read

UPDATED 2019: Updated code in Swift 4.2

This is an updated post to Implementing NSFetchedResultsController with MagicalRecord, with these changes:

1. Code is in Swift
2. No longer using MagicalRecord

State of Magical Record

Magical Record is an awesome library that makes life much easier when you use Core Data.

However, the project is pretty stagnant.

One year ago, they started work on version 3, yet it is still not finished. And it will continue to be in Objective-C.

As such, I have deprecated Magical Record for my new projects. Instead, I use Core Data stack directly, which actually isn’t too bad after having experience studying how Magical Record (and others) work.

1. Setup NSFetchedResultsController

We use the example of a Food model, which has the attributes type and createdAt.

lazy var fetchedResultsController: NSFetchedResultsController = {
    let fetchRequest = NSFetchRequest(entityName: "Food")
    fetchRequest.fetchLimit = 100
    fetchRequest.fetchBatchSize = 20

    // Filter Food where type is breastmilk
    var predicate = NSPredicate(format: "%K == %@", "type", "breastmilk")
    fetchRequest.predicate = predicate

    // Sort by createdAt
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "createdAt", ascending: false)]

    let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: DaRecord.mainContext, sectionNameKeyPath: nil, cacheName: nil)
    frc.delegate = self
    return frc
}()

Thanks to Swift, we now can simply declare fetchedResultsController property as lazy.

2. Setup Your View Controller viewDidLoad

override func viewDidLoad() {
    super.viewDidLoad()

    do {
        try fetchedResultsController.performFetch()
    } catch {
        print("Error")
    }
}

When the view is loaded, we perform the fetch once.

3. The Delegates

There will be a 3 delegates needed (for NSFetchedResultsController and UITableView):

class YourViewController: UIViewController, NSFetchedResultsControllerDelegate, UITableViewDataSource, UITableViewDelegate

We will see how to implement them in the next 3 sections.

3a. UITableViewDataSource

UITableViewDataSource will ask for the data.

Not surprisingly, in all of the methods to implement, we will ask fetchedResultsController, which holds the fetched data.

func numberOfSectionsInTableView(tableView: UITableView) -> Int {
    if let sections = fetchedResultsController.sections {
        return sections.count
    }
    return 0
}

func tableView(tableView: UITableView, titleForHeaderInSection section: Int) -> String? {
    if let currSection = fetchedResultsController.sections?[section] {
        return currSection.name
    }
    return nil
}

func tableView(tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
    if let currSection = fetchedResultsController.sections?[section] {
        return currSection.numberOfObjects
    }
    return 0
}

func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
    let cell = tableView.dequeueReusableCellWithIdentifier("food-cell", forIndexPath: indexPath)
    configureCell(cell, atIndexPath: indexPath)
    return cell
}

// A private method to configure cell at indexPath
func configureCell(cell: UITableViewCell, atIndexPath indexPath: NSIndexPath) {
    // Configure cell with the food model
    let food = fetchedResultsController.objectAtIndexPath(indexPath) as! Food
    cell.textLabel?.text = food.type
    cell.detailTextLabel?.text = String(format: "%@", food.createdAt!)
}

3b. NSFetchedResultsControllerDelegate

Using NSFetchedResultsControllerDelegate, you can know if a model is inserted/deleted/updated/moved, then update the table view.

Quite a chunk of code that you can simply copy and paste, without any modification:

func controller(controller: NSFetchedResultsController, didChangeSection sectionInfo: NSFetchedResultsSectionInfo, atIndex sectionIndex: Int, forChangeType type: NSFetchedResultsChangeType) {
    switch(type) {
    case .Insert:
        tableView.insertSections(NSIndexSet(index: sectionIndex), withRowAnimation: UITableViewRowAnimation.Fade)
    case .Delete:
        tableView.deleteSections(NSIndexSet(index: sectionIndex), withRowAnimation: UITableViewRowAnimation.Fade)
    default:
        break
    }
}

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

func controllerWillChangeContent(controller: NSFetchedResultsController) {
    tableView.beginUpdates()
}

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

3c. UITableViewDelegate

If you require deleting, add this:

func tableView(tableView: UITableView, commitEditingStyle editingStyle: UITableViewCellEditingStyle, forRowAtIndexPath indexPath: NSIndexPath) {
    switch editingStyle {
    case .Delete:
        print("TODO: Delete model")
    case .Insert:
        print("TODO: Insert model")
    default: break
    }
}