使用块在表格中加载图像 - dispatch_async返回值

Question

我环顾了SO和网页，并没有找到具体的答案。

很简单，我有一个表格从Flickr加载图片信息。我想在每个单元格的左侧显示图像的缩略图。

为了做到这一点，而不会阻塞主（UI）线程，我使用的块：

- (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，这是行不通的。因为返回单元格可能会在执行块之前发生。但是与此同时，我不能返回主队列块中的单元格，因为该块不接受返回参数。

我想避免创建一个UITableViewCell子类。

使用块的任何简单的答案？

感谢 九巴

Answer 1

用做cell.imageView.image = downloadedThumbImage;的问题是，这个小区现在可以重复使用，用于另一行。

相反，你需要更新的具体indexPath当前小区（indexPath将是相同的）

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

或者有时候我要做的就是更新模型，然后在特定indexPath重装电池：

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; 
}