Update UITableViewCell when lazy image is done downloading...

Question

I am having some issues with updating my UITableViewCells after asynchronously downloading the cell image. I am using custom UITableViewCells like so:

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

    if (cell == nil) {
        cell = [[[MainCustomCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"productCell"] autorelease];
    }
}

I have a class that is the UITableViewCell and then a class that does all the drawing. It's the sample code from Apple called "AdvancedTableViewCells" and I am doing composite.

I have images in my cells, which I download asynchronously using Apple's sample code called "LazyTableImages". Here's the delegate that should be updating the cell:

- (void)coverImageDidLoad:(NSIndexPath *)indexPath {
    CoverImageAsyncLoader *coverImageAsyncLoader = [imageDownloadsInProgress objectForKey:indexPath];
    if (coverImageAsyncLoader != nil) {
        MainTableViewCell *cell = (MainTableViewCell *)[self.tableView cellForRowAtIndexPath:coverImageAsyncLoader.indexPathInTableView];

        // Display the newly loaded image
  if (coverImageAsyncLoader.products.coverImage != nil) {
   cell.productCover = coverImageAsyncLoader.products.coverImage;
  } else {
   cell.productCover = blankCoverImage;
  }
    }
}

But nothing happens. A friend tells me I cannot update the UI from a background thread, but since I am doing it through a delegate I am not sure why it isn't updating. I have tried:

 [cell setNeedsDisplay];
 [cell.contentView setNeedsDisplay];

and also to set cell as:

 cell = [[[MainCustomCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"productCell"] autorelease];

and then update the cell's display.

But nothing works :( I can post more code, but the post is already a bit long.

Answer 1

If you know the index path's row and section to your cell in question (e.g. myCellIndexPath.section and myCellIndexPath.row), take a look at reloading a UITableView cell with the table view's -reloadRowsAtIndexPaths:withRowAnimation: method.

For example:

[tableView beginUpdates];
NSUInteger _path[2] = {myCellIndexPath.section, myCellIndexPath.row};
NSIndexPath *_indexPath = [[NSIndexPath alloc] initWithIndexes:_path length:2];
NSArray *_indexPaths = [[NSArray alloc] initWithObjects:_indexPath, nil];
[_indexPath release];
[tableView reloadRowsAtIndexPaths:_indexPaths withRowAnimation:UITableViewRowAnimationRight];
[_indexPaths release];
[tableView endUpdates];

As this updates the UI, you will want this in a method that is run on the main thread.

There are other row animations, depending on taste (UITableViewRowAnimationFade, UITableViewRowAnimationNone, etc.). Search your help on the UITableViewRowAnimation struct.