多线程GCD的使用

dispatch_async(dispatch_get_global_queue(0, 0), ^{
// 处理耗时操作的代码块... 

//通知主线程刷新
dispatch_async(dispatch_get_main_queue(), ^{
//回调或者说是通知主线程刷新，
}); 

});

dispatch_async开启一个异步操作，第一个参数是指定一个gcd队列，第二个参数是分配一个处理事物的程序块到该队列。 
dispatch_get_global_queue(0, 0)，指用了全局队列。 
一般来说系统本身会有3个队列。 
global_queue，current_queue,以及main_queue. 
获取一个全局队列是接受两个参数，第一个是我分配的事物处理程序块队列优先级。分高低和默认，0为默认2为高，-2为低

#define DISPATCH_QUEUE_PRIORITY_HIGH     2
#define DISPATCH_QUEUE_PRIORITY_DEFAULT  0
#define DISPATCH_QUEUE_PRIORITY_LOW     (-2)

GCD异步并行下载

dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_group_t group = dispatch_group_create();
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"group1");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:2];
NSLog(@"group2");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"group3");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"updateUi");
});
dispatch_release(group); 

dispatch_group_async可以实现监听一组任务是否完成，完成后得到通知执行其他的操作。这个方法很有用，比如你执行三个下载任务，当三个任务都下载完成后你才通知界面说完成的了

打印：

[43328:11303] group1
[43328:12a1b] group2
[43328:13003] group3
[43328:f803] updateUi

dispatch_async是在前面的任务执行结束后它才执行，而且它后面的任务等它执行完成之后才会执行

dispatch_queue_t queue = dispatch_queue_create("**test.rongfzh.yc", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:2];
NSLog(@"dispatch_async1");
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:4];
NSLog(@"dispatch_async2");
});
dispatch_barrier_async(queue, ^{
NSLog(@"dispatch_barrier_async");
[NSThread sleepForTimeInterval:4];
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"dispatch_async3");
});