使用GCD加载异步UITableViewCell图像

Question

我目前正在尝试加载Flickr Photo的UITableView列表(cs193p iOS Stanford,作业5).为了避免UI阻塞事件,我将每个单元的缩略图下载推迟到不同的队列(但是在主队列中更新UI).此代码不会异步加载图像,但是一旦我单击UITableViewCell行,就会添加缩略图.(见下面的截图).知道我做错了什么吗？

PS:我已经看过其他一些stackoverflow问题和Apple的LazyTableImages示例,但我仍然相信这是达到预期结果的最简洁方法.

谢谢!

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

    // Configure the cell
    NSDictionary *photo = [self.photoList objectAtIndex:indexPath.row];


    if (photo != nil) {
        if ([[photo objectForKey:@"title"] length] > 0) {
            cell.textLabel.text = [photo objectForKey:@"title"];
        } else if ([[[photo objectForKey:@"description"] objectForKey:@"_content"] length] > 0) {
            cell.textLabel.text = [[photo objectForKey:@"description"] objectForKey:@"_content"];
        } else {
            cell.textLabel.text = @"Unknown";
        }
    }
    cell.imageView.image = [[UIImage alloc] initWithCIImage:nil];

    // Fetch using GCD
    dispatch_queue_t downloadThumbnailQueue = dispatch_queue_create("Get Photo Thumbnail", NULL);
    dispatch_async(downloadThumbnailQueue, ^{
        UIImage *image = [self getTopPlacePhotoThumbnail:photo];
        dispatch_async(dispatch_get_main_queue(), ^{
            if ([self.tableView.visibleCells containsObject:cell]) {
                [cell.imageView setImage:image];
            }
        });
    });
    dispatch_release(downloadThumbnailQueue);

    return cell;
}

点击一行之前

选择行后

更新:对于那些感兴趣的人,这是我使用的最终代码:

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

  // Configure the cell
  NSDictionary *photo = [self.photoList objectAtIndex:indexPath.row];

  if (photo != nil) {
    if ([[photo objectForKey:@"title"] length] > 0) {
        cell.textLabel.text = [photo objectForKey:@"title"];
    } else if ([[[photo objectForKey:@"description"] objectForKey:@"_content"] length] > 0) {
        cell.textLabel.text = [[photo objectForKey:@"description"] objectForKey:@"_content"];
    } else {
        cell.textLabel.text = @"Unknown";
    }
  }
  cell.imageView.image = [[UIImage alloc] initWithCIImage:nil];

  // Fetch using GCD
  dispatch_queue_t downloadThumbnailQueue = dispatch_queue_create("Get Photo Thumbnail", NULL);
  dispatch_async(downloadThumbnailQueue, ^{
    UIImage *image = [self getTopPlacePhotoThumbnail:photo];
    dispatch_async(dispatch_get_main_queue(), ^{
        UITableViewCell *cellToUpdate = [self.tableView cellForRowAtIndexPath:indexPath]; // create a copy of the cell to avoid keeping a strong pointer to "cell" since that one may have been reused by the time the block is ready to update it. 
        if (cellToUpdate != nil) {
            [cellToUpdate.imageView setImage:image];
            [cellToUpdate setNeedsLayout];
        }
    });
  });
  dispatch_release(downloadThumbnailQueue);

  return cell;
}

Answer 1

您应该setNeedsLayout在设置缩略图后调用单元格.

来自Apple Doc:

"当你想调整视图的子视图的布局时,在应用程序的主线程上调用此方法.此方法记录请求并立即返回.因为此方法不强制立即更新,而是等待下一次更新在更新任何视图之前,您可以使用它来使多个视图的布局无效.这种行为允许您将所有布局更新合并到一个更新周期,这通常会提高性能."