核心数据合并策略有时会删除对象

分类：编程问答

如何解决核心数据合并策略有时会删除对象

我正在尝试创建一个简单的应用程序，该应用程序在拆分视图的左侧具有对话，在右侧具有消息。左侧的对话列表将显示每个对话的recordID，描述和最新消息的详细信息。消息视图具有“添加消息”按钮，该按钮将模拟向会话中添加消息。这将始终使用相同的消息ID。这是为了测试合并策略，它是NSMergeByPropertyObjectTrumpMergePolicy。我面临的问题是，在您添加新消息时，latestMessage变为50％。这将导致应用程序崩溃，因为会话列表正在使用LatestMessage属性来显示LatestMessage详细信息。

我正在寻找一种解决方案，该方法如何有效地在对话列表中显示LatestMessage而不导致崩溃。我也想首先了解导致此问题的原因。我的假设是它与合并策略有关，但是我不确定为什么，因为我所有其他对象都遵守合并策略而没有问题。

所有必需的代码在下面。如果您想下载该项目，请点击link。

Core Data Relationships

struct ContentView: View {
    var body: some View {
        NavigationView{
            ConversationsView()
            Text("Select a conversation")
        }
    }
}

struct MessagesView: View{
    @StateObject var model: MessagesModel
    @ObservedObject var conversation: Conversation
    
    init(conversation: Conversation) {
        _model = StateObject(wrappedValue: MessagesModel(conversation: conversation))
        self.conversation = conversation
    }
    
    var body: some View{
        List(model.messages){ message in
            VStack(alignment: .leading){
                Text("ID: \(message.id)")
                Text("Message: \(message.message)")
                Text("Person: \(message.person.firstName)")
            }
        }
        .navigationTitle("Messages \(model.messages.count)")
        .toolbar(content: {
            ToolbarItem(placement: .navigationBarTrailing){
                Button("Add Messages"){
                    model.addMessage(to: conversation)
                }
            }
        })
    }
}

struct ConversationsView: View{
    @StateObject var model = ConversationModel()
    
    var body: some View{
        List(model.conversations){ convo in
            NavigationLink(
                destination: MessagesView(conversation: convo),label: {
                    VStack(alignment: .leading){
                        Text("Record id: \(convo.id)")
                            .font(.headline)
                        Text("Description: \(convo.myDescription)")
                            .font(.headline)
                        Text("\(convo.latestMessage.person.fullName): \(convo.latestMessage.message)")
                            .font(.subheadline)
                    }
                })
        }.navigationTitle("Conversations \(model.conversations.count)")
    }
}

class ConversationModel: NSObject,ObservableObject,NSFetchedResultsControllerDelegate{
    @Published var conversations = [Conversation]()
    private let conversationsController: NSFetchedResultsController<Conversation>
    
    override init(){
        conversationsController = NSFetchedResultsController(fetchRequest: Conversation.getAllConversations(),managedObjectContext: PersistentStore.shared.context,sectionNameKeyPath: nil,cacheName: nil)
        super.init()
        conversationsController.delegate = self
        
        do {
            try conversationsController.performFetch()
            conversations = conversationsController.fetchedObjects ?? []
            createConversations()
        } catch {
            print("Error: \(error.localizedDescription)")
        }
    }
    
    private func createConversations(){
        let context = PersistentStore.shared.context
        
        let conversation = Conversation(context: context)
        let ricky        = Person(context: context)
        let message      = Message(context: context)
        
        ricky.firstName = "Ricky"
        ricky.lastName = "W"
        ricky.title = "iOS Dev"
        
        message.id = 1
        message.message = "Testing 123"
        message.timeStamp = Date().timeIntervalSince1970
        message.person = ricky
        
        conversation.myDescription = "This is a conversation"
        conversation.id = 1
        conversation.activeParticipants.insert(ricky)
        conversation.latestMessage = message
        conversation.messages.insert(message)
        
        PersistentStore.shared.saveContext()
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        if let items = controller.fetchedObjects as? [Conversation]{
            conversations = items
        }
    }
}

class MessagesModel: NSObject,NSFetchedResultsControllerDelegate{
    @Published var messages = [Message]()
    private let messagesController: NSFetchedResultsController<Message>
    
    init(conversation: Conversation){
        messagesController = NSFetchedResultsController(fetchRequest: Message.getMessagesFor(conversation),cacheName: nil)
        super.init()
        messagesController.delegate = self
        
        do {
            try messagesController.performFetch()
            messages = messagesController.fetchedObjects ?? []
        } catch {
            print("Error: \(error.localizedDescription)")
        }
    }
    
    func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
        if let items = controller.fetchedObjects as? [Message]{
            messages = items
        }
    }
    
    func addMessage(to conversation: Conversation){
        let context = PersistentStore.shared.context
        
        let tim = Person(context: context)
        tim.firstName = "Tim"
        tim.lastName = "Apple"
        tim.title = "CEO"
        
        let message = Message(context: context)
        message.id = 2
        message.message = "I Am Tim Apple"
        message.timeStamp = Date().timeIntervalSince1970
        message.person = tim
        
        conversation.messages.insert(message)
        conversation.latestMessage = message
        
        PersistentStore.shared.saveContext()
    }
}

class PersistentStore: ObservableObject {
    var context: NSManagedObjectContext {
        persistentContainer.viewContext
    }
    
    static let shared = PersistentStore()
    private init() {}
    
    // MARK: - Core Data stack
    let persistentContainer: NSPersistentContainer = {
        
        let container = NSPersistentCloudKitContainer(name: "CoreDataTesting")
        
        // Enable remote notifications
        guard let description = container.persistentStoreDescriptions.first else {
            fatalError("###\(#function): Failed to retrieve a persistent store description.")
        }
        
        description.setOption(true as NSNumber,forKey: NSPersistentStoreRemoteChangeNotificationPostOptionKey)
        
        container.loadPersistentStores(completionHandler: { (storeDescription,error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error),\(error.userInfo)")
            }
        })
            
        container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        container.viewContext.automaticallyMergesChangesFromParent = true
        
        return container
    }()
    
    
    // MARK: - Core Data Saving support

    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)")
            }
        }
    }
    
    // MARK: - Delete For Debug
    
    func deleteEverythingForDebug(){
        let conversationsRequest: NSFetchRequest<NSFetchRequestResult> = NSFetchRequest(entityName: "Conversation")
        let messagesRequest: NSFetchRequest<NSFetchRequestResult>      = NSFetchRequest(entityName: "Message")
        let personRequest: NSFetchRequest<NSFetchRequestResult>        = NSFetchRequest(entityName: "Person")

        let conversationsDeleteRequest = NSBatchDeleteRequest(fetchRequest: conversationsRequest)
        let messagesDeleteRequest      = NSBatchDeleteRequest(fetchRequest: messagesRequest)
        let personDeleteRequest        = NSBatchDeleteRequest(fetchRequest: personRequest)

        do {
            try persistentContainer.viewContext.execute(conversationsDeleteRequest)
            try persistentContainer.viewContext.execute(messagesDeleteRequest)
            try persistentContainer.viewContext.execute(personDeleteRequest)
            print("Deleted all of Core Data objects.")
        } catch {
            fatalError("Error deleting everything: \(error)")
        }
        
        saveContext()
    }
}


public class Conversation : NSManagedObject,Identifiable{
    @NSManaged public var activeParticipants   : Set<Person>
    @NSManaged public var myDescription        : String
    @NSManaged public var id                   : Int
    @NSManaged public var messages             : Set<Message>
    @NSManaged public var latestMessage        : Message
    
    static func getAllConversations() -> NSFetchRequest<Conversation> {
        let request : NSFetchRequest<Conversation> = Conversation.fetchRequest() as! NSFetchRequest<Conversation>
        request.sortDescriptors = [NSSortDescriptor(key: "latestMessage.timeStamp",ascending: false)]
        return request
    }
}



public class Message : NSManagedObject,Identifiable {
    @NSManaged public var id           : Int
    @NSManaged public var message      : String
    @NSManaged public var timeStamp    : Double
    @NSManaged public var person       : Person
    @NSManaged public var conversation : Conversation
    @NSManaged public var latestConversation : Conversation
    
    static func getMessagesFor(_ conversation: Conversation) -> NSFetchRequest<Message> {
        let request : NSFetchRequest<Message> = Message.fetchRequest() as! NSFetchRequest<Message>
        request.sortDescriptors = [NSSortDescriptor(key: "timeStamp",ascending: true)]
        request.predicate = NSPredicate(format: "conversation == %@",conversation)
        return request
    }
    
    static func getAllMessages() -> NSFetchRequest<Message> {
        let request : NSFetchRequest<Message> = Message.fetchRequest() as! NSFetchRequest<Message>
        request.sortDescriptors = [NSSortDescriptor(key: "timeStamp",ascending: true)]
        return request
    }
}


public class Person: NSManagedObject,Identifiable {
    @NSManaged public var firstName: String
    @NSManaged public var lastName: String
    @NSManaged public var title: String
    @NSManaged public var conversations: Set<Conversation>
    @NSManaged public var messages: Set<Message>
    
    var fullName : String{
        "\(firstName) \(lastName)"
    }
    
    static func getAllPeopleWith(title: String) -> NSFetchRequest<Person> {
        let request : NSFetchRequest<Person> = Person.fetchRequest() as! NSFetchRequest<Person>
        request.sortDescriptors = [NSSortDescriptor(key: "firstName",ascending: false)]
        request.predicate = NSPredicate(format: "title == %@",title)
        return request
    }
    
    static func getAllPeople() -> NSFetchRequest<Person> {
        let request : NSFetchRequest<Person> = Person.fetchRequest() as! NSFetchRequest<Person>
        request.sortDescriptors = [NSSortDescriptor(key: "firstName",ascending: false)]
        return request
    }
}

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