c40>{number = 6, name = (null)}
2018-08-25 09:24:47.781974+0800 GrandCentralDispatch[62753:12022074] <NSThread: 0x6000004642c0>{number = 11, name = (null)}
2018-08-25 09:24:47.782391+0800 GrandCentralDispatch[62753:12022075] <NSThread: 0x604000471bc0>{number = 12, name = (null)}
2018-08-25 09:24:47.782562+0800 GrandCentralDispatch[62753:12022076] <NSThread: 0x600000470c00>{number = 13, name = (null)}
2018-08-25 09:24:47.782618+0800 GrandCentralDispatch[62753:12022079] <NSThread: 0x600000471100>{number = 14, name = (null)}
2018-08-25 09:24:47.782648+0800 GrandCentralDispatch[62753:12022078] <NSThread: 0x600000470dc0>{number = 15, name = (null)}
2018-08-25 09:24:47.782971+0800 GrandCentralDispatch[62753:12022081] <NSThread: 0x600000470f80>{number = 16, name = (null)}
2018-08-25 09:24:47.783029+0800 GrandCentralDispatch[62753:12022082] <NSThread: 0x600000470fc0>{number = 17, name = (null)}
2018-08-25 09:24:47.783250+0800 GrandCentralDispatch[62753:12022084] <NSThread: 0x600000470e40>{number = 19, name = (null)}
2018-08-25 09:24:47.783145+0800 GrandCentralDispatch[62753:12022083] <NSThread: 0x600000471080>{number = 18, name = (null)}
2018-08-25 09:24:47.783324+0800 GrandCentralDispatch[62753:12022085] <NSThread: 0x600000471180>{number = 20, name = (null)}
2018-08-25 09:24:47.783698+0800 GrandCentralDispatch[62753:12022080] <NSThread: 0x600000471400>{number = 23, name = (null)}
2018-08-25 09:24:47.783634+0800 GrandCentralDispatch[62753:12022086] <NSThread: 0x600000471380>{number = 21, name = (null)}
2018-08-25 09:24:47.783715+0800 GrandCentralDispatch[62753:12022087] <NSThread: 0x6000004713c0>{number = 22, name = (null)}
2018-08-25 09:24:47.783697+0800 GrandCentralDispatch[62753:12022088] <NSThread: 0x600000471480>{number = 24, name = (null)}
2018-08-25 09:24:47.799717+0800 GrandCentralDispatch[62753:12021945] <NSThread: 0x60000046f680>{number = 4, name = (null)}
2018-08-25 09:24:47.799818+0800 GrandCentralDispatch[62753:12021947] <NSThread: 0x600000470c40>{number = 6, name = (null)}
2018-08-25 09:24:47.799900+0800 GrandCentralDispatch[62753:12021951] <NSThread: 0x600000470b80>{number = 5, name = (null)}
2018-08-25 09:24:47.799937+0800 GrandCentralDispatch[62753:12021949] <NSThread: 0x600000470c80>{number = 7, name = (null)}
2018-08-25 09:24:47.800006+0800 GrandCentralDispatch[62753:12022072] <NSThread: 0x604000468b00>{number = 9, name = (null)}
2018-08-25 09:24:47.799948+0800 GrandCentralDispatch[62753:12022071] <NSThread: 0x600000470d00>{number = 8, name = (null)}
2018-08-25 09:24:47.800015+0800 GrandCentralDispatch[62753:12022073] <NSThread: 0x6000004644c0>{number = 10, name = (null)}
2018-08-25 09:24:47.800072+0800 GrandCentralDispatch[62753:12022074] <NSThread: 0x6000004642c0>{number = 11, name = (null)}
2018-08-25 09:24:47.800766+0800 GrandCentralDispatch[62753:12022076] <NSThread: 0x600000470c00>{number = 13, name = (null)}
2018-08-25 09:24:47.800308+0800 GrandCentralDispatch[62753:12022075] <NSThread: 0x604000471bc0>{number = 12, name = (null)}
2018-08-25 09:24:47.801315+0800 GrandCentralDispatch[62753:12022079] <NSThread: 0x600000471100>{number = 14, name = (null)}
2018-08-25 09:24:47.801393+0800 GrandCentralDispatch[62753:12022078] <NSThread: 0x600000470dc0>{number = 15, name = (null)}
2018-08-25 09:24:47.801674+0800 GrandCentralDispatch[62753:12022091] <NSThread: 0x600000471b80>{number = 25, name = (null)}
2018-08-25 09:24:47.801770+0800 GrandCentralDispatch[62753:12022090] <NSThread: 0x604000473ac0>{number = 26, name = (null)}
2018-08-25 09:24:47.801938+0800 GrandCentralDispatch[62753:12022092] <NSThread: 0x604000473b40>{number = 27, name = (null)}
2018-08-25 09:24:47.802161+0800 GrandCentralDispatch[62753:12022081] <NSThread: 0x600000470f80>{number = 16, name = (null)}
2018-08-25 09:24:47.802713+0800 GrandCentralDispatch[62753:12022080] <NSThread: 0x600000471400>{number = 23, name = (null)}
2018-08-25 09:24:47.802922+0800 GrandCentralDispatch[62753:12022082] <NSThread: 0x600000470fc0>{number = 17, name = (null)}
2018-08-25 09:24:47.803142+0800 GrandCentralDispatch[62753:12022084] <NSThread: 0x600000470e40>{number = 19, name = (null)}
2018-08-25 09:24:47.803394+0800 GrandCentralDispatch[62753:12022083] <NSThread: 0x600000471080>{number = 18, name = (null)}

并行队列的异步任务则看不错什么规律。。。

从上面的很多例子我们能总结出一下的结论

同步不会开新线程

异步可能开新线程

串行队列的所有同步任务是队列所处的线程上执行

串行队列的所有异步任务是在同一条线程上执行的

并行队列的所有同步任务在队列所处线程上执行

并行队列的所有异步任务是根据系统负载决定在某条线程执行

但是这些结论是根据实验得到了 由于还没看GCD源码 待看过后会更正结论中错误的 如有大佬能斧正 感激不尽

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