swiftui - 当我的 @Published NSManagedObjects 数组更改时，我的 SwiftUI 列表不会更新

Question

我正在构建一个基本的笔记应用程序，我的应用程序的主页应显示用户笔记列表。注释用 Note 类表示，这是一个 Core Data 生成的类。 (我的最终目标是通过 NSPersistentCloudKitContainer 与 CloudKit 同步的笔记应用程序。)

到目前为止，当用户加载应用程序时，列表会显示正确的笔记数据。但是，当我尝试通过点击 newNoteButton 创建新笔记时，笔记数组发生变化，但我的 UI 没有改变。我必须重新加载应用程序才能看到新笔记。我做错了什么？抱歉下面的乱码:

NoteList.swift

struct NoteList: View {

  @EnvironmentObject var userNotes: UserNotes

  var newNoteButton: some View {
    Button(action: {
      self.userNotes.createNewNote()
      self.userNotes.objectWillChange.send()
    }) {
      Image(systemName: "plus")
        .imageScale(.large)
        .accessibility(label: Text("New Note"))
    }
  }

  var body: some View {
    NavigationView {
      List {
        ForEach(self.userNotes.notes) { note in
          NavigationLink(destination: NoteDetail(note: self.$userNotes.notes[self.userNotes.notes.firstIndex(of: note)!])) {
            Text(note.unsecuredContent!)
          }
        }
      }
      .navigationBarTitle(Text("Notes"), displayMode: .inline)
      .navigationBarItems(trailing: newNoteButton)
    }
  }

}

UserNotes.swift

class UserNotes: NSObject, ObservableObject {

  @Published var notes: [Note] = []

  var managedObjectContext: NSManagedObjectContext? = nil

  var fetchedResultsController: NSFetchedResultsController<Note> {
    if _fetchedResultsController != nil {
      return _fetchedResultsController!
    }

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

    // Set the batch size to a suitable number.
    fetchRequest.fetchBatchSize = 20

    // Edit the sort key as appropriate.
    let sortDescriptor = NSSortDescriptor(key: "unsecuredContent", ascending: false)

    fetchRequest.sortDescriptors = [sortDescriptor]

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest,
                                                               managedObjectContext: self.managedObjectContext!,
                                                               sectionNameKeyPath: nil, cacheName: "Master")
    aFetchedResultsController.delegate = self
    _fetchedResultsController = aFetchedResultsController

    do {
      try _fetchedResultsController!.performFetch()
    } 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)")
    }

    return _fetchedResultsController!
  }
  var _fetchedResultsController: NSFetchedResultsController<Note>? = nil

  override init() {
    super.init()
    managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    notes = fetchedResultsController.sections![0].objects as! [Note]
  }

  func createNewNote() {
    let newNote = Note(context: managedObjectContext!)

    // If appropriate, configure the new managed object.
    newNote.unsecuredContent = "New CloudKit note"

    // Save the context.
    do {
      try managedObjectContext!.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)")
    }
  }

}

extension UserNotes: NSFetchedResultsControllerDelegate {

  func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    notes = controller.sections![0].objects as! [Note]
  }

}

Note.swift(由 Core Data 生成)

//  This file was automatically generated and should not be edited.
//

import Foundation
import CoreData

@objc(Note)
public class Note: NSManagedObject {

}

Note.swift(扩展)

extension Note: Identifiable {}

Answer 1

在@dfd 的帮助下(参见 here)我能够通过将 Combine 导入我的 UserNotes 类、添加 objectWillChange 并调用 objectWillChange.send()< 来解决这个问题:

import Foundation
import UIKit
import CoreData
import Combine

class UserNotes: NSObject, ObservableObject {

  var objectWillChange = PassthroughSubject<Void, Never>()

  @Published var notes: [Note] = [] {
    willSet {
      objectWillChange.send()
    }
  }

  var managedObjectContext: NSManagedObjectContext? = nil

  var fetchedResultsController: NSFetchedResultsController<Note> {
    if _fetchedResultsController != nil {
      return _fetchedResultsController!
    }

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

    // Set the batch size to a suitable number.
    fetchRequest.fetchBatchSize = 20

    // Edit the sort key as appropriate.
    let sortDescriptor = NSSortDescriptor(key: "unsecuredContent", ascending: false)

    fetchRequest.sortDescriptors = [sortDescriptor]

    // Edit the section name key path and cache name if appropriate.
    // nil for section name key path means "no sections".
    let aFetchedResultsController = NSFetchedResultsController(fetchRequest: fetchRequest,
                                                               managedObjectContext: self.managedObjectContext!,
                                                               sectionNameKeyPath: nil, cacheName: "Master")
    aFetchedResultsController.delegate = self
    _fetchedResultsController = aFetchedResultsController

    do {
      try _fetchedResultsController!.performFetch()
    } 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)")
    }

    return _fetchedResultsController!
  }
  var _fetchedResultsController: NSFetchedResultsController<Note>? = nil

  override init() {
    super.init()
    managedObjectContext = (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    notes = fetchedResultsController.sections![0].objects as! [Note]
  }

  func createNewNote() {
    let newNote = Note(context: managedObjectContext!)

    // If appropriate, configure the new managed object.
    newNote.unsecuredContent = UUID().uuidString // Just some random crap

    // Save the context.
    do {
      try managedObjectContext!.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)")
    }
  }

}

extension UserNotes: NSFetchedResultsControllerDelegate {

  func controllerDidChangeContent(_ controller: NSFetchedResultsController<NSFetchRequestResult>) {
    notes = controller.sections![0].objects as! [Note]
  }

}