fetchedResultsController 返回 nil

Question

尝试创建简单的应用程序以将数据从核心数据显示到表视图。代码构建成功 - 每次加载时都会添加新条目，目前超过 20 个 - 但是在尝试构建表时它会崩溃。

这里有 ViewController：

import UIKit
import CoreData

class ViewController: UIViewController, UITableViewDataSource, UITableViewDelegate  {

     @IBOutlet weak var habitTable: UITableView!

    override func viewDidLoad() {
        super.viewDidLoad()

        let habitClassName:String = String(describing: Habits.self)

        let habit:Habits = NSEntityDescription.insertNewObject(forEntityName: habitClassName, into: DatabaseController.getContext()) as! Habits

        habit.habitsPic = ":("
        habit.positive = false
        habit.theHabit = "do bad"

        DatabaseController.saveContext()

        let fetchRequest:NSFetchRequest<Habits> = Habits.fetchRequest()

        do{
            let searchResults = try DatabaseController.getContext().fetch(fetchRequest)
            print("number of results: \(searchResults.count)")

        }
        catch{
            print("Error \(error)")
        }

    }

    //CONSTRUCT TABLE VIEW
    func numberOfSectionsInTableView(tableView: UITableView)-> Int {
        return fetchedResultsController.sections!.count
    }

    func tableView(_ tableView: UITableView, numberOfRowsInSection section: Int) -> Int {
        guard let sections = fetchedResultsController.sections else {
            fatalError("No sections in fetchedResultsController")
        }
        let sectionInfo = sections[section]
        return sectionInfo.numberOfObjects
    }

    func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath)-> UITableViewCell {
        let cell = tableView.dequeueReusableCell(withIdentifier: "positiveCell", for: indexPath as IndexPath)
        let habit = fetchedResultsController.object(at: indexPath as IndexPath) as! Habits

        cell.textLabel?.text = habit.theHabit
        cell.detailTextLabel?.text = habit.habitsPic

        return cell
    }


    override func didReceiveMemoryWarning() {
        super.didReceiveMemoryWarning()
        // Dispose of any resources that can be recreated.
    }


    //FOR DISPLAYING DATA IN TABLE
    var fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult>!

    func initializeFetchedResultsController() {
        let request = NSFetchRequest<NSFetchRequestResult>(entityName: "Habits")
        let departmentSort = NSSortDescriptor(key: "theHabit", ascending: true)
        let lastNameSort = NSSortDescriptor(key: "positive", ascending: true)
        request.sortDescriptors = [departmentSort, lastNameSort]


        let moc = DatabaseController.getContext()
        fetchedResultsController = NSFetchedResultsController(fetchRequest: request, managedObjectContext: moc, sectionNameKeyPath: nil, cacheName: nil)
        fetchedResultsController.delegate = (self as! NSFetchedResultsControllerDelegate)

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

}

和数据库控制器：

import Foundation
import CoreData


class DatabaseController {
    private init(){

    }

    class func getContext() -> NSManagedObjectContext{
        return DatabaseController.persistentContainer.viewContext
    }


    let container = NSPersistentContainer(name: "ada")
    container.loadPersistentStores(completionHandler: { (storeDescription, error) in
        if let error = error as NSError? {

            fatalError("Unresolved error \(error), \(error.userInfo)")
        }
    })
    return container
}()

// MARK: - Core Data Saving support

class func saveContext () {
    let context = persistentContainer.viewContext
    if context.hasChanges {
        do {
            try context.save()
        } catch {
            let nserror = error as NSError
            fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
        }
    }
}
}

它不会构建表格视图，因为 fetchedResultsController 返回 nil。我认为这是“moc”变量的问题，但是什么？

编辑

错误： 致命错误：在展开可选值时意外发现 nil

行： 守卫让sections = fetchedResultsController.sections else {

Answer 1

您将获取的结果控制器声明为隐式展开的可选：

var fetchedResultsController: NSFetchedResultsController<NSFetchRequestResult>!

它的初始值为nil。

您还有一个名为initializeFetchedResultsController 的方法，它为该属性分配一个值，但您永远不会调用该方法。在评论中你说你在这段代码中调用它：

guard let sections = fetchedResultsController.sections else {
    fatalError("No sections in fetchedResultsController")
}

但该代码从未提及initializeFetchedResultsController 方法，因此它从未调用该方法。

你有一个可选变量，但你从来没有给它赋值。所以它永远不会获得价值。所以它是零。这不是核心数据问题，而是基本的 Swift。除非您给属性赋值，否则属性不会获得值，而您并没有这样做。