GCD有三种queue
main queue: 主线程队列。是一个串行队列。一般用来更新UI。
global queue: 全局队列，是一个并行队列。使用方法相信大家都知道。
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT,0),^{
[self test];
}
上面的意思就是开启一个异步线程，在全局队列中执行。
custom queue:自定义队列。有两种自定义队列。”
dispatch_queue_t serial_queue = dispatch_queue_create("com.reviewcode.www", DISPATCH_QUEUE_SERIAL);
dispatch_queue_t concurrent_queue = dispatch_queue_create("com.concurrent.www", DISPATCH_QUEUE_CONCURRENT);
serial_queue即是我自定义的一个串行队列。上面提到的主线程队列也是一个串行队列。
concurrent_queue是我自定义的一个并行队列。上面提到的global queue就是一个并行队列。
现在我们来讨论2个问题。
1.dispatch_async里使用串行队列和并行队列的效果。
2.dispatch_sync里使用串行队列和并行队列的效果。
串行队列和并行队列的创建如下。
serial_queue = dispatch_queue_create("com.reviewcode.www", DISPATCH_QUEUE_SERIAL);
concurrent_queue = dispatch_queue_create("com.LiSiGuang.www", DISPATCH_QUEUE_CONCURRENT);
讨论的问题：
“在dispatch_async使用串行队列”
代码如下：
-(void)testSerialQueueWithAsync
{
for ( int index =0; index < 10; index++)
{
dispatch_async(serial_queue,^{
NSLog(@"index =%d",index);
NSLog(@" current thread is %@",[NSThread currentThread]);
});
}
NSLog(@"Running on main Thread");
}
然后在viewDidLoad()方法里打印，打印结果如下。
index =0
Running on main Thread
current thread is<NSThred:0x600000466b40>{number = 4, name = (null)}
index =1
current thread is{number = 4, name = (null)}
index =2
current thread is{number = 4, name = (null)}
index =3
current thread is{number = 4, name = (null)}
index =4
current thread is{number = 4, name = (null)}
index =5
current thread is{number = 4, name = (null)}
index =6
current thread is{number = 4, name = (null)}
index =7
current thread is{number = 4, name = (null)}
index =8
current thread is{number = 4, name = (null)}
index =9
current thread is{number = 4, name = (null)}
打印结果的几个特征。
在dispatch_async使用的所有Thread均为同一个Thread。因为他们的指针地址完全相同。
输出结果是按顺序输出，符合我们对串行队列的期待。即FIFO。先进先出原则。
Running on main Thread这句话并没有在最后执行，而是会出现在随机的位置，这也符合我们对dispatch_async的期待，因为他会开辟一个新的线程执行，不会阻塞主线程。 ok，让我们测试下一个。
在dispatch_async中使用并行队列
2018-01-31 10:07:40.409923+0800 TextD[22242:494553] index =22018-01-31 10:07:40.409922+0800 TextD[22242:494552] index =02018-01-31 10:07:40.409922+0800 TextD[22242:494551] index =12018-01-31 10:07:40.409922+0800 TextD[22242:494487] Running on main Thread2018-01-31 10:07:40.409980+0800 TextD[22242:494554] index =32018-01-31 10:07:40.410040+0800 TextD[22242:494561] index =42018-01-31 10:07:40.410284+0800 TextD[22242:494551] current thread is{number = 3, name = (null)}2018-01-31 10:07:40.410285+0800 TextD[22242:494552] current thread is{number = 5, name = (null)}2018-01-31 10:07:40.410292+0800 TextD[22242:494553] current thread is{number = 4, name = (null)}2018-01-31 10:07:40.410349+0800 TextD[22242:494554] current thread is{number = 6, name = (null)}2018-01-31 10:07:40.410422+0800 TextD[22242:494551] index =52018-01-31 10:07:40.410435+0800 TextD[22242:494561] current thread is{number = 7, name = (null)}2018-01-31 10:07:40.410780+0800 TextD[22242:494562] index =62018-01-31 10:07:40.410881+0800 TextD[22242:494564] index =82018-01-31 10:07:40.410882+0800 TextD[22242:494563] index =72018-01-31 10:07:40.410929+0800 TextD[22242:494565] index =92018-01-31 10:07:40.412670+0800 TextD[22242:494551] current thread is{number = 3, name = (null)}2018-01-31 10:07:40.437759+0800 TextD[22242:494563] current thread is{number = 9, name = (null)}2018-01-31 10:07:40.437768+0800 TextD[22242:494562] current thread is{number = 8, name = (null)}2018-01-31 10:07:40.437787+0800 TextD[22242:494564] current thread is{number = 10, name = (null)}2018-01-31 10:07:40.437830+0800 TextD[22242:494565] current thread is{number = 11, name = (null)}
打印结果的特征如下：
输出的结果是乱序的，说明我们的输出语句是并发的，由多个线程共同执行的。
Running on main Thread这句话依然没有被阻塞，直接输出了。
每次打印语句的Thread均不相同。
仔细比对两次打印结果的异同点。提出问题。
串行队列如何保证在异步线程中遵守先进先出原则（从Demo里看，也就是顺序打印我们的结果）？
很简单，它会保证每次dispatch_async开辟线程执行串行队列中的任务时，总是使用同一个异步线程。这也是为什么我们的第一次打印结果中，NSThread总是同一个。
在dispatch_async中放入并行队列并执行的时候，为什么执行顺序总是乱序的？
因为在并行对列中，每执行一次任务的时候，dispatch_async总会为我们开辟一个新的线程（当然，开辟线程的[…]”
在dispatch_sync使用串行队列：
-(void)testSerialQueueWithsync
{ dispatch_queue_t serial_queue = dispatch_queue_create("com.reviewcode.www", DISPATCH_QUEUE_SERIAL);
for ( int index =0; index < 10; index++)
{ dispatch_sync(serial_queue,^{
NSLog(@"index =%d",index);
NSLog(@" current thread is %@",[NSThread currentThread]);
});
}
NSLog(@"Running on main Thread");
}
1.dispatch_sync并没有开辟一个新的线程，直接在当前线程中执行代码（即main线程）。所以会阻塞当前线程。
2.Running on main Thread在最后输出。
也就是说，当使用dispatch_sync执行串行队列的任务时，不会开辟新的线程，会直接使用当前线程执行队列中的任务。
在dispatch_sync中使用并行队列
-(void)testConcurrentQueueWithsync
{ dispatch_queue_t Concurrent_queue = dispatch_queue_create("com.reviewcode.www", DISPATCH_QUEUE_CONCURRENT);
for ( int index =0; index < 10; index++)
{
dispatch_sync(Concurrent_queue,^{
NSLog(@"index =%d",index);
NSLog(@" current thread is %@",[NSThread currentThread]);
});
}
NSLog(@"Running on main Thread");
}
打印结果如下：
2018-01-31 10:30:19.728025+0800 TextD[22520:524950] index =02018-01-31 10:30:19.728294+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.728425+0800 TextD[22520:524950] index =12018-01-31 10:30:19.728605+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.728753+0800 TextD[22520:524950] index =22018-01-31 10:30:19.728875+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.728990+0800 TextD[22520:524950] index =32018-01-31 10:30:19.729108+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.729223+0800 TextD[22520:524950] index =42018-01-31 10:30:19.729351+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.729474+0800 TextD[22520:524950] index =52018-01-31 10:30:19.729597+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.729697+0800 TextD[22520:524950] index =62018-01-31 10:30:19.729831+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.729952+0800 TextD[22520:524950] index =72018-01-31 10:30:19.730147+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.730316+0800 TextD[22520:524950] index =82018-01-31 10:30:19.730617+0800 TextD[22520:524950] current thread is{number = 1, name = main}2018-01-31 10:30:19.730786+0800 TextD[22520:524950] index =92018-01-31 10:30:19.730986+0800 TextD[22520:524950] current thread is{number = 1, name = main}
2018-01-31 10:30:19.731201+0800 TextD[22520:524950] Running on main Thread
总结：
结果很奇怪，和在串行队列执行的效果一模一样。按我们的思考，并行队列里执行任务不应该是多个线程同时跑么？其实是由于dispatch_sync并不会开辟新的线程执行任务，所以导致了执行并行队列任务的线程总会是一个线程，自然，结果是一样的。
