NSFetchedResultsController 'didChange' 使用新的 CollectionDiffing 导致奇怪的更改操作导致崩溃

NSFetchedResultsController 'didChange' using new CollectionDiffing causes strange change operations that lead to a crash

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


我有一个简单的 CoreData 设置,有两个 entities:

例如,我添加了两个 Cities 和一个 Road "connects" 两个城市。在我的 ViewController 中,我设置了两个 NSFetchedResultsController ,它们负责使所述视图控制器中的两个数组 var cities: [City]var roads: [Road] 保持最新。

当我继续删除两个 cities 之一,而 NSFetchedResultsController 观察所有内容时,应用程序因 Index out of range error 而崩溃,同时尝试更新 didChange 获取结果控制器委托的方法:

Fatal error: Index out of range

我正在使用新的 API 获取结果控制器的委托来通过 CollectionDifference:

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

并且在删除了其中一个cities之后,那个方法被调用了很多次,有奇怪的插入和删除(我认为这对post 这些没有调试器的上下文。我在下面添加了一个示例项目)。

正在使用看起来很奇怪的 diff 调用该方法:

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))

我期望的 diff 只是删除数组中的第一个元素(删除的第一个 city)。但这对我来说没有任何意义。我什至不明白这些实际上试图告诉我什么。第一个remove是和自己关联的?最后一个插入与第二个删除相关联。我不明白。我认为问题就出在这里。有什么想法吗?

我的视图控制器看起来像这样(我评论了发生崩溃的地方):

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。

好吧,事实证明我只是个傻瓜。在遍历 diff 中的更改时,我错误地认为 .insert 情况下的 associatedWith 是应该移动的元素的旧索引:

...
            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)
                    }
            ...

这是导致崩溃的原因,因为 associatedWith 实际上是 .remove 与其相关联的更改的索引,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)
...

有了这个:

case .insert(offset: let newPosition, element: let objectID, associatedWith: let associatedWith):
...
    let city = cities.first(where: {[=12=].objectID == objectID})!
....

这不是最优的,它为我不喜欢的方法增加了线性时间复杂度。但是我找不到获取元素实际旧位置的方法。由于删除了 city 对象,移动操作变得更加复杂。我不明白为什么首先要采取行动。它所要做的就是删除一个城市,仅此而已。但是我要为此打开另一个问题,因为这是题外话。