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我在 SO 和网络上环顾四周，并没有找到具体的答案。

很简单，我有一个从 Flickr 加载图像信息的表格。我想在每个单元格的左侧显示图像的缩略图。

为了在不阻塞主（UI）线程的情况下做到这一点，我使用了块：

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
  static NSString *CellIdentifier = @"top50places";
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

  //getting the selected row image 
  NSDictionary* currentImageDictionary=[self.topfifty objectAtIndex:indexPath.row];//topFifty is an array of image dictionaries


  //creating the download queue 
  dispatch_queue_t downloadQueue=dispatch_queue_create("thumbnailImage", NULL);

  dispatch_async(downloadQueue, ^{

    UIImage *downloadedThumbImage=[self getImage:currentImageDictionary] ;

    //Need to go back to the main thread since this is UI related
    dispatch_async(dispatch_get_main_queue(), ^{

        cell.imageView.image = downloadedThumbImage ;

    });

  });

  dispatch_release(downloadQueue);

  return cell;

}

现在这行不通了。因为返回单元格可能会在执行块之前发生。但同时，我无法返回主队列块中的单元格，因为该块不接受返回参数。

我想避免创建 UITableViewCell 子类。

任何使用块的简单答案？

谢谢九巴

4

1 回答 1

5

这样做的问题cell.imageView.image = downloadedThumbImage;是这个单元格现在可能被重用并用于另一行。

相反，您需要更新特定 indexPath 中的当前单元格（indexPath 将相同） 

UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
cell.imageView.image = image;

或者有时我所做的是更新模型，然后在特定的 indexPath 处重新加载单元格： 

myModel.image = downloadedThumbImage;
[self.tableView reloadRowsAtIndexPaths:[NSArray arrayWithObject:indexPath]
                                      withRowAnimation:UITableViewRowAnimationNone];  


- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
  static NSString *CellIdentifier = @"top50places";
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:CellIdentifier];

  //getting the selected row image 
  NSDictionary* currentImageDictionary=[self.topfifty objectAtIndex:indexPath.row];//topFifty is an array of image dictionaries

  UIImage *currentImage = [currentImageDictionary objectForKey:@"image"];

  if (currentImage) { 
      // we already fetched the image, so we just set it to the cell's image view
      cell.imageView.image = currentImage;
  }
  else {
      // we don't have the image, so we need to fetch it from the server  

      // In the meantime, we can set some place holder image
      UIImage *palceholderImage = [UIImage imageNamed:@"placeholder.png"];
      cell.imageView.image = palceholderImage;

      // set the placeholder as the current image to your model, so you won't 
      // download the image multiple times (can happen if you reload this cell while 
      // download is in progress)
      [currentImageDictionary setObject:palceholderImage forKey:@"image"];

      // then download the image
      // creating the download queue 
      dispatch_queue_t downloadQueue=dispatch_queue_create("thumbnailImage", NULL);

      dispatch_async(downloadQueue, ^{
         UIImage *downloadedThumbImage=[self getImage:currentImageDictionary] ;

         //Need to go back to the main thread since this is UI related
         dispatch_async(dispatch_get_main_queue(), ^{
                // store the downloaded image in your model
                [currentImageDictionary setObject:image forKey:@"image"];

                // update UI
                UITableViewCell *cell = [tableView cellForRowAtIndexPath:indexPath];
                cell.imageView.image = image;    
         });
      });
      dispatch_release(downloadQueue);
  }

  return cell;
}
于 2012-08-09T13:16:35.837 回答