ios - 下载缩略图后重新加载 UItableViewCell

Question

我有一个应用程序，在其中的 tableViewCell 中下载图像并将其设置为缩略图。我的问题是，我可以看到缩略图刷新的唯一方法是单击另一个选项卡，然后返回到 tableViewCell 所在的选项卡。下面是设置缩略图的代码。

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
    static NSString *cellIdentifier = @"Cell";
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:cellIdentifier forIndexPath:indexPath];
    PFObject *message = [self.messages objectAtIndex:indexPath.row];
    cell.textLabel.text = [message objectForKey:@"username"];
    cell.imageView.image = [UIImage imageNamed:@"Treehouse.png"];
    PFFile *thumbnail = nil;

   if (thumbnailArray.count > 0){
    thumbnail = [[thumbnailArray objectAtIndex:indexPath.row] objectForKey:@"imageFile"];
   dispatch_async(backgroundQueue, ^{

           NSURL *thumbnailURL = [[NSURL alloc]initWithString:thumbnail.url];
           NSData *thumbnailData = [NSData dataWithContentsOfURL:thumbnailURL];

          [[cell imageView]setImage:[UIImage imageWithData:thumbnailData]];
       [tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone];


   });
   }


    return cell;
}

Answer 1

问题是.,

1. 单元格已加载。
2. 图像下载被触发。
3. 下载图像后，将重新加载单元格。
4. 图像再次下载。
5. 没有图像缓存或加载图像的逻辑

声明一个 NSMutableDictionary @property (strong) NSMutableDictionary *imagesDictionary; 并实例化它 self.imagesDictionary = [NSMutableDictionary dictionary];

在索引路径行的单元格中，在下载之前检查缓存是否存在。

PFFile *thumbnail = nil;

if (thumbnailArray.count > 0)
{
 //If image is present in cache load from cache
UIImage *image = [self.imagesDictionary objectForKey:indexPath];
if (image)
{
    [cell.imageView setImage:image];
}
else
{
    //If Image is not present fire a download
     thumbnail = [[thumbnailArray objectAtIndex:indexPath.row] objectForKey:@"imageFile"];
     dispatch_async(backgroundQueue, ^{

        NSURL *thumbnailURL = [[NSURL alloc]initWithString:thumbnail.url];
        NSData *thumbnailData = [NSData dataWithContentsOfURL:thumbnailURL];

         //Cahce it to your image dictionary
        [self.imagesDictionary setObject:[UIImage imageWithData:thumbnailData] forKey:indexPath];
         //Reload the dictionary
        [tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationNone];


    });


}