将 iOS 核心数据添加到现有项目 - 展开可选值时出错

Question

我正在尝试将 CoreData 添加到现有项目中。因此，我从一个新项目中复制了数据模型 (SWS.xcdatamodel) 和 AppDelegate 文件。然后我在 AppDelegate 方法中更改了项目名称和数据模型名称

import UIKit
import CoreData

@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {

    var window: UIWindow?


    func application(application: UIApplication, didFinishLaunchingWithOptions launchOptions: [NSObject: AnyObject]?) -> Bool {
        // Override point for customization after application launch.
        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 "at.meinhard-kissich.SWSCore" in the application's documents Application Support directory.
        let urls = NSFileManager.defaultManager().URLsForDirectory(.DocumentDirectory, inDomains: .UserDomainMask)
        return urls[urls.count-1]
        }()

    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("SWS", withExtension: "momd")!
        return NSManagedObjectModel(contentsOfURL: modelURL)!
        }()

    lazy var persistentStoreCoordinator: NSPersistentStoreCoordinator = {
        // The persistent store coordinator for the application. This implementation creates and returns 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
        let coordinator = NSPersistentStoreCoordinator(managedObjectModel: self.managedObjectModel)
        let url = self.applicationDocumentsDirectory.URLByAppendingPathComponent("SWS.sqlite")
        var failureReason = "There was an error creating or loading the application's saved data."
        do {
            try coordinator.addPersistentStoreWithType(NSSQLiteStoreType, configuration: nil, URL: url, options: nil)
        } catch {
            // 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 as NSError
            let wrappedError = 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 \(wrappedError), \(wrappedError.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
        var managedObjectContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)
        managedObjectContext.persistentStoreCoordinator = coordinator
        return managedObjectContext
        }()

    // MARK: - Core Data Saving support

    func saveContext () {
        if managedObjectContext.hasChanges {
            do {
                try managedObjectContext.save()
            } catch {
                // 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.
                let nserror = error as NSError
                NSLog("Unresolved error \(nserror), \(nserror.userInfo)")
                abort()
            }
        }
    }



}

之后我尝试将新数据集写入数据库

    let appDelegate = (UIApplication.sharedApplication().delegate as! AppDelegate)
    let managedContext = appDelegate.managedObjectContext

    let entity = (NSEntityDescription.insertNewObjectForEntityForName("WeatherData", inManagedObjectContext: managedContext)) as NSManagedObject


    entity.setValue(32, forKey: "id")


    do {
        try managedContext.save()
    } catch let error as NSError {
        print("Could not save \(error), \(error.userInfo)")
    }
    print("obj saved")

不幸的是，我总是收到以下错误：

fatal error: unexpectedly found nil while unwrapping an Optional value

在这一行：

let modelURL = NSBundle.mainBundle().URLForResource("SWS", withExtension: "momd")!

问题可能出在哪里？

• 谢谢！

Answer 1

问题可能出在哪里？

这很简单：您将 NSBundle.mainBundle().URLForResource("SWS", withExtension: "momd") 返回的 Optional 值强制解包为 !，这意味着如果该值不存在，则应用程序将崩溃。实际上这里的问题是 NSBundle 找不到您的资源。

Answer 2

您正试图强制解开可选值，这不是正确的方法，因为它是可选的，它可能包含也可能不包含任何内容。当您强制尝试使用它并且如果它包含nil 那么这就是您将得到的回应。

所以正确的方法是通过首先检查它是否包含某些东西来安全地打开可选值。您可以使用if 语句轻松地做到这一点。