保存已编辑的核心数据实体未保存

Posted 2023-02-27

【中文标题】保存已编辑的核心数据实体未保存

【英文标题】：Saving an edited Core Data entity is not being saved

【发布时间】：2019-01-28 01:14:05

【问题描述】：

我有一个包含多个部分的 TableView，这些部分使用 Core Data 框架填充了实体（称为“项目”）。在 cellForRowAt 委托方法中，我调用了一个回调，该回调在用户按下“Enter”键时调用。此回调应将他们刚刚在单元格文本字段中键入的文本保存到核心数据模型，然后在其正下方添加一个新的空单元格。

所有这些都有效，除非该部分为空并且您尝试将 item.name 设置为等于 textfield.text。如果我向该部分添加多个项目，那么它将保存我刚刚创建并添加到数据模型中的所有单元格，但第一个单元格除外。

起初我正在更新项目，然后保存上下文，但我会收到一条错误消息，类似于“错误：从上下文中删除托管对象后对其进行变异”。请参阅下面的代码。

    /// Customize the contents of the cell
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell 
    let cell = tableView.dequeueReusableCell(withIdentifier: homeCellId, for: indexPath) as! HomeTableViewCell
    cell.delegate = self
    let item = items[indexPath.section][indexPath.row]

    cell.nameText.text = item.name

    /// Called when user hits Enter on the keyboard
    /// This allows them to enter a new item
    cell.addNewCell =  [weak self] newTitle in
        guard let `self` = self else return // Capture self

        if cell.nameText.text != "" 

            let newTitle: String = cell.nameText.text! // Grab the name øf the item the user just entered
            let itemToUpdate = self.items[indexPath.section][indexPath.row] // Grab the item they typed

            itemToUpdate.name = cell.nameText.text
            self.saveContext()


            // Create reference to indexpath below the existing cell
            let newIndexPath = IndexPath(row: indexPath.row+1, section: indexPath.section) // Set the indexPath to the cell below
            // Create new dummy item
            let newPlaceholderItem = self.coreDataManager.addItem(toCategory: self.categories[indexPath.section], withItemName: "")
            // Add dummy item to tableview array
            self.items[indexPath.section].append(newPlaceholderItem)
            self.tableView.insertRows(at: [newIndexPath], with: .automatic) // Insert it into the tableView
         else 
            print("Can not add new cell since current cell is empty")
        
    

    return cell

经过大量谷歌搜索后，我发现解决上述错误的方法是使用下图所示的 performBackgroundTask()。

/// Customize the contents of the cell
override func tableView(_ tableView: UITableView, cellForRowAt indexPath: IndexPath) -> UITableViewCell 
    let cell = tableView.dequeueReusableCell(withIdentifier: homeCellId, for: indexPath) as! HomeTableViewCell
    cell.delegate = self
    let item = items[indexPath.section][indexPath.row]

    cell.nameText.text = item.name

    /// Called when user hits Enter on the keyboard
    /// This allows them to enter a new item
    cell.addNewCell =  [weak self] newTitle in
        guard let `self` = self else return // Capture self


        if cell.nameText.text != "" 

            let newTitle: String = cell.nameText.text! // Grab the name øf the item the user just entered
            let itemToUpdate = self.items[indexPath.section][indexPath.row] // Grab the item they typed

            let container = (UIApplication.shared.delegate as! AppDelegate).persistentContainer // Grab the persistent container

            container.performBackgroundTask  (context) in
                // Make sure the object isn't null
                guard let queueSafeItem = context.object(with: itemToUpdate.objectID) as? Item else 
                    print("Error, could not update item.")
                    return
                

                queueSafeItem.name = newTitle // Update the title of the item

                // Save the change that the user made to the cell he just edited
                do 
                    print("Saving context after modifying item")
                    try context.save()
                 catch 
                    print("Error saving when creating new cell: \(error)")
                
            

            // Create reference to indexpath below the existing cell
            let newIndexPath = IndexPath(row: indexPath.row+1, section: indexPath.section) // Set the indexPath to the cell below
            // Create new dummy item
            let newPlaceholderItem = self.coreDataManager.addItem(toCategory: self.categories[indexPath.section], withItemName: "")
            // Add dummy item to tableview array
            self.items[indexPath.section].append(newPlaceholderItem)
            self.tableView.insertRows(at: [newIndexPath], with: .automatic) // Insert it into the tableView
         else 
            print("Can not add new cell since current cell is empty")
        
    

    return cell

【参考方案1】：

经过一些调试，我似乎没有正确保存“虚拟”单元格，不得不重新格式化一些代码。 Xcode 或 Swift 端没有错误 - 只是常见的程序员错误。