从服务器下载大量图像到ipad的最佳方法是什么？

Question

我们正在为我们公司开发一个企业应用程序。该应用程序应该脱机工作，这意味着我们需要将超过10K的产品图像下载到iPad。每个图像可以从几KB到4M。这样做起初我们使用以下代码下载图像：

-(BOOL)DownloadFileInBatch :(NSArray*) fileArr
{
    NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);

    NSString *documentDirectory = [paths objectAtIndex:0];

    NSOperationQueue *queue = [[NSOperationQueue alloc] init];
    queue.maxConcurrentOperationCount = 4;

    NSBlockOperation *completionOperation = [NSBlockOperation blockOperationWithBlock:^{
        [[NSOperationQueue mainQueue] addOperationWithBlock:^{
            [[NSNotificationCenter defaultCenter] postNotificationName:@"taskDone" object:self];
        }];
    }];

    @autoreleasepool
    {
        NSBlockOperation *operation;

        for(int i = 0; i < fileArr.count; i++)
        {
             operation = [NSBlockOperation blockOperationWithBlock:^{

                NSURL *url = [[NSURL alloc] initWithString:[[fileArr objectAtIndex:i] objectForKey:@"image_url"]];

                NSData *data = [NSData dataWithContentsOfURL:url];

                NSString *filename = [documentDirectory stringByAppendingString:[NSString stringWithFormat:@"/%@", [url lastPathComponent]]];

                NSLog(@"downloading file : %@ : no : %d", [documentDirectory stringByAppendingString:[NSString stringWithFormat:@"/%@", [url lastPathComponent]]], i);

                BOOL success = [data writeToFile:filename atomically:YES];

                if(!success)
                {
                   // failed to download
                }
            }];

            [completionOperation addDependency:operation];
        }
    }

    [queue addOperations:completionOperation.dependencies waitUntilFinished:NO];
    [queue addOperation:completionOperation];

    return YES;
}

但是当我们使用这段代码下载时，应用程序因内存问题而崩溃。（甚至把@autoreleasepool解决了崩溃问题）。似乎逐个下载（甚至4个图像并行）不适合这个数据量，因为它会导致我们内存问题。我只想知道从服务器下载这些数量的图像的最佳方法是什么到iPad？

提前致谢...