如何在Core Data中更新现有记录？

Question

我想在数据库中更新一条记录，但是我不知道该怎么做。具体来说，我正在尝试获取一条记录，然后在用户点击UISwitch时将该记录更新/保存为新值。 我尝试这样做的方法位于我的ViewController.swift文件中，并被称为

didChangeSwitchState(#sender: SettingCell, isOn: Bool)

我正在使用基础的 CoreData 模板，该模板将 managedObjectContext 放在 AppDelegate 中。

这是我的数据模型：

下面是生成的代码和用于创建新条目的方法：

LogItem.swift

class LogItem: NSManagedObject {

    @NSManaged var settingLabel: String
    @NSManaged var switchState: Bool

    class func createInManagedObjectContext(moc: NSManagedObjectContext, label: String, state: Bool) -> LogItem {
        let newItem = NSEntityDescription.insertNewObjectForEntityForName("LogItem", inManagedObjectContext: moc) as! LogItem
        newItem.settingLabel = label
        newItem.switchState = state

        return newItem
    }    
}

ViewController.swift

class ViewController: UITableViewController, UITableViewDataSource, UITableViewDelegate, SettingCellDelegate {

    // Retreive the managedObjectContext from AppDelegate
    let managedObjectContext = (UIApplication.sharedApplication().delegate as! AppDelegate).managedObjectContext

    // Create the table view as soon as this class loads
    //var logTableView = UITableView(frame: CGRectZero, style: .Plain)

    var logItems = [LogItem]()


    override func viewDidLoad() {
        super.viewDidLoad()

        if let moc = self.managedObjectContext {

            let fetchRequest = NSFetchRequest(entityName:"LogItem")
            var error: NSError?

            let fetchedResults = moc.executeFetchRequest(fetchRequest, error: &error) as! [NSManagedObject]?

            if (fetchedResults?.count == 0) {

                // Create some dummy data to work with
                var items = [
                    ("Best Animal",  true),
                    ("Best Language", true),
                    ("Derp", false),
                    ("Applesauce", false)
                ]

                for (settingLabel, switchState) in items {
                    LogItem.createInManagedObjectContext(moc,
                        label: settingLabel, state: switchState)
                }
            } else {
                println("data already exists")
            }
            fetchLog()
        }
    }

    func fetchLog() {
        let fetchRequest = NSFetchRequest(entityName: "LogItem")

        let sortDescriptor = NSSortDescriptor(key: "settingLabel", ascending: true)

        fetchRequest.sortDescriptors = [sortDescriptor]

        if let fetchResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: nil) as? [LogItem] {

            logItems = fetchResults // question... this seems like it would store the entire table as one item in the array... huh?
        }
    }
       func save() {
        var error : NSError?
        if(managedObjectContext!.save(&error) ) {
            println(error?.localizedDescription)
        }
    }

    override func tableView(tableView: UITableView, didSelectRowAtIndexPath indexPath: NSIndexPath) {
        let logItem = logItems[indexPath.row]
        println(logItem.switchState)
    }

    override func tableView(tableView: UITableView, cellForRowAtIndexPath indexPath: NSIndexPath) -> UITableViewCell {
        let cell = tableView.dequeueReusableCellWithIdentifier("CustomSettingCell") as! SettingCell

        let logItem = logItems[indexPath.row]

        cell.settingsLabel?.text = logItem.settingLabel
        cell.settingsSwitch.on = logItem.switchState
        cell.cellDelegate = self

        return cell
    }

    func didChangeSwitchState(#sender: SettingCell, isOn: Bool) {
        let indexPath = self.tableView.indexPathForCell(sender)
        managedObjectContext!.save(nil)
        var context = managedObjectContext

        let fetchRequest = NSFetchRequest()

        var entityName = NSEntityDescription.entityForName("LogItem", inManagedObjectContext: self.managedObjectContext!)

        fetchRequest.entity = entityName

        var error: NSError?

        if let cellName = sender.settingsLabel.text {
          fetchRequest.predicate = NSPredicate(format: "settingLabel = %@", cellName)

        }

            var fetchedResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: &error) as? [LogItem]
                if let setting = fetchedResults {

                if error != nil {
                    println("An error occurred loading the data")
                } else {

                    var saveError : NSError? = nil
                    if !managedObjectContext!.save(&saveError) {
                        println("Could not update record")
                    } else {

                        tableView.reloadData()
                    }
                }
            }
            tableView.reloadData()
    }

SettingCell.swift

class SettingCell: UITableViewCell {

    @IBOutlet weak var settingsLabel: UILabel!
    @IBOutlet weak var settingsSwitch: UISwitch!

    var cellDelegate: SettingCellDelegate?

    @IBAction func handledSwitchChange(sender: UISwitch) {

        self.cellDelegate?.didChangeSwitchState(sender: self, isOn:settingsSwitch.on)

    }

    override func setSelected(selected: Bool, animated: Bool) {
        super.setSelected(selected, animated: animated)

    } 
}

SettingItem.swift

class SettingItem: NSObject {

    var settingName : String?
    var switchState : Bool?

    override init() {
        super.init()
    }

    init (settingName: String?, switchState : Bool?) {
        super.init()
        self.settingName = settingName
        self.switchState = switchState
    }
}

SettingCellDelegate.swift

protocol SettingCellDelegate {
    func didChangeSwitchState(# sender: SettingCell, isOn: Bool)
}

最终这是我的输出结果：

- Dan Beaulieu

---

顺便说一下，问题写得很好！ - andrewbuilder

Answer 1

在保存之前，您没有更改 LogItem 的值。请相应地进行修改，此代码可能会生成编译器错误，因为我对 Swift 语言不熟悉，我只是编写了可能帮助您的伪代码。

var fetchedResults = managedObjectContext!.executeFetchRequest(fetchRequest, error: &error) as? [LogItem]
if let setting = fetchedResults {
  if error != nil {
    println("An error occurred loading the data")
  }
  else {
    //now you have array of LogItem in your case it's one (assuming that you have unique name
    //change the item's switch state and then save it
    //Please make a count check here before getting the object
    /*
    if count == 0 -->> record doesn't exist
    else -->> record exist
    */
    var settingLogItem = setting[0]
    settingLogItem.switchState = isOn

    var saveError : NSError? = nil
    if !managedObjectContext!.save(&saveError) {
      println("Could not update record")
    }
    else {
      tableView.reloadData()
    }
  }
}

- Inder Kumar Rathore