ios - NSFetchedResultsController 'didChange' 使用新的 CollectionDiffing 会导致奇怪的更改操作，从而导致崩溃

Question

演示崩溃的示例项目:https://github.com/d3mueller/TestProjectFetchedResultsController

---

我有一个简单的 CoreData 设置，其中有两个 entities :

• City ，具有一对多关系 incidentRoads至Road (删除城市时设置为Cascade)
• Road ，具有一对多关系 cities至City

作为示例，我添加了两个 Cities和一个Road “连接”两个城市。在我的ViewController我设置了两个NSFetchedResultsController负责保存两个数组 var cities: [City]和var roads: [Road]在所述 View Controller 中是最新的。

当我继续删除两个 cities 之一时而NSFetchedResultsController观察一切，应用程序因Index out of range error而崩溃在尝试更新 didChange 中的数组时获取结果 Controller 的委托(delegate)方法:

Fatal error: Index out of range

我正在使用获取结果 Controller 委托(delegate)的新 API 来通过 CollectionDifference 跟踪更改:

func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith diff: CollectionDifference<NSManagedObjectID>)

删除 cities 之一后，该方法被多次调用，并出现奇怪的插入和删除(我认为在没有调试器上下文的情况下将这些内容发布到此处没有帮助。我在下面添加了一个示例项目)。

正在使用 diff 调用该方法看起来很奇怪:

remove(offset: 1, element: 0xcb6b486b0bd166fe <x-coredata://E2AA7DB9-7A1F-4130-9AE4-A9D0DC695159/City/p2>, associatedWith: Optional(0))

remove(offset: 0, element: 0xcb6b486b0bdd66fe <x-coredata://E2AA7DB9-7A1F-4130-9AE4-A9D0DC695159/City/p1>, associatedWith: nil)

insert(offset: 0, element: 0xcb6b486b0bd166fe <x-coredata://E2AA7DB9-7A1F-4130-9AE4-A9D0DC695159/City/p2>, associatedWith: Optional(1))

我所期望的diff看起来只是删除了数组中的第一个元素(第一个 city 被删除)。但这对我来说没有任何意义。我什至不明白这些实际上想告诉我什么。第一个删除与其自身相关联？最后一次插入与第二次删除相关联。我不明白。我想问题就出在这里。有什么想法吗？

我的 View Controller 看起来像这样(我评论了崩溃发生的地方):

class ViewController: UIViewController, NSFetchedResultsControllerDelegate {

    public var cities: [City] = []
    private lazy var citiesResultsController: NSFetchedResultsController<City> = {
        let request: NSFetchRequest<City> = City.fetchRequest()
        request.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: true)]

        let controller = NSFetchedResultsController(fetchRequest: request, managedObjectContext: AppDelegate.managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

        controller.delegate = self
        return controller
    }()

    public var roads: [Road] = []
    private lazy var roadsResultsController: NSFetchedResultsController<Road> = {
        let request: NSFetchRequest<Road> = Road.fetchRequest()
        request.sortDescriptors = [NSSortDescriptor(key: "creationDate", ascending: true)]

        let controller = NSFetchedResultsController(fetchRequest: request, managedObjectContext: AppDelegate.managedObjectContext, sectionNameKeyPath: nil, cacheName: nil)

        controller.delegate = self
        return controller
    }()

    func controller(_ controller: NSFetchedResultsController<NSFetchRequestResult>, didChangeContentWith diff: CollectionDifference<NSManagedObjectID>) {
        if controller === citiesResultsController {
            for change in diff {
                switch change {
                case .insert(offset: let newPosition, element: _, associatedWith: let oldPosition):

                    if let oldPosition = oldPosition {
                        // was moved
// HERE IT CRASHES 
                        let city = cities.remove(at: oldPosition)
                        cities.insert(city, at: newPosition)
                    } else {
                        // was inserted
                        let city = citiesResultsController.object(at: IndexPath(item: newPosition, section: 0))
                        cities.insert(city, at: newPosition)
                    }
                case .remove(offset: let position, element: _, associatedWith: let associatedWith):
                    if associatedWith == nil {
                        _ = cities.remove(at: position)
                    }
                }
            }
        } else {
            for change in diff {
                switch change {
                case .insert(offset: let newPosition, element: _, associatedWith: let oldPosition):
                    if let oldPosition = oldPosition {
                        // was moved
                        let road = roads.remove(at: oldPosition)
                        roads.insert(road, at: newPosition)
                    } else {
                        // was inserted
                        let road = roadsResultsController.object(at: IndexPath(item: newPosition, section: 0))
                        roads.insert(road, at: newPosition)
                    }
                case .remove(offset: let position, element: _, associatedWith: let associatedWith):
                    if associatedWith == nil {
                        _ = roads.remove(at: position)
                    }
                }
            }
        }
    }

    override func viewDidLoad() {
        super.viewDidLoad()

        try? citiesResultsController.performFetch()
        cities = citiesResultsController.fetchedObjects ?? []

        try? roadsResultsController.performFetch()
        roads = roadsResultsController.fetchedObjects ?? []

        DispatchQueue.main.asyncAfter(deadline: .now() + .seconds(5)) {
            print("Now")
            AppDelegate.managedObjectContext.delete(AppDelegate.cityA)
            try! AppDelegate.managedObjectContext.save()
        }
    }
}

还有我的 appDelegate(我在其中设置对象和东西来测试所有内容):

//
//  AppDelegate.swift
//  TestProject
//
//  Created by Dennis Müller on 23.09.19.
//  Copyright © 2019 Dennis Müller. All rights reserved.
//

import UIKit
import CoreData

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    public static var managedObjectContext: NSManagedObjectContext {
        return (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    }

    public static var cityA: City!

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        // Override point for customization after application launch.

        if !UserDefaults.standard.bool(forKey: "setup") {
            UserDefaults.standard.set(true, forKey: "setup")

            let cityA = City(context: persistentContainer.viewContext)
            cityA.name = "cityA"
            cityA.creationDate = Date()

            let cityB = City(context: persistentContainer.viewContext)
            cityB.name = "cityB"
            cityB.creationDate = Date()

            let road = Road(context: persistentContainer.viewContext)
            road.creationDate = Date()
            road.addToCities(cityA)
            road.addToCities(cityB)

            saveContext()

        }

        let request: NSFetchRequest<City> = City.fetchRequest()
        request.predicate = NSPredicate(format: "name == %@", "cityA")
        AppDelegate.cityA = try! persistentContainer.viewContext.fetch(request).first!

        return true
    }

    // MARK: UISceneSession Lifecycle

    func application(_ application: UIApplication, configurationForConnecting connectingSceneSession: UISceneSession, options: UIScene.ConnectionOptions) -> UISceneConfiguration {
        // Called when a new scene session is being created.
        // Use this method to select a configuration to create the new scene with.
        return UISceneConfiguration(name: "Default Configuration", sessionRole: connectingSceneSession.role)
    }

    func application(_ application: UIApplication, didDiscardSceneSessions sceneSessions: Set<UISceneSession>) {
        // Called when the user discards a scene session.
        // If any sessions were discarded while the application was not running, this will be called shortly after application:didFinishLaunchingWithOptions.
        // Use this method to release any resources that were specific to the discarded scenes, as they will not return.
    }

    // MARK: - Core Data stack

    lazy var persistentContainer: NSPersistentContainer = {
        /*
         The persistent container for the application. This implementation
         creates and returns a container, having loaded the store for the
         application to it. This property is optional since there are legitimate
         error conditions that could cause the creation of the store to fail.
        */
        let container = NSPersistentContainer(name: "TestProject")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.

                /*
                 Typical reasons for an error here include:
                 * The parent directory does not exist, cannot be created, or disallows writing.
                 * The persistent store is not accessible, due to permissions or data protection when the device is locked.
                 * The device is out of space.
                 * The store could not be migrated to the current model version.
                 Check the error message to determine what the actual problem was.
                 */
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        return container
    }()

    // MARK: - Core Data Saving support

    func saveContext () {
        let context = persistentContainer.viewContext
        if context.hasChanges {
            do {
                try context.save()
            } catch {
                // Replace this implementation with code to handle the error appropriately.
                // fatalError() causes the application to generate a crash log and terminate. You should not use this function in a shipping application, although it may be useful during development.
                let nserror = error as NSError
                fatalError("Unresolved error \(nserror), \(nserror.userInfo)")
            }
        }
    }

}

抱歉，如果这有点令人困惑，我不知道如何正确解释这个问题(我什至不知道问题是什么)。如果您需要更多信息，请告诉我。

非常感谢您的帮助。

Answer 1

好吧，事实证明我只是个傻子。当迭代 diff 中的更改时，我错误地认为 .insert 案例的 linkedWith 是该元素的旧索引应该移动:

...
            case .insert(offset: let newPosition, element: _, associatedWith: let oldPosition):

                    // I called the associatedWith 'oldPosition'
                    if let oldPosition = oldPosition {
                        // was moved
// HERE IT CRASHES 
                        let city = cities.remove(at: oldPosition)
                        cities.insert(city, at: newPosition)
                    } else {
                        // was inserted
                        let city = citiesResultsController.object(at: IndexPath(item: newPosition, section: 0))
                        cities.insert(city, at: newPosition)
                    }
            ...

这导致了崩溃，因为 linkedWith 实际上是连接到它的 .remove 更改的索引，表明它实际上是移动而不是插入。

所以我替换了这个:

case .insert(offset: let newPosition, element: _, associatedWith: let oldPosition):
...
   let city = cities.remove(at: oldPosition)
...

这样:

case .insert(offset: let newPosition, element: let objectID, associatedWith: let associatedWith):
...
    let city = cities.first(where: {$0.objectID == objectID})!
....

这不是最佳的，它增加了我不喜欢的方法的线性时间复杂度。但我找不到一种方法来获取元素的实际旧位置。由于删除了city对象，移动操作变得更加复杂。我不明白为什么会有这样的举动。它所要做的就是删除一个城市，仅此而已。但我要为此提出另一个问题，因为这是题外话。