Ch5. NSFetchedResultsController

NSFetchedResultsController의 세 가지 장점

1. Section
2. Cache
3. Monitoring

이전 챕터에서 봤듯, CoreData는 UITableView와 잘 어울린다.

NSFetchedResultsController를 사용하면 UITableView와의 조합을 더 잘 활용할 수 있다.

NSFetchedResultsController는 NSFetchRequest를 감싸며 FetchResult를 내부에 갖고 있다. 그래서 fetch하는 메소드의 리턴타입이 Void다.

lazy var fetchedResultsController: NSFetchedResultsController<Team> = {
    let fetchRequest: NSFetchRequest<Team> = Team.fetchRequest()
    let zoneSort = NSSortDescriptor(key: #keyPath(Team.qualifyingZone), ascending: true)
    let scoreSort = NSSortDescriptor(key: #keyPath(Team.wins), ascending: false)
    let nameSort = NSSortDescriptor(key: #keyPath(Team.teamName), ascending: true)
    fetchRequest.sortDescriptors = [zoneSort, scoreSort, nameSort]
    
    let fetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest,
                                                              managedObjectContext: coreDataStack.managedContext,
                                                              sectionNameKeyPath: #keyPath(Team.qualifyingZone),
                                                              cacheName: "worldCup")
    return fetchedResultsController
  }()

// 리턴값을 사용하지 않음
do {
  try fetchedResultsController.performFetch()
} catch let error as NSError {
  print("Fetching error: \\(error), \\(error.userInfo)")
}

// fetchedResultsController는 내부에 sections를 갖고 있음
// open var sections: [NSFetchedResultsSectionInfo]? { get }
func numberOfSections(in tableView: UITableView) -> Int {
  fetchedResultsController.sections?.count ?? 0
}

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

Modifying data

NSFetchRequest 쓸 때랑 비슷하다.

func tableView(_ tableView: UITableView,
               didSelectRowAt indexPath: IndexPath) {
  let team = fetchedResultsController.object(at: indexPath)
  team.wins += 1
  coreDataStack.saveContext()
}

Cache

on-disk 캐시다. 앱을 재실행해도 캐시의 혜택을 볼 수 있다.

NSFetchedResultsController를 선언할 때, cacheName을 지정할 수 있음.

캐시는 변화에 민감하기 때문에, 변경사항이 있다면 캐시 이름을 바꾸거나 deleteCache(withName:)를 사용하여 캐시를 삭제하자.

Grouping a set of fetched results into sections is an expensive operation. Avoid having to compute sections multiple times by specifying a cache name on your fetched results controller.

Monitoring changes

NSFetchedResultsControllerDelegate를 구현하여 변화를 감지할 수 있다. 하지만 NSFetchedResultsController를 생성할 때 사용된 ManagedObjectContext에서 발생한 변화만 감지가능하다.

extension ViewController: NSFetchedResultsControllerDelegate {
  func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    tableView.reloadData()
  }
}

extension ViewController: NSFetchedResultsControllerDelegate {
  func controllerWillChangeContent(_ controller:
    NSFetchedResultsController<NSFetchRequestResult>) {
      tableView.beginUpdates()
  }
  func controller(_ controller:
                  NSFetchedResultsController<NSFetchRequestResult>,
                  didChange anObject: Any,
                  at indexPath: IndexPath?,
                  for type: NSFetchedResultsChangeType,
                  newIndexPath: IndexPath?) {
    switch type {
    case .insert:
      tableView.insertRows(at: [newIndexPath!], with: .automatic)
    case .delete:
      tableView.deleteRows(at: [indexPath!], with: .automatic)
    case .update:
      let cell = tableView.cellForRow(at: indexPath!) as! TeamCell
      configure(cell: cell, for: indexPath!)
    case .move:
      tableView.deleteRows(at: [indexPath!], with: .automatic)
      tableView.insertRows(at: [newIndexPath!], with: .automatic)
    @unknown default:
      print("Unexpected NSFetchedResultsChangeType")
    }
  }
  
  func controller(_ controller:
                  NSFetchedResultsController<NSFetchRequestResult>,
                  didChange sectionInfo: NSFetchedResultsSectionInfo,
                  atSectionIndex sectionIndex: Int,
                  for type: NSFetchedResultsChangeType) {
    let indexSet = IndexSet(integer: sectionIndex)
    switch type {
    case .insert:
      tableView.insertSections(indexSet, with: .automatic)
    case .delete:
      tableView.deleteSections(indexSet, with: .automatic)
    default: break
    }
  }
  
  func controllerDidChangeContent(_ controller:
    NSFetchedResultsController<NSFetchRequestResult>) {
      tableView.endUpdates()
  }
}

Inserting an underdog

let team = Team(context: self.coreDataStack.managedContext)
team.teamName = nameTextField.text
team.qualifyingZone = zoneTextField.text
team.imageName = "wenderland-flag"
self.coreDataStack.saveContext()