Three20 TTURLRequest 同步缓存（使其异步或线程化）？

Question

我目前正在使用 Three20 库来为 iPhone 项目实现一些缓存。它对加快我的表格视图和页面加载速度有很大帮助，但是我遇到了一个小问题：

我对 TTURLImageResponse 进行了子类化，以在缓存之前调整远程检索的图像的大小/裁剪图像，以便每次我从服务器/缓存取回它们时，我不必调整它们的大小。这工作得很好，图像加载得很快，但是当图像从 Three20 库中的缓存返回时，它们会同步返回。当触发从缓存返回的 10-20 个图像请求时，这会导致明显的延迟。由于它们是同步返回的，因此它们会阻塞我的 UI 线程，并且用户需要等待几秒钟才能看到从缓存返回的图像。

我尝试通过执行以下操作来对操作进行线程化：

- (void) setupImages
{   

    if(imageList != nil &&
       [imageList count] > 0)
    {
        [NSThread detachNewThreadSelector:@selector(fillImages) toTarget:self withObject:nil];
    }
    else
    {
        lblMessage.hidden = NO;
        lblMessage.text = @"There are no images. Try refreshing the current list.";
        lblTitle.text = @"";
        lblAuthor.text = @"";
    }
}

- (void)fillImages
{
    NSAutoreleasePool *pool = [[NSAutoreleasePool alloc] init];

    int i=0;

    TTURLRequest *request;

    for (MyImage *imageObj in imageList)
    {
        NSString *imageUrl = [[[imageObj thumbnail_lg] image_url] absoluteString];
        if(imageUrl != nil)
        {           
            request = [TTURLRequest requestWithURL:imageUrl delegate:self];
            MyTTURLImageResponse *responseObj = [[[MyTTURLImageResponse alloc] init] autorelease];
            responseObj.index = i;

            request.cachePolicy = TTURLRequestCachePolicyDisk;
            request.response = responseObj;
            request.httpMethod = @"GET";
            [request send];

            i++;
        }
    }

    [pool release];
}

//When the fillImages selector is spawned on its own thread it does not callback to this method, therefore images are never loaded!!
- (void)requestDidFinishLoad:(TTURLRequest*)request 
{
    MyTTURLImageResponse *response = request.response;

    UIImage *loadedImage = response.image;
    int imageIndex = response.index;

    //this now happens as the image is cached :)
    //loadedImage = [loadedImage cropCenterAndScaleImageToSize:CGSizeMake(200, 200)];

    [myImageDisplay setImage:loadedImage forIndex:imageIndex];
}

但是线程似乎不起作用（回调位于主线程上...）。有谁知道一种从 Three20 缓存中异步检索的方法或一种线程化 Three20 TTURLRequest 的方法，以便它至少在后台执行此操作？