Question

我有一个应用程序，其中在tableViewCell中下载图像而不是设置为缩略图。我的问题是，我可以看到缩略图图像刷新的唯一方法是，如果我单击另一个选项卡，然后返回到正在保持tableViewCell的选项卡。以下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。，

正在加载单元格。
触发图像下载。
下载图像后，将重新加载单元格。
再次下载图片。
没有用于加载图像的图像或逻辑的缓存

声明NSMutableDictionary @property (strong) NSMutableDictionary *imagesDictionary;并将其实例化self.imagesDictionary = [NSMutableDictionary dictionary];

在索引路径的Row的单元格中，在下载之前检查是否存在缓存。

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];


    });


}

Answer 2

我建议您使用AFNetworkings类别UIImage + AFNetworking，它允许您只管理[imageView setImageWithURL：（NSURL）]来管理后台的所有内容。您不必担心加载图像，缓存图像或类似的东西。它会为你缓存它。

https://github.com/AFNetworking/AFNetworking

Answer 3

以下代码有助于解决我的问题

- (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reloadInputViews，我的问题就解决了。

缩略图下载后重新加载UItableViewCell

3 个答案: