为什么在 Core Data container.performBackgroundTask() 之后不触发 tableView.reloadData()

Question

我正在使用 Swift 4 构建具有 UITableViewController 的单视图 iOS 11 应用程序，该应用程序也定义为 delegate 用于 NSFetchedResultsController。

class MyTVC: UITableViewController, NSFetchedResultsControllerDeleagate {
    var container:NSPersistentContainer? = 
           (UIApplication.shared.delegate as? AppDelegate)?.persistentContainer

   var frc : NSFetchedResultsController<Student>? 

   override func viewDidLoad() {
      container?.performBackgroundTask { context in 
          // adds 100 dummy records in background
          for i in 1...100 {
             let student = Student(context: context)
             student.name = "student \(i)"
          }
          try? context.save()   // this works because count is printed below
          if let count = try? context.count(for: Student.fetchRequest()) {
               print("Number of students in core data: \(count)")  // prints 100
          }
      }  // end of background inserting.

      // now defining frc:
      if let context = container?.viewContext {
          let request:NSFetchRequest<Student> = Student.fetchRequest()
          request.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
          frc = NSFetchedResultsController<Student> (
               fetchRequest: request,
               managedObjectContext: context,
               sectionNameKeyPath: nil,
               cacheName: nil )

          try? frc?.performFetch()   // this works and I get no errors
          tableView.reloadData()
          frc.delegate = self
      }  // end of frc definition
   }
}

如果我使用viewContext 添加一行Student，frc 将触发所需的方法以在tableView 中显示它。然而，100 个虚拟行未显示。事实上，如果我试图在插入完成后告诉 tableview 重新加载，我的应用程序开始表现得很奇怪并变得有问题，并且没有做它应该做的事情（即：不删除行，不编辑等） .

但是如果我重新启动我的应用程序，而不调用虚拟插入，我可以看到前一次运行插入的 100 行。

唯一的问题是我无法从后台线程调用tableView.reloadData()，所以我尝试这样做：

// after printing the count, I did this:
DispatchQueue.main.async { [weak self] in
    self?.tableView.reloadData()   // causes UI to behave weirdly 
}

然后我尝试调用viewContext.perform 在正确的线程中重新加载表格视图

func viewDidLoad() {

   // code for inserting 100 dummy rows in background thread

   // code for defining frc and setting self as delegate

   if let context = container?.viewContext {
      context.perform { [weak self] in
         self?.tableView.reloadData()    // that also causes UI to behave weirdly

      }
   }
}

如何告诉我的 tableview 以线程安全的方式重新加载和显示 100 个虚拟行？

Answer 1

override func viewDidLoad() {
    super.viewDidLoad()

    //Always need your delegate for the UI to be set before calling the UI's delegate functions.
    frc.delegate = self

    //First we can grab any already stored values.
    goFetch()

    //This chunk just saves. I would consider putting it into a separate function such as "goSave()" and then call that from an event handler.
    container?.performBackgroundTask { context in
        //We are in a different queue than the main queue, hence "backgroundTask".
        for i in 1...100 {
            let student = Student(context: context)
            student.name = "student \(i)"
        }
        try? context.save()   // this works because count is printed below
        if let count = try? context.count(for: Student.fetchRequest()) {
            print("Number of students in core data: \(count)")  // prints 100
        }
        //Now that we are done saving its ok to fetch again.

        goFetch()

    }

    //goFetch(); Your other code was running here would start executing before the backgroundTask is done. bad idea.
    //The reason it works if you restart the app because that data you didn't let finish saving is persisted
    //So the second time Even though its saving another 100 in another queue there were still at least 100 records to fetch at time of fetch.

}


func goFetch() {

    if let context = container?.viewContext {
        let request:NSFetchRequest<Student> = Student.fetchRequest()
        request.sortDescriptors = [NSSortDescriptor(key: "name", ascending: true)]
        frc = NSFetchedResultsController<Student> (
            fetchRequest: request,
            managedObjectContext: context,
            sectionNameKeyPath: nil,
            cacheName: nil )

        try? frc?.performFetch()

        //Now that records are both stored and fetched its safe for our delegate to access the data on the main thread.
        //To me it would make sense to do a tableView reload everytime data is fetched so I placed this inside o `goFetch()`
        DispatchQueue.main.async { [weak self] in
            self?.tableView.reloadData()
        }
    }
}

Answer 2

在大量阅读有关 NSFetchedResultsController 和 NSPersistentContainer 并最终在 SO 找到重要信息后，我想我有一个可行的示例。

我的代码略有不同，因为我为此使用了一个项目。无论如何，这就是我所做的：

在我的视图控制器中，我的容器有一个属性

private var persistentContainer = NSPersistentContainer(name: coreDataModelName)

在 viewDidLoad 中，我加载了持久存储并创建了 100 条记录。

persistentContainer.loadPersistentStores { persistentStoreDescription, error in
  if let error = error {
    print("Unable to add Persistent Store [\(error)][\(error.localizedDescription)]")
  } else {
    self.createFakeNotes() // Here 100 elements get created
    DispatchQueue.main.async {
      self.setupView() // other stuff, not relevant
      self.fetchNotes() // fetch using fetch result controller
      self.tableView.reloadData()
    }
  }
}

下面是 createFakeNotes()，我在其中使用单独的上下文在后台线程中插入元素，这段代码几乎取自 Apple's Core Data programming guide，但为了更新 UI，我需要将自动MergesChangesFromParent 设置为 true，我发现出this SO answer

为了方便测试，我也先删除旧笔记。

private func createFakeNotes() {
  let deleteRequest = NSBatchDeleteRequest(fetchRequest: Note.fetchRequest())
  do {
    try persistentContainer.persistentStoreCoordinator.execute(deleteRequest, with: persistentContainer.viewContext)
  } catch {
    print("Delete error [\(error)]")
    return
  }

  let privateContext = persistentContainer.newBackgroundContext()
  privateContext.automaticallyMergesChangesFromParent = true //Important!!!

  privateContext.perform {
    let createDate = Date()
    for i in 1...100 {
      let note = Note(context: privateContext)
      note.title = String(format: "Title %2d", i)
      note.contents = "Content"
      note.createdAt = createDate
      note.updatedAt = createDate
    }
    do {
      try privateContext.save()
      do {
        try self.persistentContainer.viewContext.save()
      } catch {
        print("Fail saving main context [\(error.localizedDescription)")
      }
    } catch {
      print("Fail saving private context [\(error.localizedDescription)")
  }
}

}

Answer 3

您应该通过从 viewwillappear 调用数据来获取数据，然后尝试重新加载您的 tableview。

override func viewWillAppear(_ animated: Bool) {
    getdata()
    tableView.reloadData()
}
 func getdata() {
    let context =  (UIApplication.shared.delegate as! AppDelegate).persistentContainer.viewContext
    do{
    persons = try context.fetch(Person.fetchRequest()) 
    }
    catch {
        print("fetching failed")
    }
}