GCD作为日常开发中的使用已经非常普遍，基于C的API为应用在多核硬件上高效运行提供了有力支持。本文分场景写了几个测试Demo，方便大家理解与应用。
　　1，dispatch_get_global_queue与dispatch_get_main_queue交互
　　很多应用场景需要后台读写大量数据，通过dispatch_get_global_queue函数可以获取全局队列来并发执行后台任务，并再结束后更新UI，保证应用的流程，避免主线程阻塞。代码如下：
- (void)test1
{
dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT， 0);
dispatch_group_t group = dispatch_group_create();
for (int i = 0; i < 10; i++) {
dispatch_group_async(group， queue， ^{
NSLog(@"___%i %@"， i， [NSThread currentThread]);
});
}
dispatch_group_notify(group， queue， ^{
NSLog(@"currentThread1 %@"，[NSThread currentThread]);
NSLog(@"执行完毕");
dispatch_async(dispatch_get_main_queue()， ^{
NSLog(@"currentThread2 %@"，[NSThread currentThread]);
NSLog(@"main_queue 执行完毕");
});
});
}
　　执行结果：
　　test1：
　　2016-03-10 17:21:37.293 GCDSample[3892:2238505] ___0 <NSThread: 0x12e53e570>{number = 2， name = (null)}
　　2016-03-10 17:21:37.293 GCDSample[3892:2238506] ___2 <NSThread: 0x12e67f1e0>{number = 4， name = (null)}
　　2016-03-10 17:21:37.293 GCDSample[3892:2238504] ___3 <NSThread: 0x12e689760>{number = 3， name = (null)}
　　2016-03-10 17:21:37.293 GCDSample[3892:2238507] ___1 <NSThread: 0x12e683590>{number = 5， name = (null)}
　　2016-03-10 17:21:37.294 GCDSample[3892:2238505] ___4 <NSThread: 0x12e53e570>{number = 2， name = (null)}
　　2016-03-10 17:21:37.295 GCDSample[3892:2238506] ___5 <NSThread: 0x12e67f1e0>{number = 4， name = (null)}
　　2016-03-10 17:21:37.295 GCDSample[3892:2238504] ___6 <NSThread: 0x12e689760>{number = 3， name = (null)}
　　2016-03-10 17:21:37.295 GCDSample[3892:2238507] ___7 <NSThread: 0x12e683590>{number = 5， name = (null)}
　　2016-03-10 17:21:37.295 GCDSample[3892:2238505] ___8 <NSThread: 0x12e53e570>{number = 2， name = (null)}
　　2016-03-10 17:21:37.298 GCDSample[3892:2238506] ___9 <NSThread: 0x12e67f1e0>{number = 4， name = (null)}
　　2016-03-10 17:21:37.299 GCDSample[3892:2238506] currentThread1 <NSThread: 0x12e67f1e0>{number = 4， name = (null)}
　　2016-03-10 17:21:37.299 GCDSample[3892:2238506] 执行完毕
　　2016-03-10 17:21:37.302 GCDSample[3892:2238481] currentThread2 <NSThread: 0x12e50c390>{number = 1， name = main}
　　2016-03-10 17:21:37.303 GCDSample[3892:2238481] main_queue 执行完毕
　　2，异步线程中串行执行任务
　　test1中dispatch_group_async以并发的方式开启异步线程，不能保证执行顺序，如果想在并发线程中串行执行任务该如何做呢？只需要创建一个串行队列，加入group任务即可。
- (void)test1
{
NSLog(@"code begin");
NSLog(@"currentThread0 %@"，[NSThread currentThread]);
dispatch_group_t group = dispatch_group_create();
dispatch_queue_t serialQueue = dispatch_queue_create(NULL， DISPATCH_QUEUE_SERIAL);
for (int i = 0; i < 10; i++) {
dispatch_group_async(group， serialQueue， ^{
[NSThread sleepForTimeInterval:.1f];
NSLog(@"___%i %@"， i， [NSThread currentThread]);
});
}
dispatch_group_notify(group， serialQueue， ^{
NSLog(@"currentThread1 %@"，[NSThread currentThread]);
NSLog(@"执行完毕");
});
NSLog(@"code end");
}
　　执行结果：
　　test2:
　　2016-03-10 17:07:10.366 GCDSample[3879:2235420] code begin
　　2016-03-10 17:07:10.367 GCDSample[3879:2235420] currentThread0 <NSThread: 0x14c50c350>{number = 1， name = main}
　　2016-03-10 17:07:10.367 GCDSample[3879:2235420] code end
　　2016-03-10 17:07:10.472 GCDSample[3879:2235438] ___0 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:10.577 GCDSample[3879:2235438] ___1 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:10.683 GCDSample[3879:2235438] ___2 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:10.784 GCDSample[3879:2235438] ___3 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:10.890 GCDSample[3879:2235438] ___4 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:10.995 GCDSample[3879:2235438] ___5 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.101 GCDSample[3879:2235438] ___6 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.206 GCDSample[3879:2235438] ___7 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.309 GCDSample[3879:2235438] ___8 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.411 GCDSample[3879:2235438] ___9 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.411 GCDSample[3879:2235438] currentThread1 <NSThread: 0x14c692ed0>{number = 2， name = (null)}
　　2016-03-10 17:07:11.411 GCDSample[3879:2235438] 执行完毕
　　3，dispatch_barrier_async，分割多任务线程
　　如果有10个并发任务，想要分成两组，比如指定前五个任务执行完之后，优先执行第六个任务，再执行剩下的操作，可以通过dispatch_barrier_async来分割开，示例：
- (void)test3
{
dispatch_queue_t concurrentQueue = ({
dispatch_queue_t queue = dispatch_queue_create("concurrentQueue"， DISPATCH_QUEUE_CONCURRENT);
queue;
});
for (int i = 0; i < 10; i++) {
if (i != 5) {
dispatch_async(concurrentQueue， ^{
[NSThread sleepForTimeInterval:arc4random_uniform(3)];
NSLog(@"dispatch_async %i"， i);
});
}else{
dispatch_barrier_async(concurrentQueue， ^{
NSLog(@"dispatch_barrier_async");
});
}
}
}
　　执行结果：
　　test3
　　2016-03-10 17:47:32.397 GCDSample[3949:2246479] dispatch_async 1
　　2016-03-10 17:47:32.400 GCDSample[3949:2246481] dispatch_async 3
　　2016-03-10 17:47:33.403 GCDSample[3949:2246480] dispatch_async 2
　　2016-03-10 17:47:34.400 GCDSample[3949:2246477] dispatch_async 0
　　2016-03-10 17:47:34.405 GCDSample[3949:2246479] dispatch_async 4
　　2016-03-10 17:47:34.406 GCDSample[3949:2246477] dispatch_barrier_async
　　2016-03-10 17:47:34.406 GCDSample[3949:2246479] dispatch_async 6
　　2016-03-10 17:47:35.411 GCDSample[3949:2246479] dispatch_async 7
　　2016-03-10 17:47:35.412 GCDSample[3949:2246480] dispatch_async 9
　　2016-03-10 17:47:35.411 GCDSample[3949:2246477] dispatch_async 8
　　4，串行队列死锁
- (void)test4
{
NSLog(@"1");
dispatch_sync(dispatch_get_main_queue()， ^{
NSLog(@"2");
});
NSLog(@"3");
}
- (void)test4_1 {
NSLog(@"1");
dispatch_async(dispatch_get_global_queue(0， 0)， ^{
NSLog(@"2 %@"，[NSThread currentThread]);
dispatch_sync(dispatch_get_main_queue()， ^{
NSLog(@"3 %@"，[NSThread currentThread]);
});
NSLog(@"4 %@"，[NSThread currentThread]);
});
NSLog(@"5");
}
　　方法test4会造成死锁，因为main queue需要等待dispatch_sync函数中block返回才能继续执行，而通过dispatch_sync放入main queue的block按照FIFO的原则（先入先出）现在得不到执行，造成了相互等待的局面，产生死锁。将同步的串行队列放到另外一个异步线程能够解决（如方法test4_1所示）。所以在使用dispatch_sync的时候需要很谨慎，需要先执行[NSThread isMainThread]判断下当前任务是否在mian queue中调用，比如有一段代码在后台执行，而它需要从界面控制层获取一个值。那么你可以使用dispatch_sync简单办到。执行结果：
　　test4
　　2016-03-10 17:23:30.020 GCDSample[3896:2239119] 1
　　test4_1
　　2016-03-11 17:30:11.900 GCDSample[1197:730552] 1
　　2016-03-11 17:30:11.901 GCDSample[1197:730552] 5
　　2016-03-11 17:30:11.903 GCDSample[1197:730564] 2 <NSThread: 0x170263080>{number = 2， name = (null)}
　　2016-03-11 17:30:11.933 GCDSample[1197:730552] 3 <NSThread: 0x174075400>{number = 1， name = main}
　　2016-03-11 17:30:11.933 GCDSample[1197:730564] 4 <NSThread: 0x170263080>{number = 2， name = (null)}