iOS: Updating tableViewCell infinite loop - ios

Question

This is the problem: I update my tableview after saving and fetching an thumbnail to Core Data and then I tell the cell to update itself - so it can show a thumbnail when the image has been loaded into Core Data. I Use two different threads as Core Data is not threadsafe and all GUI elements of course needs to happen in the main thread.
But this whole method just keep looping forever, and what is causing it is when I reload the thread:
[self.tableView reloadRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationNone];
Why? and how do I fix this?
- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
{
UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:#"Photo"];
Photo *photo = [self.fetchedResultsController objectAtIndexPath:indexPath];
cell.textLabel.text = photo.title;
cell.detailTextLabel.text = photo.subtitle;
NSLog(#"Context %#", self.photographer.managedObjectContext);
[self.photographer.managedObjectContext performBlock:^{
[Photo setThumbnailForPhoto:photo];
dispatch_async(dispatch_get_main_queue(), ^{
cell.imageView.image = [UIImage imageWithData:photo.thumbnail];
[self.tableView reloadRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationNone];
});
}];
return cell;
}

Answer 1

The infinite loop is caused by calling the cellForRowAtIndexPath: again after the fetch is loaded and the reload on a certain cell is called.
A reload will force the cellForRowAtIndexPath: to be called again... and in your case again and again to infinity.
The solution is simple... Do not reload your cell in the cellForRowAtIndexPath but rather in a callback-method of the fetchrequest. Then reload it there rather then the creation of the cell.
Rather do not load the image inside the cellForRowAtIndexpath: at all.
Whenever your table is instantiated make a method that loops over your datasource and get the respective cell for each item. Then load an image for each cell you deem needed. And reload the cell whenever the fetching of the item is done (for instance a callback method).
If you do want the image to be loaded inside the creation of the cell as you have done now (Although I don't think that is the proper way to do it). You can surround the whole performBlock: with an if-statement checking if the image has already been set or not.

Answer 2

As already been said by others, there should be no need to call reloadRowsAtIndexPaths when the image has been loaded. Assigning a new image to cell.imageView.image is sufficient.
But there is another problem with your code. I assume that self.photographer.managedObjectContext is a managed object context of the "private concurrency type", so that performBlock executes on a background thread. (Otherwise there would be no need to use dispatch_async(dispatch_get_main_queue(), ...).)
So performBlock: executes the code asynchronously, on a background thread. (Which is good because the UI is not blocked.) But when the image has been fetched after some time, the cell might have been reused for a different row. (This happens if you scroll the table view so that the row becomes invisible while the image is fetched.)
Therefore, you have to check if the cell is still at the same position in the tableview:
[self.photographer.managedObjectContext performBlock:^{
[Photo setThumbnailForPhoto:photo];
dispatch_async(dispatch_get_main_queue(), ^{
if ([[tableView indexPathForCell:cell] isEqualTo:indexPath]) {
cell.imageView.image = [UIImage imageWithData:photo.thumbnail];
}
});
}];

Answer 3

I'd replace the following code:
cell.imageView.image = [UIImage imageWithData:photo.thumbnail];
[self.tableView reloadRowsAtIndexPaths:#[indexPath] withRowAnimation:UITableViewRowAnimationNone];
With:
UITableViewCell *blockCell = [tableView cellForRowAtIndexPath:indexPath];
blockCell.imageView.image = [UIImage imageWithData:photo.thumbnail];
[blockCell setNeedsLayout];
This solves the issue of reloading recursively, as well as the possibility that cell has gotten reused in the interim. I'd also check for whether or not you've loaded the photo data already and not update if so.
Since you are using an NSFetchedResultsController though, you may be better off going with Totomus Maximus' answer, if the delegate callbacks are already informing you when the photo data updates. This way just updates the image view and wouldn't update any other information that the Photo update method could change.