使用dispatch_group来进行线程同步

我的上篇文章iOS中多个网络请求的同步问题总结中用到了dispatch_group来进行线程同步，对用法不是特别熟悉所以整理这篇文章来加深记忆(闲着也是闲着)。

一、简单介绍下将会用到的一些东西

英语不好就不翻译官方文档了..

1、dispatch_group_async

* Submits a block to a dispatch queue and associates the block with the given
* dispatch group
//将一个block(代码块)加入到dispatch_queue_t queue中并和dispatch_group_t group相关联
void dispatch_group_async(dispatch_group_t group, dispatch_queue_t queue, dispatch_block_t block);

个人理解：将代码块dispatch_block_t block放入队列dispatch_queue_t queue中执行；并和调度组dispatch_group_t group相互关联；如果提交到dispatch_queue_t queue中的block全都执行完毕会调用dispatch_group_notify并且dispatch_group_wait会停止等待；

2、dispatch_group_enter(group)、dispatch_group_leave(group)

* Calling this function indicates another block has joined the group through
* a means other than dispatch_group_async(). Calls to this function must be
* balanced with dispatch_group_leave().
调用这个方法标志着一个代码块被加入了group，和dispatch_group_async功能类似；dispatch_group_enter()、dispatch_group_leave()必须成对出现
void dispatch_group_enter(dispatch_group_t group);

个人理解：和内存管理的引用计数类似，我们可以认为group也持有一个整形变量(只是假设)，当调用enter时计数加1，调用leave时计数减1，当计数为0时会调用dispatch_group_notify并且dispatch_group_wait会停止等待；

3、dispatch_group_notify

void dispatch_group_notify(dispatch_group_t group,dispatch_queue_t queue,dispatch_block_t block);

个人理解：当关联到dispatch_group_t上的dispatch_group_async任务执行完毕或者是关联在上面的dispatch_group_enter、dispatch_group_leave成对出现了。参数中的dispatch_block_t block会被提交到dispatch_queue_t queue中执行。

4、dispatch_group_wait

long dispatch_group_wait(dispatch_group_t group, dispatch_time_t timeout);

个人理解：和dispatch_group_notify功能类似(多了一个dispatch_time_t参数可以设置超时时间)，在group上任务完成前，dispatch_group_wait会阻塞当前线程(所以不能放在主线程调用)一直等待；当group上任务完成，或者等待时间超过设置的超时时间会结束等待；

二、dispatch_group_async代码示例

- (void)groupSync
{
    dispatch_queue_t disqueue =  dispatch_queue_create("com.shidaiyinuo.NetWorkStudy", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_t disgroup = dispatch_group_create();
    dispatch_group_async(disgroup, disqueue, ^{
        
        NSLog(@"任务一完成");
    });
    
    dispatch_group_async(disgroup, disqueue, ^{
        
        sleep(8);
        NSLog(@"任务二完成");
    });
    
    dispatch_group_notify(disgroup, disqueue, ^{
        
        NSLog(@"dispatch_group_notify 执行");
    });
    
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        dispatch_group_wait(disgroup, dispatch_time(DISPATCH_TIME_NOW, 5 * NSEC_PER_SEC));
        NSLog(@"dispatch_group_wait 结束");
    });
}

向group中放入两个任务(准确讲是将任务加入到了并行队列disqueue中执行，然后队列和group关联当队列上任务执行完毕时group会进行同步)，第二个任务会等待8秒所以第一个任务会先完成；会先打印任务一完成再打印任务二完成，当两个任务都完成时dispatch_group_notify中的block会执行；会接着打印dispatch_group_notify 执行；dispatch_group_wait设置了超时时间为5秒所以它会在5秒后停止等待打印dispatch_group_wait 结束(任务二会等待8秒所以它会在任务二完成前打印)；

- (void)groupSync2
{
    dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
    dispatch_group_t dispatchGroup = dispatch_group_create();
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
        
        sleep(5);
        NSLog(@"任务一完成");
    });
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
        
        sleep(6);
        NSLog(@"任务二完成");
    });
    
    dispatch_group_async(dispatchGroup, globalQueue, ^{
        
        sleep(10);
        NSLog(@"任务三完成");
    });
    
    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
        NSLog(@"notify：任务都完成了");
    });
}

上面的代码里有两个队列一个是我自己创建的并行队列dispatchQueue，另一个是系统提供的并行队列globalQueue；dispatch_group_notify会等这两个队列上的任务都执行完毕才会执行自己的代码块。

三、dispatch_group_enter、dispatch_group_level示例

和dispatch_async相比，当我们调用n次dispatch_group_enter后再调用n次dispatch_group_level时，dispatch_group_notify和dispatch_group_wait会收到同步信号；这个特点使得它非常适合处理异步任务的同步当异步任务开始前调用dispatch_group_enter异步任务结束后调用dispatch_group_leve；
下面是代码示例：

- (void)groupSync
{
    dispatch_group_t group = dispatch_group_create();
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
       
        sleep(5);
        NSLog(@"任务一完成");
        dispatch_group_leave(group);
    });
    dispatch_group_enter(group);
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        
        sleep(8);
        NSLog(@"任务二完成");
        dispatch_group_leave(group);
    });
    dispatch_group_notify(group, dispatch_get_global_queue(0, 0), ^{
        
        NSLog(@"任务完成");
    });
}

示例代码中在global_queue上执行sleep任务模拟网络请求。

- (void)groupSync2
{
    dispatch_queue_t dispatchQueue = dispatch_queue_create("ted.queue.next1", DISPATCH_QUEUE_CONCURRENT);
    dispatch_queue_t globalQueue = dispatch_get_global_queue(0, 0);
    dispatch_group_t dispatchGroup = dispatch_group_create();
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
        
        dispatch_async(globalQueue, ^{
           
            sleep(5);
            NSLog(@"任务一完成");
        });
    });
    dispatch_group_async(dispatchGroup, dispatchQueue, ^(){
        
        dispatch_async(globalQueue, ^{
            
            sleep(8);
            NSLog(@"任务二完成");
        });
    });
    dispatch_group_notify(dispatchGroup, dispatch_get_main_queue(), ^(){
        NSLog(@"notify：任务都完成了");
    });
}

如果dispatch_group_async里执行的是异步代码dispatch_group_notify会直接触发而不会等待异步任务完成，而dispatch_group_enter、和dispatch_group_leave则不会有这个问题，它们只需要在任务开始前enter结束后leave即可达到线程同步的效果。