需求：异步上传5张图片到服务器A,上传成功后将返回的url以及相关信息传给服务器B。【保证 上传给B的url顺序与图片的上传顺序相同】

方法1：

- (void)semaphoreA{
    NSLog(@"============= semaphoreA =======");
    NSArray *arr = @[@"aaa",@"bbb",@"ccc",@"ddd"];
    //有同时被写入数据的情况 需要加线程锁 此处用dispatch_semaphore_t锁住 以保证数据安全
    NSMutableArray *addedArray = [NSMutableArray array];
    dispatch_semaphore_t semaLock = dispatch_semaphore_create(1);
    NSMutableArray *replaceArray = [NSMutableArray arrayWithArray:arr];
    for (int i = 0; i < arr.count; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            NSObject *obj = [arr objectAtIndex:i];
            sleep(2);
            NSLog(@"image %@ index %tu has uploaded",obj,i);
            NSString *newObj = [NSString stringWithFormat:@"%@_%tu",obj,i];
            [replaceArray replaceObjectAtIndex:i withObject:newObj];
            dispatch_semaphore_wait(semaLock, DISPATCH_TIME_FOREVER);
            [addedArray addObject:newObj];
            dispatch_semaphore_signal(semaLock);
            
            if(addedArray.count == arr.count){
                NSLog(@"done");
                NSLog(@"replaceArray = %@",replaceArray);
                NSLog(@"addedArray = %@",addedArray);
            }

        });
    }
}

image.png

方法2.

- (void)semaphoreAA{
    
    NSLog(@"============= semaphoreAA =======");
    NSArray *arr = @[@"aaa",@"bbb",@"ccc",@"ddd"];
    //有同时被写入数据的情况 需要加线程锁 此处用dispatch_semaphore_t锁住 以保证数据安全
    NSMutableArray *addedArray = [NSMutableArray array];
    dispatch_semaphore_t semaLock = dispatch_semaphore_create(1);
    dispatch_semaphore_t sema = dispatch_semaphore_create(0);
    NSMutableArray *replaceArray = [NSMutableArray arrayWithArray:arr];
    for (int i = 0; i < arr.count; i++) {
        dispatch_async(dispatch_get_global_queue(0, 0), ^{
            NSObject *obj = [arr objectAtIndex:i];
            sleep(2);
            NSLog(@"image %@ index %tu has uploaded",obj,i);
            NSString *newObj = [NSString stringWithFormat:@"%@_%tu",obj,i];
            [replaceArray replaceObjectAtIndex:i withObject:newObj];
            dispatch_semaphore_wait(semaLock, DISPATCH_TIME_FOREVER);
            [addedArray addObject:newObj];
            dispatch_semaphore_signal(semaLock);
            
            if(addedArray.count == arr.count){
                dispatch_semaphore_signal(sema);
            }
        });
    }
    dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
    NSLog(@"done");
    NSLog(@"replaceArray = %@",replaceArray);
    NSLog(@"addedArray = %@",addedArray);
}

image.png

用两个数组是为了显示 add方法图片顺序问题。
在实际的上传图片网络请求中，往往会有成功&失败两个回调，用上述方法都需要一个计数量判断，麻烦易出错不说还显得很不美观，于是有了方法3.

方法3：

- (void)groupA{
    
    NSLog(@"============= groupA =======");
    dispatch_group_t group = dispatch_group_create();
    NSArray *arr = @[@"aaa",@"bbb",@"ccc",@"ddd"];
    //有同时被写入数据的情况 需要加线程锁 此处用dispatch_semaphore_t锁住 以保证数据安全
    NSMutableArray *addedArray = [NSMutableArray array];
    dispatch_semaphore_t semaLock = dispatch_semaphore_create(1);
    NSMutableArray *replaceArray = [NSMutableArray arrayWithArray:arr];
    for (int i = 0; i < arr.count; i++) {

        dispatch_group_async(group, dispatch_get_global_queue(0, 0), ^{
            dispatch_semaphore_t sema = dispatch_semaphore_create(0);
            dispatch_async(dispatch_get_global_queue(0, 0), ^{
                NSObject *obj = [arr objectAtIndex:i];
                sleep(2);
                NSLog(@"image %@ index %tu has uploaded",obj,i);
                NSString *newObj = [NSString stringWithFormat:@"%@_%tu",obj,i];
                [replaceArray replaceObjectAtIndex:i withObject:newObj];
                dispatch_semaphore_wait(semaLock, DISPATCH_TIME_FOREVER);
                [addedArray addObject:newObj];
                dispatch_semaphore_signal(semaLock);
                dispatch_semaphore_signal(sema);
            });
            dispatch_semaphore_wait(sema, DISPATCH_TIME_FOREVER);
        });

    }
    //会阻碍主线程
   // dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{

    NSLog(@"done");
    NSLog(@"replaceArray = %@",replaceArray);
    NSLog(@"addedArray = %@",addedArray);
});
}

如果是成功失败回调，则

success{
                dispatch_semaphore_signal(sema);
}failure{
                dispatch_semaphore_signal(sema);
}

image.png

参考资料：iOS之利用GCD信号量控制并发网络请求