GCD可以设置线程最大并发量

-(void)dispatchSignal{

    //crate的value表示，最多几个资源可访问

    dispatch_semaphore_t semaphore = dispatch_semaphore_create(2);   

    dispatch_queue_t quene = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);

     

    //任务1

    dispatch_async(quene, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"run task 1");

        sleep(1);

        NSLog(@"complete task 1");

        dispatch_semaphore_signal(semaphore);       

    });<br>

    //任务2

    dispatch_async(quene, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"run task 2");

        sleep(1);

        NSLog(@"complete task 2");

        dispatch_semaphore_signal(semaphore);       

    });<br>

    //任务3

    dispatch_async(quene, ^{

        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

        NSLog(@"run task 3");

        sleep(1);

        NSLog(@"complete task 3");

        dispatch_semaphore_signal(semaphore);       

    });   

}