ios - 核心数据 - NSManagedObject 返回 nil

Question

我在 Appdelegate 类中编写了两个单独的函数 fetchRecords 和 displayRecords。 fetchRecords 函数将从实体中获取所有记录并且它工作正常。displayRecords 函数接受来自 fetchRecords 函数的返回值并打印所有记录逐个。

我有一个 View Controller 调用这两个函数来完成所需的任务。我的问题是 displayRecords 中的 result.count 显示获取的记录中可用的记录总数。当一条一条地打印记录时，该值为 nil。

override func viewDidLoad() {
super.viewDidLoad()
    let fetchedRecords = AppDelegate().fetchRecords(fromEntity: "Dashboard")
    AppDelegate().displayRecords(fromFetchedRecords: fetchedRecords)   
}

这里是 AppDelegate 类中编写的 fetchRecords 和 displayRecords 函数

  func fetchRecords(fromEntity entity:String) -> Array<AnyObject> {
    var fetchedResult:Array<AnyObject> = []
    let fetchRequest = NSFetchRequest()
    let entityDescription = NSEntityDescription.entityForName(entity, inManagedObjectContext: self.managedObjectContext)!
    fetchRequest.entity = entityDescription
    do{
        fetchedResult = try self.managedObjectContext.executeFetchRequest(fetchRequest)              
    }catch{
        let fetchError = error as NSError?
        print(fetchError!)
    }
   return fetchedResult
}

func displayRecords(fromFetchedRecords result:Array<AnyObject>) {
    print("Total records:\(result.count)")
    if (result.count > 0) {
        for data in result {
        let dashboard = data as! NSManagedObject
        print("Value: \(dashboard.valueForKey("count"))")
        }
    }
}

在这里添加我的数据模型

我也会分享数据插入代码。

func saveDashBoardData(dictionary: Dictionary<String, String>) {
    print(NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask))

    //Create Manage Object
    let entityDescription: NSEntityDescription = NSEntityDescription.entityForName("Dashboard", inManagedObjectContext: self.managedObjectContext)!
    for data in dictionary {
        let dashboardObject = Dashboard(entity: entityDescription,insertIntoManagedObjectContext: self.managedObjectContext)
        dashboardObject.type  = data.0
        dashboardObject.count = data.1
        do {
            try self.managedObjectContext.save()
            print("Data saved succesfully")
        }
        catch {
            print(error)
        }
    }
}

Answer 1

问题是 AppDelegate() 总是创建一个类的新实例，它与应用程序的“硬编码”委托(delegate)实例不同。

你必须写

let appDelegate = UIApplication.sharedApplication().delegate as! AppDelegate
let fetchedRecords = appDelegate.fetchRecords(fromEntity: "Dashboard")
appDelegate.displayRecords(fromFetchedRecords: fetchedRecords)

因为您已经创建了 NSManagedObject 的自定义子类，所以将其用作类型而不是 NSManagedObject 或 - 更糟的是 - 未指定的 AnyObject。它使很多事情变得容易得多:

func fetchRecords(fromEntity entity:String) -> [Dashboard] {
  let fetchRequest = NSFetchRequest()
  let entityDescription = NSEntityDescription.entityForName(entity, inManagedObjectContext: self.managedObjectContext)!
  fetchRequest.entity = entityDescription
  do {
     return try self.managedObjectContext.executeFetchRequest(fetchRequest) as! [Dashboard]              
  } catch let fetchError as NSError {
     print(fetchError!)
  }
  return [Dashboard]()
}

func displayRecords(fromFetchedRecords result:[Dashboard]) {
    print("Total records:\(result.count)")

    for dashboard in result { // the check for empty is not needed
       print("Value: \(dashboard.count)")
    }
}