How to implement re-ordering of CoreData records?
core-data, ios
Solution
FetchedResultsController and its delegate are not meant to be used for user-driven model changes. See the Apple reference doc. Look for User-Driven Updates part. So if you look for some magical, one-line way, there's not such, sadly.
What you need to do is make updates in this method:
- (void)tableView:(UITableView *)tableView moveRowAtIndexPath:(NSIndexPath *)fromIndexPath toIndexPath:(NSIndexPath *)toIndexPath {
userDrivenDataModelChange = YES;
...[UPDATE THE MODEL then SAVE CONTEXT]...
userDrivenDataModelChange = NO;
}
and also prevent the notifications to do anything, as changes are already done by the user:
- (void)controllerWillChangeContent:(NSFetchedResultsController *)controller {
if (userDrivenDataModelChange) return;
...
}
- (void)controller:(NSFetchedResultsController *)controller didChangeObject:(id)anObject atIndexPath:(NSIndexPath *)indexPath forChangeType:(NSFetchedResultsChangeType)type newIndexPath:(NSIndexPath *)newIndexPath {
if (userDrivenDataModelChange) return;
...
}
- (void)controllerDidChangeContent:(NSFetchedResultsController *)controller {
if (userDrivenDataModelChange) return;
...
}
I have just implemented this in my to-do app (Quickie) and it works fine.
Problem
I am using CoreData for my iPhone app, but CoreData doesn't provide an automatic way of allowing you to reorder the records. I thought of using another column to store the order info, but using contiguous numbers for ordering index has a problem. if I am dealing with lots of data, reordering a record potentially involves updating a lot of records on the ordering info (it's sorta like changing the order of an array element) What's the best way to implement an efficient ordering scheme?