GCD之dispatch queue深入浅出

iOS中多线程编程工具主要有：

 NSThread
NSOperation
GCD
这三种方法都简单易用,各有千秋.但无疑GCD是最有诱惑力的,因为其本身是apple为多核的并行运算提出的解决方案.虽然当前移动平台用双核的不多,但不影响GCD作为多线程编程的利器(ipad2已经是双核了,这无疑是一个趋势).

http://www.cnblogs.com/scorpiozj/archive/2011/07/25/2116459.html

GCD是和block紧密相连的,所以最好先了解下block(可以查看这里).GCD是C level的函数,这意味着它也提供了C的函数指针作为参数,方便了C程序员.

一、下面首先来看GCD的使用:

dispatch_async(dispatch_queue_t queue, dispatch_block_t block);
async表明异步运行,block代表的是你要做的事情,queue则是你把任务交给谁来处理了.(除了async,还有sync,delay,本文以async为例).

之所以程序中会用到多线程是因为程序往往会需要读取数据,然后更新UI.为了良好的用户体验,读取数据的操作会倾向于在后台运行,这样以避免阻塞主线程.GCD里就有三种queue来处理.

       先来介绍一下 Main queue：

　　顾名思义,运行在主线程,由dispatch_get_main_queue获得.和ui相关的就要使用Main Queue.

[cpp] view plaincopy
//GCD下载图片刷新主界面的例子  
/*
- (IBAction)touchUpInsideByThreadOne:(id)sender {
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];
        NSData * data = [[NSData alloc]initWithContentsOfURL:url];
        UIImage *image = [[UIImage alloc]initWithData:data];
        if (data != nil) {
            dispatch_async(dispatch_get_main_queue(), ^{
                self.imageView.image = image;
            });
        }
    });
}*/  

 通过与线程池的配合，dispatch queue分为下面两种：而系统默认就有一个串行队列main_queue和并行队列global_queue：

     Serial Dispatch Queue -- 线程池只提供一个线程用来执行任务，所以后一个任务必须等到前一个任务执行结束才能开始。
     Concurrent Dispatch Queue -- 线程池提供多个线程来执行任务，所以可以按序启动多个任务并发执行。
而系统默认就有一个串行队列main_queue和并行队列global_queue：

[cpp] view plaincopy
dispatch_queue_t globalQ = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);  
dispatch_queue_t mainQ = dispatch_get_main_queue();  
通常，我们可以在global_queue中做一些long-running的任务，完成后在main_queue中更新UI，避免UI阻塞，无法响应用户操作：
[cpp] view plaincopy
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{  
        // long-running task  
        dispatch_async(dispatch_get_main_queue(), ^{  
            // update UI  
        });  
    });  


1.Serial quque(private dispatch queue)

　　每次运行一个任务,可以添加多个,执行次序FIFO. 通常是指程序员生成的,比如:

NSDate *da = [NSDate date];
NSString *daStr = [da description];
const char *queueName = [daStr UTF8String];
dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_PRIORITY_DEFAULT);
下面还是下载图片例子：

[cpp] view plaincopy
- (IBAction)touchUpInsideByThreadOne:(id)sender {  
    NSDate *da = [NSDate date];  
    NSString *daStr = [da description];  
    const char *queueName = [daStr UTF8String];  
    dispatch_queue_t myQueue = dispatch_queue_create(queueName, NULL);  
      
    dispatch_async(myQueue, ^{  
        NSURL * url = [NSURL URLWithString:@"http://avatar.csdn.net/2/C/D/1_totogo2010.jpg"];  
        NSData * data = [[NSData alloc]initWithContentsOfURL:url];  
        UIImage *image = [[UIImage alloc]initWithData:data];  
        if (data != nil) {  
            dispatch_async(dispatch_get_main_queue(), ^{  
                self.imageView.image = image;  
            });  
        }  
    });  
      
    dispatch_release(myQueue);  
}  
为了验证Serial queue的FIFO特性，写了如下的验证代码：发现的确是顺序执行的。

[cpp] view plaincopy
- (IBAction)touchUpInsideByThreadOne:(id)sender {  
    NSDate *da = [NSDate date];  
    NSString *daStr = [da description];  
    const char *queueName = [daStr UTF8String];  
    dispatch_queue_t myQueue = dispatch_queue_create(queueName, DISPATCH_QUEUE_SERIAL);  
      
    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];");  
    });  
      
    dispatch_release(myQueue);  
}  
运行结果为：

[cpp] view plaincopy
2013-07-24 16:37:14.397 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:6];  
2013-07-24 16:37:17.399 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:3];  
2013-07-24 16:37:18.401 NSThreadAndBlockDemo[1924:12303] [NSThread sleepForTimeInterval:1];  

3. Concurrent queue(global dispatch queue):

可以同时运行多个任务,每个任务的启动时间是按照加入queue的顺序,结束的顺序依赖各自的任务.使用dispatch_get_global_queue获得.

[cpp] view plaincopy
- (IBAction)touchUpInsideByThreadOne:(id)sender {  
    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];");  
    });  
      
    dispatch_release(myQueue);  
}  
运行的结果为：
[cpp] view plaincopy
2013-07-24 16:38:41.660 NSThreadAndBlockDemo[1944:12e03] [NSThread sleepForTimeInterval:1];  
2013-07-24 16:38:43.660 NSThreadAndBlockDemo[1944:12b03] [NSThread sleepForTimeInterval:3];  
2013-07-24 16:38:46.660 NSThreadAndBlockDemo[1944:12303] [NSThread sleepForTimeInterval: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。下面是一段例子代码：

[cpp] view plaincopy
- (IBAction)touchUpInsideByThreadOne:(id)sender {  
    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.");  
    });  
    dispatch_release(group);  
}  
执行结果为：
[cpp] view plaincopy
2013-07-24 16:48:23.063 NSThreadAndBlockDemo[2004:12e03] group3 [NSThread sleepForTimeInterval:1];  
2013-07-24 16:48:25.063 NSThreadAndBlockDemo[2004:12b03] group2 [NSThread sleepForTimeInterval:3];  
2013-07-24 16:48:28.063 NSThreadAndBlockDemo[2004:12303] group1 [NSThread sleepForTimeInterval:6];  
2013-07-24 16:48:28.065 NSThreadAndBlockDemo[2004:11303] main thread.  

果然，dispatch_group_async只会监听最终的结果完成后，并通知main queue，那如果是我们需要顺序执行的话呢？请看下面的dispatch_barrier_async。

3、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

例子代码如下：

[cpp] view plaincopy
- (IBAction)touchUpInsideByThreadOne:(id)sender {  
    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：
[cpp] view plaincopy
2013-07-24 17:01:54.580 NSThreadAndBlockDemo[2153:12b03] dispatch_async2  
2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_async1  
2013-07-24 17:01:56.580 NSThreadAndBlockDemo[2153:12303] dispatch_barrier_async  
2013-07-24 17:01:58.083 NSThreadAndBlockDemo[2153:12303] dispatch_async3  
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 plaincopy
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);  

由此可见,GCD的使用非常简单,以我的使用经验来看,以后会逐步淘汰使用NSOperation而改用GCD.