MVVM CoreData FetchedReusltsController

Question

I am trying to start using MVVM with Objective-c but I get some problems with CoreData. I don't know who should handle the fetchedResultsControllerDelegate methods. The viewModel or the viewController?

I think that the viewModel should handle it, but I see too much code to do the same.

Answer 1

Let's say you have an edit button in a view. Edit button is hidden when there are no objects in fetchResultsController and you want to show it when any objects are added.

If you implement NSFetchedResultsControllerDelegate in viewController:

• ViewController gets notifications about changes of model
• ViewController must notify ViewModel that something happened with the model
• ViewModel has to change value of the editButtonShown property
• ViewModel must notify the viewController to update the view

If you implement NSFetchedResultsControllerDelegate in viewModel:

• model notifies the viewModel about the change
• viewModel changes the editButtonShown property and sends notification to viewController

If you implement these methods in viewController then it gets reference to the model and this should not happened in MVVM

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

I think that correct solution is to implement the NSFetchedResultsControllerDelegatein ViewModel. Then you should have similar protocol to notify the viewController to update the view, but it should work with viewModels, not with the models