Ios 下载缩略图图像后重新加载UItableViewCell

我有一个应用程序，在tableViewCell中下载图像，然后将其设置为缩略图。我的问题是，我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;
}

---

我建议您使用AFNetworkings类别UIImage+AFNetworking，它允许您只需执行[imageView setImageWithURL:（NSURL）]，即可在后台管理所有内容。您不必担心加载图像、缓存图像或诸如此类的事情。它将为您缓存它

问题是

单元格已加载

图像下载已启动

图像下载后，单元格将重新加载

图像将再次下载

没有缓存图像或加载图像的逻辑

声明NSMutableDictionary

@property（strong）NSMutableDictionary*imagesDictionary并实例化它
self.imagesDictionary=[NSMutableDictionary]

在索引路径行的单元格中，在下载缓存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];


    });


}

下面的代码帮助解决了我的问题

- (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];
           dispatch_sync(dispatch_get_main_queue(), ^{
               [[cell imageView]setImage:[UIImage imageWithData:thumbnailData]];
               [tableView reloadInputViews];
           });
   });
   }


    return cell;
}

问题是重新加载TableView是在一个单独的线程上调用的。一旦我将图像的设置移动到主线程并从那里调用ReloadInputView，我的问题就解决了
 
[需要单元格布局]我已放置[单元格布局需要]；在声明单元格变量之后，我的问题仍然存在。您建议我将该方法放在何处？您正在后台线程上更新UI。大禁忌。将其分派到主线程。我放入Dispatch_sync以获取主线程，然后调用[talbeView reloadInputViews]；它起作用了。改用SDWebImage。