问题
My application has two tab bars. The first one presents a list of games added on view controller and save them on the core data database. Switching on the second tab/view reads from the database and presents it inside a table view. I implemented the NSFetchedResultsControllerDelegate with a fetch method. When I add the first item to the context on the first tab and switch to second tab, FRC delegate methods (controllerWillChangeContent(_:), controller(_:didChange:at:for:newIndexPath:), controllerDidChangeContent(_:)) are not getting called and the table view is empty while I can see arrayOfGamesCount = 1. But when I add a second item, I can see all FRC delegate methods are getting call when I switch to second tab bar. And TableView display one rows while arrayOfGamesCount = 2
The first tab bar have 2 view controllers.(AddGameViewController and WelcomeViewController) AddGameVC is used to grab data from textfields and send it to welcomeVC.
import UIKit
import CoreData
class WelcomeViewController: UIViewController,SendGameDataDelegate, UIAdaptivePresentationControllerDelegate {
var games : [Game]? = []
var gamesMo: [GameMo]? = []
var gamed: GameMo?
var game : Game?
func ShouldSendGame(game: Game) {
self.game = game
print("\(game)")
games?.append(game)
}
@IBAction func endWLButton(_ sender: UIButton) {
saveDataToCoreData()
print("number of games from gamesMoCount is \(gamesMo?.count ?? 0)")
games?.removeAll()
reloadCollectionViewData()
}
func saveDataToCoreData (){
if let appDelegate = UIApplication.shared.delegate as? AppDelegate {
gamed = GameMo(context: appDelegate.persistentContainer.viewContext)
if games != nil {
for game in games! {
gamed?.goal = Int32(game.goal ?? 0 )
gamed?.rivalGoal = Int32(game.rivalGoal ?? 0)
gamed?.shot = Int32(game.shots ?? 0)
gamed?.rivalShot = Int32(game.rivalGoal ?? 0)
gamed?.rivalCorners = Int32(game.rivalsCorner ?? 0)
gamed?.corners = Int32(game.corners ?? 0)
gamesMo?.append(gamed!)
}
print("Saving data to context ....")
appDelegate.saveContext()
}
}
}
}
extension WelcomeViewController: UICollectionViewDelegate, UICollectionViewDataSource {
func collectionView(_ collectionView: UICollectionView, numberOfItemsInSection section: Int) -> Int {
return games?.count ?? 0
}
func collectionView(_ collectionView: UICollectionView, cellForItemAt indexPath: IndexPath) -> UICollectionViewCell {
if let gameIndex = games?[indexPath.row] {
let userGameScore = gameIndex.goal
let rivalGameScore = gameIndex.rivalGoal
if let cell = collectionView.dequeueReusableCell(withReuseIdentifier: "FormCell", for: indexPath) as? FormCollectionViewCell {
cell.setCell(userScores: userGameScore!, rivalScores: rivalGameScore! )
return cell
}
}
return UICollectionViewCell ()
}
}
The second tab bar have only one VC: AllWLeagueController used to display items from the the database.
import UIKit
import CoreData
class AllWLeagueController : UITableViewController {
var fetchRequestController : NSFetchedResultsController<GameMo>!
var arrayOfGamesModel : [[GameMo]]? = []
var gameMo: GameMo?
var gamesMo: [GameMo] = []
override func viewDidLoad() {
validation(object: arrayOfGamesModel)
}
override func viewWillAppear(_ animated: Bool) {
super.viewWillAppear(animated)
fetchRequest()
}
override func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
print("arrayOfGamesModelcount est \(arrayOfGamesModel?.count ?? 0)")
return arrayOfGamesModel?.count ?? 0
}
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell {
if let weekL = arrayOfGamesModel?[indexPath.row] {
if let cell = tableView.dequeueReusableCell(withIdentifier: "WL") as? AllWLeaguesTableViewCell {
let winCounts = WLManager.winCountMethod(from: weekL)
let lossCounts = WLManager.lossCountMethod(from:weekL)
cell.setOulet(win: winCounts, loss: lossCounts, rankName: rankString)
cellLayer(with: cell)
return cell
}
}
}
extension AllWLeagueController: NSFetchedResultsControllerDelegate {
func fetchRequest () {
let fetchRequest = NSFetchRequest<GameMo>(entityName: "Game")
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "win", ascending: true)]
if let appDelegate = (UIApplication.shared.delegate as? AppDelegate){
let context = appDelegate.persistentContainer.viewContext
// fetch result controller
fetchRequestController = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
fetchRequestController.delegate = self
do{
try fetchRequestController.performFetch()
if let fetchedObjects = fetchRequestController.fetchedObjects {
gamesMo = fetchedObjects
print("Fetech Request Activated")
print(gamesMo)
}
}catch{
fatalError("Failed to fetch entities: \(error)")
}
}
}
func controllerWillChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
print("TableView beginupdates")
tableView.beginUpdates()
}
func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChange anObject: Any, at indexPath: IndexPath?, for type: NSFetchedResultsChangeType, newIndexPath: IndexPath?) {
switch type {
case .insert:
if let newIndexPath = newIndexPath {
print("insert")
tableView.insertRows(at: [newIndexPath], with: .fade)
}
case .delete:
if let indexPath = indexPath {
print("delete")
tableView.deleteRows(at: [indexPath], with: .fade)
}
case .update:
if let indexPath = indexPath {
print("update")
tableView.reloadRows(at: [indexPath], with: .fade)
}
default:
tableView.reloadData()
}
if let fetchedObjects = controller.fetchedObjects {
gamesMo = fetchedObjects as! [GameMo]
print("we are about to append arrayOfGamesModel")
arrayOfGamesModel?.append(gamesMo)
}
}
func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
print("TableView endupdates")
tableView.endUpdates()
}
}
回答1:
You are making a fatal mistake. In saveDataToCoreData only one instance is created and then it's being overwritten with the game data in each iteration of the array. So your array gamesMo may contain multiple items but it's always the same instance and only one instance is saved into the context.
Replace saveDataToCoreData with
func saveDataToCoreData (){
let appDelegate = UIApplication.shared.delegate as! AppDelegate
guard let games = games else { return }
for game in games {
let newGame = GameMo(context: appDelegate.persistentContainer.viewContext)
newGame.goal = Int32(game.goal ?? 0 )
newGame.rivalGoal = Int32(game.rivalGoal ?? 0)
newGame.shot = Int32(game.shots ?? 0)
newGame.rivalShot = Int32(game.rivalGoal ?? 0)
newGame.rivalCorners = Int32(game.rivalsCorner ?? 0)
newGame.corners = Int32(game.corners ?? 0)
gamesMo.append(newGame)
}
print("Saving data to context ....")
appDelegate.saveContext()
}
Another bad practice is to create new fetch results controllers in viewWillAppear. It's highly recommended to create one controller as lazy instantiated property – as well as the managed object context – for example
lazy var context : NSManagedObjectContext = {
let appDelegate = UIApplication.shared.delegate as! AppDelegate
return appDelegate.persistentContainer.viewContext
}()
lazy var fetchRequestController : NSFetchedResultsController<GameMo> = {
let fetchRequest = NSFetchRequest<GameMo>(entityName: "Game")
fetchRequest.sortDescriptors = [NSSortDescriptor(key: "win", ascending: true)]
// fetch result controller
let frc = NSFetchedResultsController(fetchRequest: fetchRequest, managedObjectContext: context, sectionNameKeyPath: nil, cacheName: nil)
frc.delegate = self
do {
try frc.performFetch()
if let fetchedObjects = frc.fetchedObjects {
self.gamesMo = fetchedObjects
print("Fetech Request Activated")
print(gamesMo)
}
} catch{
fatalError("Failed to fetch entities: \(error)")
}
return frc
}()
Force unwrapping AppDelegate is perfectly fine. Your app won't even launch if AppDelegate was missing.
I recommend also to use less ambiguous variable names. games, gamesMo, gamed and game look very similar and can cause confusion.
来源：https://stackoverflow.com/questions/61233538/nsfetchedresultscontrollerdelegate-not-triggered-when-adding-first-item-to-db-tr