Updating sections with NSFetchedResultsController

Question

I have an entity named ToDoItem. Which has three properties:

@NSManaged var toDoString: String?
@NSManaged var creationDate: NSDate?
@NSManaged var isComplete: NSNumber?

I'm trying to create a NSFetchedResultsController that will display with two section depending on the isComplete variable. I'm able to do that successfully buy doing this:

lazy var fetchedResultsController: NSFetchedResultsController = {

    let questionAnswerFetchRequest = NSFetchRequest(entityName: "ToDoItem")

    let questionAnswerFetchSortDescriptor = NSSortDescriptor(key: "creationDate", ascending: false)
    questionAnswerFetchRequest.sortDescriptors = [questionAnswerFetchSortDescriptor]

    let frc = NSFetchedResultsController(
        fetchRequest: questionAnswerFetchRequest,
        managedObjectContext: (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext,
        sectionNameKeyPath:"isComplete",
        cacheName: nil)

    frc.delegate = self
    return frc
}()

I've also implemented the delete methods like this:

//
// MARK: Fetched Results Controller Delegate Methods
//
func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

func controllerDidChangeContent(controller: NSFetchedResultsController) {
    tableView.endUpdates()
}

func controller(controller: NSFetchedResultsController,
                didChange sectionInfo: NSFetchedResultsSectionInfo,
                          atSectionIndex sectionIndex: Int,
                                         for type: NSFetchedResultsChangeType) {

    switch (type) {
    case .Insert:
        self.tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        break
    case .Delete:
        self.tableView.deleteSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        break
    default:
        break
    }

}

func controller(controller: NSFetchedResultsController, didChangeObject anObject: AnyObject, atIndexPath indexPath: NSIndexPath?, forChangeType type: NSFetchedResultsChangeType, newIndexPath: NSIndexPath?) {
    switch (type) {
    case .Insert:
        if let indexPath = newIndexPath {
            tableView.insertRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }
        break;
    case .Delete:
        if let indexPath = indexPath {
            tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }
        break;
    case .Update:
        if let indexPath = indexPath {
            tableView.reloadRowsAtIndexPaths([indexPath], withRowAnimation: .Automatic)
        }
        break;
    case .Move:
        if let indexPath = indexPath {
            tableView.deleteRowsAtIndexPaths([indexPath], withRowAnimation: .Fade)
        }

        if let newIndexPath = newIndexPath {
            tableView.insertRowsAtIndexPaths([newIndexPath], withRowAnimation: .Fade)
        }
        break;
    }
}

The problem is that when I update the isComplete property I get this error:

CoreData: error: Serious application error. An exception was caught from the delegate of NSFetchedResultsController during a call to -controllerDidChangeContent:. Invalid update: invalid number of sections. The number of sections contained in the table view after the update (1) must be equal to the number of sections contained in the table view before the update (0), plus or minus the number of sections inserted or deleted (0 inserted, 0 deleted). with userInfo (null)

Which I understand what this is saying, I just don't understand why it causing that error. I've implemented the delegate methods per the documentation.....

---

I've updated my NSFetchedResultsController creation to add another sortDescriptor per @wain's answer like so:

lazy var fetchedResultsController: NSFetchedResultsController = {

        let questionAnswerFetchRequest = NSFetchRequest(entityName: "ToDoItem")

        let isCompleteSortDescriptor   = NSSortDescriptor(key: "isComplete", ascending: false)
        let creationDateSortDescriptor = NSSortDescriptor(key: "creationDate", ascending: false)

        questionAnswerFetchRequest.sortDescriptors = [isCompleteSortDescriptor, creationDateSortDescriptor]

        let frc = NSFetchedResultsController(
            fetchRequest: questionAnswerFetchRequest,
            managedObjectContext: (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext,
            sectionNameKeyPath:"isComplete",
            cacheName: nil)

        frc.delegate = self
        return frc
    }()

I've added some breakpoints and it turns out that this delegate method is never being called:

func controller(controller: NSFetchedResultsController,
                didChange sectionInfo: NSFetchedResultsSectionInfo,
                          atSectionIndex sectionIndex: Int,
                                         for type: NSFetchedResultsChangeType) {

    switch (type) {
    case .Insert:
        self.tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        break
    case .Delete:
        self.tableView.deleteSections(NSIndexSet(index: sectionIndex), withRowAnimation: .Fade)
        break
    default:
        break
    }
}

Which is leading to the same error as posted above. Any ideas on why that would never be called??

Answer 1

You need to add a sort descriptor for isComplete. It should be the first sort descriptor, then have your date one.

Basically, the sort and the sections need to be compatible, and yours currently aren't.