珊栏函数 iOS之多线程GCD（三）

场景需求：需要异步完成三个任务。任务一、任务二、任务三。要求：任务三必须在任务一、任务二完成之后触发。这就需要使用dispatch_barrier_async。

特点：像一堵围墙、成为任务的分割线。

代码如下：

dispatch_queue_t queue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        //任务1
        for (int i = 0; i < 2; i++) {
            NSLog(@"我是任务一、来自线程：%@",[NSThread currentThread]);
        }
    });
    dispatch_async(queue, ^{
        //任务2
        for (int i = 0; i < 2 ; i++) {
            NSLog(@"我是任务二、来自线程：%@",[NSThread currentThread]);
        }
    });
    
    
    dispatch_barrier_async(queue, ^{
        //珊栏
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是分割线、来自线程：%@",[NSThread currentThread]);
        }
    });
    
    dispatch_async(queue, ^{
        //任务3
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是任务三、来自线程：%@",[NSThread currentThread]);
        }
    });
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分析：任务一和任务二相互交错执行、由于珊栏函数存在、任务三最后执行。这三个任务都是异步完成、在各自的线程里。

dispatch_barrier_async 、dispatch_barrier_sync区别

直接看如下代码：

1.dispatch_barrier_async

NSLog(@"开始啦");
    dispatch_queue_t queue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        //任务1
        for (int i = 0; i < 2; i++) {
            NSLog(@"我是任务一、来自线程：%@",[NSThread currentThread]);
        }
    });
    dispatch_async(queue, ^{
        //任务2
        for (int i = 0; i < 2 ; i++) {
            NSLog(@"我是任务二、来自线程：%@",[NSThread currentThread]);
        }
    });
    
      NSLog(@"动啊动啊");
    dispatch_barrier_async(queue, ^{
        //珊栏
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是分割线、来自线程：%@",[NSThread currentThread]);
        }
    });
    
    dispatch_async(queue, ^{
        //任务3
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是任务三、来自线程：%@",[NSThread currentThread]);
        }
    });
    NSLog(@"结束啦");
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打印结果：

2019-01-25 13:47:17.656139+0800 多线程demo[4912:51826] 开始啦
2019-01-25 13:47:17.656348+0800 多线程demo[4912:51826] 动啊动啊
2019-01-25 13:47:17.656463+0800 多线程demo[4912:51882] 我是任务一、来自线程：<NSThread: 0x60000047f740>{number = 3, name = (null)}
2019-01-25 13:47:17.656470+0800 多线程demo[4912:51884] 我是任务二、来自线程：<NSThread: 0x60000047ed00>{number = 4, name = (null)}
2019-01-25 13:47:17.656599+0800 多线程demo[4912:51826] 结束啦
2019-01-25 13:47:17.656654+0800 多线程demo[4912:51884] 我是任务二、来自线程：<NSThread: 0x60000047ed00>{number = 4, name = (null)}
2019-01-25 13:47:17.656654+0800 多线程demo[4912:51882] 我是任务一、来自线程：<NSThread: 0x60000047f740>{number = 3, name = (null)}
2019-01-25 13:47:17.657139+0800 多线程demo[4912:51882] 我是分割线、来自线程：<NSThread: 0x60000047f740>{number = 3, name = (null)}
2019-01-25 13:47:17.657313+0800 多线程demo[4912:51882] 我是任务三、来自线程：<NSThread: 0x60000047f740>{number = 3, name = (null)}

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分析：任务三确实在任务一、任务二之后完成。使用dispatch_barrier_async、体现了异步的特点、不做任何等待、直接返回。所以主线程中的打印、不会在珊栏函数后面。

2.dispatch_barrier_sync

代码如下：

NSLog(@"开始啦");
    dispatch_queue_t queue = dispatch_queue_create("myQueue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(queue, ^{
        //任务1
        for (int i = 0; i < 2; i++) {
            NSLog(@"我是任务一、来自线程：%@",[NSThread currentThread]);
        }
    });
    dispatch_async(queue, ^{
        //任务2
        for (int i = 0; i < 2 ; i++) {
            NSLog(@"我是任务二、来自线程：%@",[NSThread currentThread]);
        }
    });
    
    NSLog(@"动次打次：%@",[NSThread currentThread]);
    dispatch_barrier_sync(queue, ^{
        //珊栏
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是分割线、来自线程：%@",[NSThread currentThread]);
        }
    });
    
    dispatch_async(queue, ^{
        //任务3
        for (int i = 0; i < 1 ; i++) {
            NSLog(@"我是任务三、来自线程：%@",[NSThread currentThread]);
        }
    });
    NSLog(@"结束啦");
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打印结果：

2019-01-25 13:54:09.045223+0800 多线程demo[5014:57352] 开始啦
2019-01-25 13:54:09.045461+0800 多线程demo[5014:57352] 动次打次：<NSThread: 0x600000072f00>{number = 1, name = main}
2019-01-25 13:54:09.045486+0800 多线程demo[5014:57400] 我是任务二、来自线程：<NSThread: 0x60000026dd80>{number = 4, name = (null)}
2019-01-25 13:54:09.045496+0800 多线程demo[5014:57404] 我是任务一、来自线程：<NSThread: 0x600000260b80>{number = 3, name = (null)}
2019-01-25 13:54:09.045758+0800 多线程demo[5014:57400] 我是任务二、来自线程：<NSThread: 0x60000026dd80>{number = 4, name = (null)}
2019-01-25 13:54:09.045830+0800 多线程demo[5014:57404] 我是任务一、来自线程：<NSThread: 0x600000260b80>{number = 3, name = (null)}
2019-01-25 13:54:09.045991+0800 多线程demo[5014:57352] 我是分割线、来自线程：<NSThread: 0x600000072f00>{number = 1, name = main}
2019-01-25 13:54:09.046099+0800 多线程demo[5014:57352] 结束啦
2019-01-25 13:54:09.046124+0800 多线程demo[5014:57404] 我是任务三、来自线程：<NSThread: 0x600000260b80>{number = 3, name = (null)}
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分析：确实分割了前后执行的任务