Swift Core Data NSFetchRequest 没有结果

Posted 2023-03-12

【中文标题】Swift Core Data NSFetchRequest 没有结果

【英文标题】：Swift Core Data NSFetchRequest no results

【发布时间】：2015-02-14 13:54:20

【问题描述】：

我在使用 Core Data 时遇到了问题。我有这个名为“Test.xcdatamodeld”的核心数据模型，它只包含一个名为“Data”的实体。此数据实体包含一个名为“currCount”的属性，类型为Int32。

我正在尝试读取“currCount”的值，但是当我尝试从实体“数据”中获取某些内容时，没有结果。

这是我的获取代码：

var appDel:AppDelegate = (UIApplication.sharedApplication().delegate as AppDelegate)
var context:NSManagedObjectContext = appDel.managedObjectContext!

var request = NSFetchRequest(entityName: "Data")
request.returnsObjectsAsFaults = false
var results: NSArray = context.executeFetchRequest(request, error: nil)!
var currCount:Int32 = 0
if results.count > 0 
    var res = results[0] as NSManagedObject
    currCount = Int32(res.valueForKey("currCount") as Int)
    println("Local Count: \(currCount)")
    return currCount

else 
    println("firstTime no Questions")            
    return 0

我总是以 else 语句结束。我不明白我做错了什么。我在另一个项目中一直采用这种方式，并且效果很好。

我的AppDelegate.swift如下：

import UIKit
import CoreData

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate 

    var window: UIWindow?

    func application(application: UIApplication!, didFinishLaunchingWithOptions launchOptions: NSDictionary!) -> Bool 
        return true
    

    func applicationWillResignActive(application: UIApplication!) 
        // Sent when the application is about to move from active to inactive state. This can occur for certain types of temporary interruptions (such as an incoming phone call or SMS message) or when the user quits the application and it begins the transition to the background state.
        // Use this method to pause ongoing tasks, disable timers, and throttle down OpenGL ES frame rates. Games should use this method to pause the game.
    

    func applicationDidEnterBackground(application: UIApplication!) 
        // Use this method to release shared resources, save user data, invalidate timers, and store enough application state information to restore your application to its current state in case it is terminated later.
        // If your application supports background execution, this method is called instead of applicationWillTerminate: when the user quits.
    

    func applicationWillEnterForeground(application: UIApplication!) 
        // Called as part of the transition from the background to the inactive state; here you can undo many of the changes made on entering the background.
    

    func applicationDidBecomeActive(application: UIApplication!) 
        // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    

    func applicationWillTerminate(application: UIApplication!) 
        // Called when the application is about to terminate. Save data if appropriate. See also applicationDidEnterBackground:.
    

    // MARK: - Core Data stack

    lazy var applicationDocumentsDirectory: NSURL = 
        // The directory the application uses to store the Core Data store file. This code uses a directory named "com.jqsoftware.MyLog" in the application's documents Application Support directory.
        let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
        return urls[urls.count-1] as NSURL
        ()

    lazy var managedObjectModel: NSManagedObjectModel = 
        // The managed object model for the application. This property is not optional. It is a fatal error for the application not to be able to find and load its model.
        let modelURL = NSBundle.mainBundle().URLForResource("Test", withExtension: "momd")!
        return NSManagedObjectModel(contentsOfURL: modelURL)!
        ()

    lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator? = 
        // The persistent store coordinator for the application. This implementation creates and return a coordinator, having added the store for the application to it. This property is optional since there are legitimate error conditions that could cause the creation of the store to fail.
        // Create the coordinator and store
        var coordinator: NSPersistentStoreCoordinator? = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
        let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("Test.sqlite")
        var error: NSError? = nil
        var failureReason = "There was an error creating or loading the application's saved data."
        if coordinator!.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil, error: &error) == nil 
            coordinator = nil
            // Report any error we got.
            var dict = [String: AnyObject]()
            dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
            dict[NSLocalizedFailureReasonErrorKey] = failureReason
            dict[NSUnderlyingErrorKey] = error
            error = NSError(domain: "YOUR_ERROR_DOMAIN", code: 9999, userInfo: dict)
            // Replace this with code to handle the error appropriately.
            // abort() 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.
            NSLog("Unresolved error \(error), \(error!.userInfo)")
            abort()
        

        return coordinator
        ()

    lazy var managedObjectContext: NSManagedObjectContext? = 
        // Returns the managed object context for the application (which is already bound to the persistent store coordinator for the application.) This property is optional since there are legitimate error conditions that could cause the creation of the context to fail.
        let coordinator = self.persistentStoreCoordinator
        if coordinator == nil 
            return nil
        
        var managedObjectContext = NSManagedObjectContext()
        managedObjectContext.persistentStoreCoordinator = coordinator
        return managedObjectContext
        ()

    // MARK: - Core Data Saving support

    func saveContext () 
        if let moc = self.managedObjectContext 
            var error: NSError? = nil
            if moc.hasChanges && !moc.save(&error) 
                // Replace this implementation with code to handle the error appropriately.
                // abort() 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.
                NSLog("Unresolved error \(error), \(error!.userInfo)")
                abort()
            
        
    

编辑 即使我在再次获取之前将某些内容保存到 currCount，我也会得到 0 个结果。这是我用来保存的代码（在上面的代码之前）：

var appDel:AppDelegate = (UIApplication.sharedApplication().delegate as AppDelegate)
var context:NSManagedObjectContext = appDel.managedObjectContext!

var request = NSFetchRequest(entityName: "Data")
request.returnsObjectsAsFaults = false
var results: NSArray = context.executeFetchRequest(request, error: nil)!
var currentCount:Int32 = 5
var newCount = NSEntityDescription.insertNewObjectForEntityForName("Data", inManagedObjectContext: context) as NSManagedObject
newHp.setValue(currentCount, forKey: "currCount")

【问题讨论】：

在哪里插入对象到上下文中并保存？

我还没有插入任何东西，但是即使我尝试这样做，下次我尝试获取它也不会在那里，它会说 0 结果。

请出示代码。

@flexaddicted 好的，请检查我对原始帖子的编辑！

【参考方案1】：

一旦你在你的上下文中插入了一些东西，你应该保存它以使更改永久化（我想你正在使用 SQLite 存储）。

换句话说，您需要运行context.save(&error) 方法。

更新 1

var error: NSError?
if !context.save(&error) 
    println("Error saving context: \(error?.localizedDescription)\n\(error?.userInfo)")

【讨论】：

什么是moc？我可以在 setValue 之后输入 moc.save(&error) 吗？

我已经编辑了这个问题。 moc 表示托管对象上下文。和context一样。