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ios中得多线程技术主要使用3种：NSThread、NSOperation和GCD,这里主要讲GCD
GCD:（Grand Central Dispatch）是一种多核编码技术，用纯C语言编写。
异步：具备开启线程的功能
同步：不具备开启线程的功能
并行队列：多个任务可以同时执行
串行队列：执行完一个任务后再执行下一个任务
下面来进行验证：
1 // 自定义异步方法 2 - (void)async:(dispatch_queue_t)queue 3 { 4 // 异步执行 5 dispatch_async(queue, ^{ 6 7 NSLog(@"im1-%@",[NSThread currentThread]); 8 }); 9 dispatch_async(queue, ^{ 10 11 NSLog(@"im2-%@",[NSThread currentThread]); 12 }); 13 dispatch_async(queue, ^{ 14 15 NSLog(@"im3-%@",[NSThread currentThread]); 16 }); 17 dispatch_async(queue, ^{ 18 19 NSLog(@"im4-%@",[NSThread currentThread]); 20 }); 21 22 }
1 // 自定义同步方法 2 - (void)sync:(dispatch_queue_t)queue 3 { 4 // 同步执行 5 dispatch_sync(queue, ^{ 6 7 NSLog(@"im1-%@",[NSThread currentThread]); 8 }); 9 dispatch_sync(queue, ^{ 10 11 NSLog(@"im2-%@",[NSThread currentThread]); 12 }); 13 dispatch_sync(queue, ^{ 14 15 NSLog(@"im3-%@",[NSThread currentThread]); 16 }); 17 dispatch_sync(queue, ^{ 18 19 NSLog(@"im4-%@",[NSThread currentThread]); 20 }); 21 22 }
调用异步执行方法：
1 // 创建一个全局并行队列 2 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); 3 4 // 异步执行 5 [self async:queue];
最终的结果为：开启了四个线程,并且执行顺序是不定
1 2015-04-02 16:29:20.438 GCD[4398:1f07] im3-<NSThread: 0x7177ee0>{name = (null), num = 5} 2 2015-04-02 16:29:20.432 GCD[4398:1303] im1-<NSThread: 0x7177d70>{name = (null), num = 3} 3 2015-04-02 16:29:20.440 GCD[4398:4307] im4-<NSThread: 0x7178490>{name = (null), num = 6} 4 2015-04-02 16:29:20.432 GCD[4398:1a03] im2-<NSThread: 0x75429f0>{name = (null), num = 4}
1 // 创建一个全局串行队列 2 dispatch_queue_t queue = dispatch_queue_create("wys", NULL); 3 4 // 异步执行 5 [self async:queue];
最终的结果为：开启了一个线程,执行顺序为从上往下依次执行
1 2015-04-02 16:32:19.415 GCD[4442:1303] im1-<NSThread: 0x71631a0>{name = (null), num = 3} 2 2015-04-02 16:32:19.428 GCD[4442:1303] im2-<NSThread: 0x71631a0>{name = (null), num = 3} 3 2015-04-02 16:32:19.437 GCD[4442:1303] im3-<NSThread: 0x71631a0>{name = (null), num = 3} 4 2015-04-02 16:32:19.450 GCD[4442:1303] im4-<NSThread: 0x71631a0>{name = (null), num = 3}
调用主队列执行方法：
1 // 创建主队列 2 dispatch_queue_t queue = dispatch_get_main_queue(); 3 4 // 异步执行 5 [self async:queue];
最红的结果为：顺序执行并且不开启线程，在主线程中执行
1 2015-04-02 16:35:45.320 GCD[4484:c07] im1-<NSThread: 0x71560c0>{name = (null), num = 1} 2 2015-04-02 16:35:45.333 GCD[4484:c07] im2-<NSThread: 0x71560c0>{name = (null), num = 1} 3 2015-04-02 16:35:45.339 GCD[4484:c07] im3-<NSThread: 0x71560c0>{name = (null), num = 1} 4 2015-04-02 16:35:45.347 GCD[4484:c07] im4-<NSThread: 0x71560c0>{name = (null), num = 1}
1 // 创建主队列 2 dispatch_queue_t queue = dispatch_get_main_queue(); 3 4 NSLog(@"start"); 5 6 // 同步执行 7 [self sync:queue]; 8 9 NSLog(@"end");
最终的结果为：执行到start就卡住了，不能往下执行
1 2015-04-02 16:38:12.856 GCD[4514:c07] start
调用同步执行方法：
1 // 创建全局并行队列 2 dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0); 3 4 // 同步执行 5 [self sync:queue];
最终的结果为：不开启线程，并且顺序执行，直接主线程执行
1 2015-04-02 16:41:27.448 GCD[4555:c07] im1-<NSThread: 0x71133c0>{name = (null), num = 1} 2 2015-04-02 16:41:27.458 GCD[4555:c07] im2-<NSThread: 0x71133c0>{name = (null), num = 1} 3 2015-04-02 16:41:27.468 GCD[4555:c07] im3-<NSThread: 0x71133c0>{name = (null), num = 1} 4 2015-04-02 16:41:27.472 GCD[4555:c07] im4-<NSThread: 0x71133c0>{name = (null), num = 1}
1 // 创建串行队列 2 dispatch_queue_t queue = dispatch_queue_create("wys", NULL); 3 4 // 同步执行 5 [self sync:queue];
最终的结果为：不开启线程，并且顺序执行，直接主线程执行
1 2015-04-02 16:43:40.609 GCD[4589:c07] im1-<NSThread: 0x713e570>{name = (null), num = 1} 2 2015-04-02 16:43:40.621 GCD[4589:c07] im2-<NSThread: 0x713e570>{name = (null), num = 1} 3 2015-04-02 16:43:40.626 GCD[4589:c07] im3-<NSThread: 0x713e570>{name = (null), num = 1} 4 2015-04-02 16:43:40.634 GCD[4589:c07] im4-<NSThread: 0x713e570>{name = (null), num = 1}
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原文地址：http://www.cnblogs.com/GeekStar/p/4387440.html
