iOS学习——并发编程GCD

 　在iOS中使用的多线程技术有四种，Pthread、NSThread、GCD、NSOperation，但GCD与OP严格来说，应该叫并发编程技术。GCD虽然是用C语言书写，但是苹果对它做了很多封装，让它使用起来及其简单方便，因此在OC开发中应用很广。而OP则是在iOS4.0之后对GCD进行了一次封装，增加了许多用GCD实现比较麻烦的功能，如最大并发数，线程暂停，取消以及依赖添加。

　　GCD的使用其实可以拆分成两步，一是队列，二是任务/指令；队列分为串行队列、并发队列、全局队列以及主队列，其中主队列只在主线程上运行，但都遵守FIFO。任务则有异步任务与同步任务。

demo1 ：

 异步调用会访问线程池调取线程，不开启自线程的情况就是主队列执行异步

//主队列  异步任务
- (void)demo1 {
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    
    dispatch_async(mainQueue, ^{
        NSLog(@"%s %@",__FUNCTION__,[NSThread currentThread]);
    });
    
    NSLog(@"结束");
/*
 2017-11-16 19:23:41.644179+0800 iOS学习——GCD[8388:387757] 结束
 2017-11-16 19:23:41.645610+0800 iOS学习——GCD[8388:387757] -[ViewController demo1]_block_invoke <NSThread: 0x604000071200>{number = 1, name = main}
 */
}

demo2:

 同步调用任务立刻执行

//主队列  同步任务（如果主线程同步，主线程正在执行demo2这个任务，同步任务又要求主线程执行block内任务，由于线程只有执行完一个任务才能进行下一个，这样资源争夺就会死锁，如果是放在子线程则没有问题）
- (void)demo2 {
    dispatch_queue_t mainQueue = dispatch_get_main_queue();
    
    [NSThread detachNewThreadWithBlock:^{
        NSLog(@"%s",__FUNCTION__ );
        dispatch_sync(mainQueue, ^{
            NSLog(@"%s %@",__FUNCTION__,[NSThread currentThread]);
        });
        NSLog(@"子线程走完 ");
    }];
    NSLog(@"结束");
/*
 2017-11-17 09:50:48.452495+0800 iOS学习——GCD[9829:634367] 结束
 2017-11-17 09:50:48.452541+0800 iOS学习——GCD[9829:635350] -[ViewController demo2]_block_invoke
 2017-11-17 09:50:48.453392+0800 iOS学习——GCD[9829:634367] -[ViewController demo2]_block_invoke_2 <NSThread: 0x60400007eac0>{number = 1, name = main}
 2017-11-17 09:50:48.453540+0800 iOS学习——GCD[9829:635350] 子线程走完
 */
}

demo3:

 串行队列同步任务阻塞主线程，由于队列FIFO，所以保持顺序

//串行队列  同步任务 （gcd已经加入arc）
- (void)demo3 {
    dispatch_queue_t serialQueue = dispatch_queue_create("iosSerial", NULL);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_sync(serialQueue, ^{
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");
    /*
     2017-11-17 09:52:58.616681+0800 iOS学习——GCD[9854:637007] 第0次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.616871+0800 iOS学习——GCD[9854:637007] 第1次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.618652+0800 iOS学习——GCD[9854:637007] 第2次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.618805+0800 iOS学习——GCD[9854:637007] 第3次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.618944+0800 iOS学习——GCD[9854:637007] 第4次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.619485+0800 iOS学习——GCD[9854:637007] 第5次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.619606+0800 iOS学习——GCD[9854:637007] 第6次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.619715+0800 iOS学习——GCD[9854:637007] 第7次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.702830+0800 iOS学习——GCD[9854:637007] 第8次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.702967+0800 iOS学习——GCD[9854:637007] 第9次，-[ViewController demo3]_block_invoke <NSThread: 0x604000072280>{number = 1, name = main}
     2017-11-17 09:52:58.703120+0800 iOS学习——GCD[9854:637007] 结束
     */
}

demo4:

   异步会调用线程池线程，不会阻塞主线程，只有执行完一个任务才能调用，由于串行队列FIFO，所以顺序执行

//串行队列  异步任务
- (void)demo4 {
    dispatch_queue_t serialQueue = dispatch_queue_create("iosSerial", NULL);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_async(serialQueue, ^{
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");
    /*
     2017-11-17 09:56:50.914235+0800 iOS学习——GCD[9874:639468] 第0次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.914099+0800 iOS学习——GCD[9874:639183] 结束
     2017-11-17 09:56:50.916152+0800 iOS学习——GCD[9874:639468] 第1次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.917729+0800 iOS学习——GCD[9874:639468] 第2次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.918009+0800 iOS学习——GCD[9874:639468] 第3次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.918256+0800 iOS学习——GCD[9874:639468] 第4次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.918381+0800 iOS学习——GCD[9874:639468] 第5次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.918494+0800 iOS学习——GCD[9874:639468] 第6次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.918600+0800 iOS学习——GCD[9874:639468] 第7次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.951677+0800 iOS学习——GCD[9874:639468] 第8次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     2017-11-17 09:56:50.951871+0800 iOS学习——GCD[9874:639468] 第9次，-[ViewController demo4]_block_invoke <NSThread: 0x60400026d200>{number = 3, name = (null)}
     */
}

demo5:

并发队列，依旧保持队列FIFO特性，但是可以在一个任务未执行完取下一个任务执行，但是由于同步任务阻塞主线程，只有主线程调用任务，所以每次只从队列中取一个任务

//并发队列  同步任务
- (void)demo5 {
    dispatch_queue_t conQueue = dispatch_queue_create("iosConcurrent", DISPATCH_QUEUE_CONCURRENT);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_sync(conQueue, ^{
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");
    /*
     2017-11-17 10:01:15.328701+0800 iOS学习——GCD[9920:645669] 第0次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.328898+0800 iOS学习——GCD[9920:645669] 第1次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.328987+0800 iOS学习——GCD[9920:645669] 第2次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329083+0800 iOS学习——GCD[9920:645669] 第3次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329206+0800 iOS学习——GCD[9920:645669] 第4次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329473+0800 iOS学习——GCD[9920:645669] 第5次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329584+0800 iOS学习——GCD[9920:645669] 第6次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329786+0800 iOS学习——GCD[9920:645669] 第7次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.329917+0800 iOS学习——GCD[9920:645669] 第8次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.330044+0800 iOS学习——GCD[9920:645669] 第9次，-[ViewController demo5]_block_invoke <NSThread: 0x600000070e80>{number = 1, name = main}
     2017-11-17 10:01:15.330191+0800 iOS学习——GCD[9920:645669] 结束

     */
}

demo6:

 从运行的次数可以看出，并发队列 异步调用的时候，每开启一个线程，都能从队列中取一个任务执行，所以没办法保证执行顺序，而根据线程的number可以判断出，线程从线程池中被拿出来执行完任务后，会回到线程池，等待再次被调用，如果任务未执行完，最先执行完的线程又回到线程池，则将会在此执行。

//并发队列  异步任务
- (void)demo6 {
    dispatch_queue_t conQueue = dispatch_queue_create("iosConcurrent", DISPATCH_QUEUE_CONCURRENT);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_async(conQueue, ^{
            if (i % 2 == 0) {
                [NSThread sleepForTimeInterval:1.f];//由于执行太快，特阻塞一秒
            }
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");
    /*
     2017-11-17 10:10:02.883676+0800 iOS学习——GCD[9990:654627] 结束
     2017-11-17 10:10:02.883749+0800 iOS学习——GCD[9990:654784] 第1次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027f8c0>{number = 3, name = (null)}
     2017-11-17 10:10:02.883773+0800 iOS学习——GCD[9990:654786] 第3次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027f900>{number = 4, name = (null)}
     2017-11-17 10:10:02.884475+0800 iOS学习——GCD[9990:655044] 第5次，-[ViewController demo6]_block_invoke <NSThread: 0x60400046efc0>{number = 5, name = (null)}
     2017-11-17 10:10:02.884740+0800 iOS学习——GCD[9990:655045] 第7次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027fcc0>{number = 6, name = (null)}
     2017-11-17 10:10:02.884875+0800 iOS学习——GCD[9990:655044] 第9次，-[ViewController demo6]_block_invoke <NSThread: 0x60400046efc0>{number = 5, name = (null)}
     2017-11-17 10:10:03.887784+0800 iOS学习——GCD[9990:654783] 第2次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027fac0>{number = 7, name = (null)}
     2017-11-17 10:10:03.887784+0800 iOS学习——GCD[9990:654784] 第6次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027f8c0>{number = 3, name = (null)}
     2017-11-17 10:10:03.887784+0800 iOS学习——GCD[9990:654786] 第8次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027f900>{number = 4, name = (null)}
     2017-11-17 10:10:03.887788+0800 iOS学习——GCD[9990:654785] 第0次，-[ViewController demo6]_block_invoke <NSThread: 0x60400027fc80>{number = 8, name = (null)}
     2017-11-17 10:10:03.887834+0800 iOS学习——GCD[9990:655043] 第4次，-[ViewController demo6]_block_invoke <NSThread: 0x60000027f980>{number = 9, name = (null)}
     */
}

demo7、8:

与并发区别在与：1.并发可以命名 2.全局队列在MRC下不需要释放，而并发在MRC下需要主动释放

//全局队列  同步任务
- (void)demo7 {
    
    //第一个参数代表优先级，
   
    /* iOS8.0后
    *  - QOS_CLASS_USER_INTERACTIVE
    *  - QOS_CLASS_USER_INITIATED
    *  - QOS_CLASS_DEFAULT
    *  - QOS_CLASS_UTILITY
     *  - QOS_CLASS_BACKGROUND
    */
    /*
     ios 7.0 与8.0比对
     *  - DISPATCH_QUEUE_PRIORITY_HIGH:         QOS_CLASS_USER_INITIATED  高优先级
     *  - DISPATCH_QUEUE_PRIORITY_DEFAULT:      QOS_CLASS_DEFAULT  默认
     *  - DISPATCH_QUEUE_PRIORITY_LOW:          QOS_CLASS_UTILITY   低优先级
     *  - DISPATCH_QUEUE_PRIORITY_BACKGROUND:   QOS_CLASS_BACKGROUND 后台执行，会很慢，需谨慎使用
     */
    
    /*
     第二个参数是占位参数，暂时没有任何意义，为苹果后续拓展使用
     */
    dispatch_queue_t conQueue = dispatch_get_global_queue(0, 0);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_sync(conQueue, ^{
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");

}

//全局队列  异步任务
- (void)demo8 {

    dispatch_queue_t conQueue = dispatch_get_global_queue(0, 0);
    
    for (NSInteger i = 0; i < 10; i ++) {
        dispatch_async(conQueue, ^{
            NSLog(@"第%ld次，%s %@ ",i,__FUNCTION__,[NSThread currentThread]);
        });
    }
    NSLog(@"结束");
}

demo9:

同步线程设置依赖关系

//登录   评论  关注 三个操作（登录后才可以评论和关注）  评论或关注后要更新UI
//设置依赖
- (void)demo9 {
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);

    dispatch_sync(queue, ^{
        NSLog(@"登录 %@ ",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"评论 %@ ",[NSThread currentThread]);
    });

    dispatch_async(queue, ^{
        NSLog(@"关注 %@ ",[NSThread currentThread]);
    });
    
    /*
     2017-11-17 11:33:48.563496+0800 iOS学习——GCD[10351:720878] 登录 <NSThread: 0x600000263f80>{number = 1, name = main}
     2017-11-17 11:33:48.563946+0800 iOS学习——GCD[10351:721028] 关注 <NSThread: 0x604000474380>{number = 3, name = (null)}
     2017-11-17 11:33:48.563946+0800 iOS学习——GCD[10351:721030] 评论 <NSThread: 0x604000474080>{number = 5, name = (null)}
     2017-11-17 11:33:48.739571+0800 iOS学习——GCD[10351:720878] 登录 <NSThread: 0x600000263f80>{number = 1, name = main}
     2017-11-17 11:33:48.740086+0800 iOS学习——GCD[10351:721030] 评论 <NSThread: 0x604000474080>{number = 5, name = (null)}
     2017-11-17 11:33:48.740100+0800 iOS学习——GCD[10351:721146] 关注 <NSThread: 0x600000473040>{number = 4, name = (null)}
     */
}

demo10:

由结果可以看出，dispatch_barrier_async可以设置依赖关系，只有之前的代码执行完毕，才执行dispatch_barrier_async内代码，所有代码都是开启子线程执行，不阻塞当前线程；但是它会设置当前内容为同步执行，只有执行dispatch_barrier_async内任务，才能继续执行。如果需要并发执行复数任务作为依赖，然后并发执行复数任务，没有合适dispatch_barrier_async执行条件，这样就必须开启一个空dispatch_barrier_async才行

- (void)demo10 {
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_async(queue, ^{
        NSLog(@"登录 %@ ",[NSThread currentThread]);
        sleep(1);
    });
    dispatch_barrier_async(queue, ^{
        sleep(5);
        NSLog(@"评论 %@ ",[NSThread currentThread]);
    });
    
    dispatch_async(queue, ^{
        sleep(1);
        NSLog(@"关注 %@ ",[NSThread currentThread]);
    });
    dispatch_async(queue, ^{
        NSLog(@"收藏 %@ ",[NSThread currentThread]);
    });
    NSLog(@"结束主线程任务  ");
    /*
     2017-11-17 11:40:15.901540+0800 iOS学习——GCD[10445:729689] 结束主线程任务
     2017-11-17 11:40:15.901692+0800 iOS学习——GCD[10445:729786] 登录 <NSThread: 0x604000471800>{number = 3, name = (null)}
     2017-11-17 11:40:21.910464+0800 iOS学习——GCD[10445:729786] 评论 <NSThread: 0x604000471800>{number = 3, name = (null)}
     2017-11-17 11:40:21.910723+0800 iOS学习——GCD[10445:729798] 收藏 <NSThread: 0x60000026ea00>{number = 4, name = (null)}
     2017-11-17 11:40:22.913477+0800 iOS学习——GCD[10445:729786] 关注 <NSThread: 0x604000471800>{number = 3, name = (null)}
     */
}

demo11：

当group内任务全部执行完毕，就会调用notify通知，所有任务都是异步执行

#pragma mark 调度组

- (void)demo11 {
    dispatch_group_t groupQueue = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_async(groupQueue, queue, ^{
        sleep(8);
            NSLog(@"登录 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
        NSLog(@"评论 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
      
            NSLog(@"关注 %@ ",[NSThread currentThread]);
            
    });
    dispatch_notify(groupQueue, dispatch_get_main_queue(), ^{//通知完成，使用主队列，让任务在主线程执行
        NSLog(@"更新UI %@ ",[NSThread currentThread]);

    });
    NSLog(@"结束主线程任务  ");
    
    /*
     2017-11-17 11:51:20.427599+0800 iOS学习——GCD[10489:740925] 结束主线程任务
     2017-11-17 11:51:20.427726+0800 iOS学习——GCD[10489:741115] 关注 <NSThread: 0x6040004765c0>{number = 4, name = (null)}
     2017-11-17 11:51:20.427726+0800 iOS学习——GCD[10489:741147] 评论 <NSThread: 0x604000475f80>{number = 3, name = (null)}
     2017-11-17 11:51:28.429514+0800 iOS学习——GCD[10489:741114] 登录 <NSThread: 0x604000475f80>{number = 5, name = (null)}
     2017-11-17 11:51:28.429725+0800 iOS学习——GCD[10489:740925] 更新UI <NSThread: 0x60000007f7c0>{number = 1, name = main}
     */
}

demo12、13:

dispatch_group_wait可以阻塞当前线程是假，等待调度组完成，第二个参数为阻塞时间，如果时间大于调度组完成时间，那么调度组完成后，阻塞取消；如果小于调度组完成时间，那么到设定的阻塞时间后，也不再阻塞当前线程,DISPATCH_TIME_FOREVER参数则保证阻塞当前线程直到调度组完成

- (void)demo12 {
    dispatch_group_t groupQueue = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_async(groupQueue, queue, ^{
        sleep(8);
        NSLog(@"登录 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
        NSLog(@"评论 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
        
        NSLog(@"关注 %@ ",[NSThread currentThread]);
        
    });
    //第一个参数： 时间起点
    //第二个参数： 时间长度
    dispatch_group_wait(groupQueue, dispatch_time(DISPATCH_TIME_NOW, 10 * NSEC_PER_SEC));
    
    dispatch_notify(groupQueue, dispatch_get_main_queue(), ^{//通知完成，使用主队列，让任务在主线程执行
        NSLog(@"更新UI %@ ",[NSThread currentThread]);
        
    });
    NSLog(@"结束主线程任务  ");
    /*
     2017-11-17 11:53:24.627643+0800 iOS学习——GCD[10507:743414] 评论 <NSThread: 0x60000026ac40>{number = 3, name = (null)}
     2017-11-17 11:53:24.627643+0800 iOS学习——GCD[10507:743373] 关注 <NSThread: 0x60000026ac80>{number = 4, name = (null)}
     2017-11-17 11:53:32.631477+0800 iOS学习——GCD[10507:743374] 登录 <NSThread: 0x604000463300>{number = 5, name = (null)}
     2017-11-17 11:53:32.631708+0800 iOS学习——GCD[10507:743191] 结束主线程任务
     2017-11-17 11:53:32.632605+0800 iOS学习——GCD[10507:743191] 更新UI <NSThread: 0x60400006fb00>{number = 1, name = main}
     */
}
- (void)demo13 {
    dispatch_group_t groupQueue = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);
    dispatch_group_async(groupQueue, queue, ^{
        sleep(8);
        NSLog(@"登录 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
        NSLog(@"评论 %@ ",[NSThread currentThread]);
    });
    dispatch_group_async(groupQueue, queue, ^{
        
        NSLog(@"关注 %@ ",[NSThread currentThread]);
        
    });
    

    dispatch_group_wait(groupQueue,  dispatch_time(DISPATCH_TIME_NOW, 4 * NSEC_PER_SEC));
    
    dispatch_notify(groupQueue, dispatch_get_main_queue(), ^{//通知完成，使用主队列，让任务在主线程执行
        NSLog(@"更新UI %@ ",[NSThread currentThread]);
        
    });
    NSLog(@"结束主线程任务  ");
/*
 2017-11-17 13:52:26.915659+0800 iOS学习——GCD[10810:807557] 评论 <NSThread: 0x604000275640>{number = 3, name = (null)}
 2017-11-17 13:52:26.915659+0800 iOS学习——GCD[10810:807556] 关注 <NSThread: 0x604000275400>{number = 4, name = (null)}
 2017-11-17 13:52:30.916019+0800 iOS学习——GCD[10810:807242] 结束主线程任务
 2017-11-17 13:52:34.919903+0800 iOS学习——GCD[10810:807559] 登录 <NSThread: 0x604000275640>{number = 5, name = (null)}
 2017-11-17 13:52:34.920194+0800 iOS学习——GCD[10810:807242] 更新UI <NSThread: 0x600000065f00>{number = 1, name = main}

 */
    
}

demo14:

     dispatch_group_enter，dispatch_group_leave在项目中必须成对出现，不然会崩溃。如果dispatch_group_async内执行异步线程，notify是无法识别到异步线程是否完成，只会认为执行完dispatch_group_async内任务就通知；但是当用上enter和leave后，只要把enter放在调度组任务创建前，然后leave放在你认为会结束的地方，那么只有等leave执行，notify才会执行

- (void)demo14 {
    dispatch_group_t groupQueue = dispatch_group_create();
    dispatch_queue_t queue = dispatch_queue_create("ios", DISPATCH_QUEUE_CONCURRENT);
    
    dispatch_group_async(groupQueue, queue, ^{

        dispatch_async(queue, ^{
            sleep(8);
            NSLog(@"登录 %@ ",[NSThread currentThread]);
        });
    });

    
    dispatch_group_enter(groupQueue);
    dispatch_group_async(groupQueue, queue, ^{
        dispatch_async(queue, ^{
            sleep(4);
            NSLog(@"评论 %@ ",[NSThread currentThread]);
            dispatch_group_leave(groupQueue);

        });
   
    });
    
    dispatch_group_enter(groupQueue);
    dispatch_group_async(groupQueue, queue, ^{
        
        NSLog(@"关注 %@ ",[NSThread currentThread]);
        
    });
    dispatch_group_leave(groupQueue);

    
    dispatch_notify(groupQueue, dispatch_get_main_queue(), ^{//通知完成，使用主队列，让任务在主线程执行
        NSLog(@"更新UI %@ ",[NSThread currentThread]);
        
    });
    NSLog(@"结束主线程任务  ");
    /*
     2017-11-17 14:05:44.459763+0800 iOS学习——GCD[10883:822533] 结束主线程任务
     2017-11-17 14:05:44.459822+0800 iOS学习——GCD[10883:822914] 关注 <NSThread: 0x60000027f000>{number = 3, name = (null)}
     2017-11-17 14:05:48.462705+0800 iOS学习——GCD[10883:822911] 评论 <NSThread: 0x6040002772c0>{number = 4, name = (null)}
     2017-11-17 14:05:48.462927+0800 iOS学习——GCD[10883:822533] 更新UI <NSThread: 0x604000071640>{number = 1, name = main}
     2017-11-17 14:05:52.460289+0800 iOS学习——GCD[10883:822938] 登录 <NSThread: 0x604000273100>{number = 5, name = (null)}
     */
}