iOS深入理解GCD 第一篇( 并行、串行、异步、同步的加深理解

并行、串行、异步、同步的加深理解

单个理解：

并行：就是队列里面的任务（代码块，block）不是一个个执行，而是同时执行，不存在先后顺序

串行：队列里面的任务一个接着一个执行，第一个执行完了才能轮到第二个

异步：具有新开线程的能力

同步：不具备有新开线程的能力，只能在当前线程执行任务（所以任务只能一个挨着一个执行，而不能同时进行）

串起来理解：

并行+异步：就是真正的并发，新开有有多个线程处理任务，任务并发执行（不按顺序执行）
串行+异步：新开一个线程，任务一个接一个执行，上一个任务处理完毕，下一个任务才可以被执行
并行+同步：不新开线程，任务一个接一个执行
串行+同步：不新开线程，任务一个接一个执行

​ 这里不得不说一下，之前看了这篇博客

http://www.cnblogs.com/ziyi--caolu/p/4900650.html 该作者成功的将我绕进了里面，其实归根究底，只要是在同步环境中，并行和串行确实就可以这么理解，

即并行+同步 和 串行+同步 都是不新开线程，并且任务一个接一个执行

下面是具体代码示例：

一：异步并行队列内回调同步串行队列

- (void)exampleFirst {
    dispatch_queue_t queue = dispatch_queue_create("com.hao123.www", DISPATCH_QUEUE_SERIAL);
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---11  %@",[NSThread currentThread]);
        dispatch_sync(queue, ^{
            //打印结果与主队列bingxingyibuqiantaozhu函数保持一致
            NSLog(@"11  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---22  %@",[NSThread currentThread]);
        dispatch_sync(queue, ^{
            NSLog(@"22  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---33  %@",[NSThread currentThread]);
        dispatch_sync(queue, ^{
            NSLog(@"33  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---44  %@",[NSThread currentThread]);
        dispatch_sync(queue, ^{
            NSLog(@"44  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---55  %@",[NSThread currentThread]);
        dispatch_sync(queue, ^{
            NSLog(@"55  %@",[NSThread currentThread]);
        });
    });
}

打印结果

2017-09-08 14:29:24.407 GcdInsightTest[63582:2978456] 外部---44  <NSThread: 0x608000072780>{number = 6, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978364] 外部---11  <NSThread: 0x608000070f40>{number = 3, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978366] 外部---33  <NSThread: 0x600000076380>{number = 5, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978363] 外部---22  <NSThread: 0x600000072f00>{number = 4, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978457] 外部---55  <NSThread: 0x608000073340>{number = 7, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978456] 44  <NSThread: 0x608000072780>{number = 6, name = (null)}
2017-09-08 14:29:24.407 GcdInsightTest[63582:2978364] 11  <NSThread: 0x608000070f40>{number = 3, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978366] 33  <NSThread: 0x600000076380>{number = 5, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978363] 22  <NSThread: 0x600000072f00>{number = 4, name = (null)}
2017-09-08 14:29:24.408 GcdInsightTest[63582:2978457] 55  <NSThread: 0x608000073340>{number = 7, name = (null)}

二：异步并行内回调同步并行队列

- (void)exampleSecond {
 dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---11  %@",[NSThread currentThread]);
        //对比异步并行回调同步串行，可以看出当外部异步并行队列未执行完毕时，有可能内部同步并行队列已经开始执行了
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"11  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---22  %@",[NSThread currentThread]);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"22  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---33  %@",[NSThread currentThread]);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"33  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---44  %@",[NSThread currentThread]);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"44  %@",[NSThread currentThread]);
        });
    });
    
    dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
        NSLog(@"外部---55  %@",[NSThread currentThread]);
        dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"55  %@",[NSThread currentThread]);
        });
    });
}

打印结果

2017-09-08 14:34:59.192 GcdInsightTest[64242:2984344] 外部---11  <NSThread: 0x608000263040>{number = 13, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984353] 外部---22  <NSThread: 0x608000262480>{number = 17, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984111] 外部---33  <NSThread: 0x608000073980>{number = 12, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984345] 外部---55  <NSThread: 0x60000007f1c0>{number = 16, name = (null)}
2017-09-08 14:34:59.192 GcdInsightTest[64242:2984346] 外部---44  <NSThread: 0x608000074d40>{number = 14, name = (null)}
2017-09-08 14:34:59.193 GcdInsightTest[64242:2984344] 11  <NSThread: 0x608000263040>{number = 13, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984353] 22  <NSThread: 0x608000262480>{number = 17, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984111] 33  <NSThread: 0x608000073980>{number = 12, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984345] 55  <NSThread: 0x60000007f1c0>{number = 16, name = (null)}
2017-09-08 14:34:59.194 GcdInsightTest[64242:2984346] 44  <NSThread: 0x608000074d40>{number = 14, name = (null)}

根据两段例子的打印结果，可以显而易见的得出结论：

一：由于外部是异步并行队列，所以外部打印结果不按顺序执行。

二：当外部所有队列执行完毕，内部队列才开始执行。

三：其内部的队列将与外部队列顺序保持一致，无论异步并行队列内回调的是同步串行队列，还是同步并行队列，只有当外部队列执行完毕，接下来内部队列才会在同一个线程继续执行直到任务结束。

根据第三条可以得出结论：
并行+同步：不新开线程，任务一个接一个执行
串行+同步：不新开线程，任务一个接一个执行
这两句话没毛病~