GCD的基本用法

1、DISPATCH_QUEUE_SERIAL 每次运行一个任务,可以添加多个,执行次序FIFO.

dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:6];
NSLog(@"[NSThread sleepForTimeInterval:6];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"[NSThread sleepForTimeInterval:3];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"[NSThread sleepForTimeInterval:1];");
});

执行顺序为6--3--1

2、DISPATCH_QUEUE_CONCURRENT 可以同时运行多个任务, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)相同

dispatch_queue_t myQueue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:6];
NSLog(@"[NSThread sleepForTimeInterval:6];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"[NSThread sleepForTimeInterval:3];");
});
dispatch_async(myQueue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"[NSThread sleepForTimeInterval:1];");
});

执行的顺序为1--3--6

二、dispatch_group_async的使用

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。

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:6];
NSLog(@"group1 [NSThread sleepForTimeInterval:6];");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"group2 [NSThread sleepForTimeInterval:3];");
});
dispatch_group_async(group, queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"group3 [NSThread sleepForTimeInterval:1];");
});
dispatch_group_notify(group, dispatch_get_main_queue(), ^{
NSLog(@"main thread.");
});

执行结果为1--3--6--main thread

dispatch_group_async只会监听最终的结果完成后，并通知main queue

三、

dispatch_barrier_async的使用

dispatch_barrier_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("gcdtest.rongfzh.yc", DISPATCH_QUEUE_CONCURRENT);
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:3];
NSLog(@"dispatch_async1");
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"dispatch_async2");
});
dispatch_barrier_async(queue, ^{
NSLog(@"dispatch_barrier_async");
[NSThread sleepForTimeInterval:0.5];
});
dispatch_async(queue, ^{
[NSThread sleepForTimeInterval:1];
NSLog(@"dispatch_async3");
});

执行结果为：2 --1--dispatch_barrier_async---3

如果使用dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);会发现运行结果为：

 
   [cpp] 
   view plain
   copy
  
 2013-07-24 17:07:17.577 NSThreadAndBlockDemo[2247:12e03] dispatch_barrier_async  
 2013-07-24 17:07:18.579 NSThreadAndBlockDemo[2247:15207] dispatch_async3  
 2013-07-24 17:07:19.578 NSThreadAndBlockDemo[2247:12b03] dispatch_async2  
 2013-07-24 17:07:20.577 NSThreadAndBlockDemo[2247:12303] dispatch_async1  

说明dispatch_barrier_async的顺序执行还是依赖queue的类型啊，必需要queue的类型为dispatch_queue_create创建的，而且attr参数值必需是DISPATCH_QUEUE_CONCURRENT类型，前面两个非dispatch_barrier_async的类型的执行是依赖其本身的执行时间的，如果attr如果是DISPATCH_QUEUE_SERIAL时，那就完全是符合Serial queue的FIFO特征了。

4、dispatch_apply

执行某个代码片段N次。

dispatch_apply(5, globalQ, ^(size_t index) {

// 执行5次

});

5、dispatch_once

dispatch_once这个函数，它可以保证整个应用程序生命周期中某段代码只被执行一次！

[cpp]view plaincopy 
   
 static dispatch_once_t onceToken;  
     dispatch_once(&onceToken, ^{  
         // code to be executed once  
     });  

6、dispatch_after

有时候我们需要等个几秒钟然后做个动画或者给个提示，这时候可以用 dispatch_after这个函数：

[cpp]view plaincopy 
    
 double delayInSeconds = 2.0;  
     dispatch_time_t popTime = dispatch_time(DISPATCH_TIME_NOW, delayInSeconds * NSEC_PER_SEC);  
     dispatch_after(popTime, dispatch_get_main_queue(), ^(void){  
         // code to be executed on the main queue after delay  
     });  

7、dispatch_set_target_queue

通过 dispatch_set_target_queue函数可以设置一个dispatch queue的优先级，或者指定一个dispatch source相应的事件处理提交到哪个queue上。

[cpp]view plaincopy 
    
 dispatch_set_target_queue(serialQ, globalQ);  

GCD的基本用法

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