每当对应于该单元格的NSFetchedResultsController中的对象发生更改时，如何使单元格的外观更新？

Question

I'm using Core Data in my app, and with it NSFetchedResultsController to use it in a table view. I can do various things to the cell/data, like swiping it left or right to delete or mark the cell as read respectively (this also deletes the data).

In cases where I say, mark the cell as read (which also sets the corresponding Core Data object's isRead attribute to YES ), I want the cell to update visually to indicate this. The visual update is lightening the cell to look read (like what a read vs an unread email looks like).

How do I have this happen? (Update the cell when the object that corresponds to it has an attribute change, that is.)

I tried in this delegate method for NSFetchedResultsController :

- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {

    UITableView *tableView = self.tableView;

    switch(type) {

        case NSFetchedResultsChangeInsert:
            [tableView insertRowsAtIndexPaths:[NSArray arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeDelete:
            [tableView deleteRowsAtIndexPaths:[NSArray arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;

        case NSFetchedResultsChangeUpdate:
            [self configureCell:(ArticleCell *)[tableView cellForRowAtIndexPath:indexPath] atIndexPath:indexPath];
            break;

        case NSFetchedResultsChangeMove:
            [tableView deleteRowsAtIndexPaths:[NSArray
                                               arrayWithObject:indexPath] withRowAnimation:UITableViewRowAnimationFade];
            [tableView insertRowsAtIndexPaths:[NSArray
                                               arrayWithObject:newIndexPath] withRowAnimation:UITableViewRowAnimationFade];
            break;
    }
}

And NSFetchedResultsChangeUpdate is the case that gets selected, so configureCell: is then called, which looks like this:

- (void)configureCell:(ArticleCell *)cell atIndexPath:(NSIndexPath *)indexPath
{
    Article *article = [self.fetchedResultsController objectAtIndexPath:indexPath];

    cell.article = article;
}

Which invokes my UITableViewCell subclass, ArticleCell 's custom article property setter. In that method I basically start off with:

- (void)setArticle:(Article *)article {
    if (_article != article) {

Comparing the new article to be set to the existing one for the cell (so it only updates if there's a change). However, the code inside this if statement is never called! It hops right past and exists the method. Within that if statement I have all the custom code to set what the cell looks like based on the properties of that article ( isRead , kind , etc.)

How should I be handling this? I just want that whenever there is an update to the object in that corresponds to that cell, the cell reflects the update. Be it a change of read state, or whatever.

Answer 1

In the if statement you're comparing two references to the same object, so it will always be true when the setter is called from a change of type NSFetchedResultsChangeUpdate .

You don't really need the if, just trust the code in the controller:didChangeObject:atIndexPath:forChangeType:newIndexPath: method and always update the cell if the method requests it.

Answer 2

It doesn't go inside that if because article object is the same. However its properties might be updated.

You might want to change the if or introduce some other method like -[ArticleCell update] that checks if any of the properties that affect the appearance of the cell are changed.