重构NSFetchedResultsController

Question

This code now sits in a UIViewController :

lazy var categoryFRC: NSFetchedResultsController<Category> = {
    let fetchRequest: NSFetchRequest<Category> = Category.fetchRequest()
    fetchRequest.sortDescriptors = [NSSortDescriptor(key: "text", ascending: true)]
    let appDelegate = (UIApplication.shared.delegate) as! AppDelegate
    let context = appDelegate.persistentContainer.viewContext
    let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
    frc.delegate = self
    do {
        try frc.performFetch()
    } catch {
        fatalError("Failed to initialize FetchedResultsController: \(error)")
    }
    return frc
}()

And this is the way I refer to the fetched categories with a for-loop:

let category = (self.categoryFRC.fetchedObjects?[index])! as Category

I would like to access the same CoreData entity from a different ViewController .

1. May I know what is the right way to restructure my code?

2. Do I create a new class?

3. And how do I handle the NSFetchedResultsControllerDelegate ?

4. Must all the ViewControllers referring to the fetchedResults be a subclass of NSFetchResultsControllerDelegate ?

Answer 1

What I've done is pull all non-view rendering/user event handling from ViewControllers and put that logic into another class. In your case you want to create a new class and put the categoryFRC logic into the new class

class DataController {
  lazy var categoryFRC: NSFetchedResultsController<Category> = {
  let fetchRequest: NSFetchRequest<Category> = Category.fetchRequest()
  fetchRequest.sortDescriptors = [NSSortDescriptor(key: "text", ascending: true)]
  let appDelegate = (UIApplication.shared.delegate) as! AppDelegate
  let context = appDelegate.persistentContainer.viewContext
  let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
  frc.delegate = self

  do {
    try frc.performFetch()
  } catch {
    fatalError("Failed to initialize FetchedResultsController: \(error)")
  }

  return frc
  }()
}

Then in the ViewController store an instance of DataController:

class ViewController {
  var dataController : DataController!

  func viewDidLoad() {
    dataController = DataController()
    dataController.categoryFRC.delegate = <Class that adopts NSFetchedResultsControllerDelegate>
  }
}

Other observations:

• Create another class that adopts NSFetchedResultsControllerDelegate. Then set the categoryFRC.delegate to the new class. This way the new class is responsible for the FRC delegate methods, not the view controller.

• pull the do/try/catch around the frc.performFetch() from the lazy categoryFRC definition. Are you sure that you want to perform a fetch each time you access categoryFRC? I'd put the do/try/catch logic into a new function.

• Look at your for loop, what happens if fetchedObjects? is nil? Wouldn't you crash because you are then unwrapping a nil value?