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GCD信号量和组队列来完成线程依赖

GCD信号量和组队列来完成线程依赖

作者: 淡暗 | 来源:发表于2018-03-19 19:06 被阅读0次

    需求是:C任务需要得到请求A,B的结果后再来执行.这种需求单独用组队列是不太好的.因为请求A,B都是异步的,完成需要有一定的耗时,而把任务放入队列组中,提交完AB两个任务后,就算完成了任务,并不会等AB两个异步请求返回请求结果.

    • dispatch_semaphore GCD中的信号量,主要解决线程间的依赖.让多个线程之间相互协调完成任务,信号量会阻塞当前任务直到任务完成.
      1.创建一个信号量,传入参数一般为0,不能传入小于0的值.
    dispatch_semaphore_t dispatch_semaphore_create(long value);
    

    2.使信号量等待,可以看做是信号量减一,后一个参数是等待时间.

    long dispatch_semaphore_wait(dispatch_semaphore_t dsema, dispatch_time_t timeout);
    

    3.放行信号量,可以看做是信号量加一,当信号量>=0的时候,程序就可以通过向下执行.

    long dispatch_semaphore_signal(dispatch_semaphore_t dsema);
    

    下面是DEMO,
    1.模拟一个耗时的操作,通过传入不同大小的参数,并把任务放入子线程来模拟AB两个异步请求的不同耗时.

    - (void)killTime:(long)num{
        for (int i=0; i<num; i++) {
            @autoreleasepool{
                NSString * str = [NSString stringWithFormat:@"iodfi"];
            }
        }
    }
    

    2.信号量和队列组结合使用

    - (void)cTask{
        
        self.taskA = @"A没完成";
        self.taskB = @"B没完成";
        //1.创建信号量,组队列
        NSLog(@"1-- %@",[NSThread currentThread]);//1
        dispatch_semaphore_t semaphore = dispatch_semaphore_create(0);
        dispatch_group_t group = dispatch_group_create();
        
        //A请求
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
           NSLog(@"A group 1-- %@",[NSThread currentThread]);//3
           //任务放入子线程模拟异步请求
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                NSLog(@"A group 子线程-- %@",[NSThread currentThread]);//5
                [self killTime:10e6];
                self.taskA = @"A完成了";
                dispatch_semaphore_signal(semaphore);
            });
            //让信号量等待
            NSLog(@"A group 2-- %@",[NSThread currentThread]);//3
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        });
        
        //B请求
        dispatch_group_async(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"B group 1 -- %@",[NSThread currentThread]);//4
            //任务放入子线程
            dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
                NSLog(@"B group 子线程-- %@",[NSThread currentThread]);//6
                [self killTime:10e3];
                self.taskB = @"B完成了";
                dispatch_semaphore_signal(semaphore);
            });
            //让信号量等待
            NSLog(@"B group 2-- %@",[NSThread currentThread]);//4
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        });
        
        //C任务
        dispatch_group_notify(group, dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            NSLog(@"%@ --- %@",self.taskA,self.taskB);
        });
        
    }
    

    打印的结果是

    2018-03-19 19:01:46.843305+0800 005-信号量[1195:77040] 1-- <NSThread: 0x600000068a40>{number = 1, name = main}
    2018-03-19 19:01:46.843634+0800 005-信号量[1195:77040] 主线程执行任务
    2018-03-19 19:01:46.843755+0800 005-信号量[1195:77120] A group 1-- <NSThread: 0x600000267780>{number = 3, name = (null)}
    2018-03-19 19:01:46.843796+0800 005-信号量[1195:77122] B group 1 -- <NSThread: 0x604000276500>{number = 4, name = (null)}
    2018-03-19 19:01:46.845369+0800 005-信号量[1195:77120] A group 2-- <NSThread: 0x600000267780>{number = 3, name = (null)}
    2018-03-19 19:01:46.845449+0800 005-信号量[1195:77121] A group 子线程-- <NSThread: 0x604000276900>{number = 5, name = (null)}
    2018-03-19 19:01:46.846946+0800 005-信号量[1195:77122] B group 2-- <NSThread: 0x604000276500>{number = 4, name = (null)}
    2018-03-19 19:01:46.847011+0800 005-信号量[1195:77123] B group 子线程-- <NSThread: 0x600000267c80>{number = 6, name = (null)}
    2018-03-19 19:01:52.813326+0800 005-信号量[1195:77122] A完成了 --- B完成了
    

    3.如何把和信号量相关的代码删除,可以看见结果是不正确的.

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