GCD信号量控制并发

例程一：控制线程数量

    //信号量控制并发
    dispatch_group_t group = dispatch_group_create();
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(10);
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    
    for(int i=0;i<100;i++){
        dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
        dispatch_group_async(group, queue, ^{
            NSLog(@"i=%i",i);
            sleep(2);
            dispatch_semaphore_signal(semaphore);
        });
    }

简单介绍一下

dispatch_semaphore_create(10): 创建信号量

dispatch_semaphore_wait(): 同步等待，对信号量减1效果

dispatch_semaphore_signal(): 对信号量加1效果

例程二：精细的同步效果

    //信号量控制并发2
    dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0);
    dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);
    NSMutableArray *array = [[NSMutableArray alloc] init];
    
    for(int i=0;i<1000;i++){
        dispatch_async(queue, ^{
            dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);
            [array addObject:[NSNumber numberWithInt:i]];
            NSLog(@"add");
            dispatch_semaphore_signal(semaphore);
        });
    }