static NSString *CellIdentifier = @"Cell";        //重用单元格    JKCallbacksTableViewCell *cell = (JKCallbacksTableViewCell *)[tableView dequeueReusableCellWithIdentifier:CellIdentifier];    if (cell == nil) {        cell = [[[JKCallbacksTableViewCell alloc] initWithStyle:UITableViewCellStyleDefault                                                reuseIdentifier:CellIdentifier] autorelease];    }        //获得要加载的文件名    NSString *imageFilename = [imageArray objectAtIndex:[indexPath row]];    NSString *imagePath = [imageFolder stringByAppendingPathComponent:imageFilename];        //图片名称    [[cell textLabel] setText:imageFilename];    [cell setSelectionStyle:UITableViewCellSelectionStyleNone];        // If we already have an image cached for the cell, use that. Otherwise we need to go into the     // background and generate it.    UIImage *image = [imageCache objectForKey:imageFilename];    if (image) {        //图片缓存不存在，加载图片        [[cell imageView] setImage:image];    } else {            dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0ul);                // Get the height of the cell to pass to the block.        CGFloat cellHeight = [tableView rowHeight];                // Now, we can’t cancel a block once it begins, so we’ll use associated objects and compare        // index paths to see if we should continue once we have a resized image.        // 创建关联对象        /**         * @param cell                          源对象         * @param kIndexPathAssociationKey      关联key         * @param indexPath                     关联的值         * @param OBJC_ASSOCIATION_RETAIN       关联方式         */        objc_setAssociatedObject(cell,                                 kIndexPathAssociationKey,                                 indexPath,                                 OBJC_ASSOCIATION_RETAIN);                /*         使用关联引用，你可以对一个对象添加数据而不需要修改这个类定义，这在你没有这个类的源代码时很有用，         或者是为了二进制兼容的原因你无法修改这个对象的时候。                  关联基于一个key，所以你可以在一个对象上添加多个关联，每个使用不同的key，关联对象也可以确保被关         联的对象是否存在，至少在源对象的生命周期内（也就是说这个对象没有将被引入到垃圾回收系统的可能性）         */                dispatch_async(queue, ^{            //子线程            UIImage *image = [UIImage imageWithContentsOfFile:imagePath];                        //调整图片大小            UIImage *resizedImage = [image resizedImageWithContentMode:UIViewContentModeScaleAspectFill                                                                bounds:CGSizeMake(cellHeight, cellHeight)                                                  interpolationQuality:kCGInterpolationHigh];                        dispatch_async(dispatch_get_main_queue(), ^{                //主线程(异步操作)                                //检索关联对象                NSIndexPath *cellIndexPath =                (NSIndexPath *)objc_getAssociatedObject(cell, kIndexPathAssociationKey);                                if ([indexPath isEqual:cellIndexPath]) {                    [[cell imageView] setImage:resizedImage];                }                //根据文件名来作为缓存的key                [imageCache setObject:resizedImage forKey:imageFilename];            });        });    }