ios - Implement CoreData into an existing project using Swift

Question

I am currently using Xcode 6 beta 7 and decided I needed CoreData after already creating my project. I have looked at some of the questions already asking this for objective C along with watching videos on the subject but I seem to be getting the same error. As for my usage of the CoreData framework itself, I am pretty sure it is correct as I created another empty project (with the CoreData box checked) and followed the same implementation and it worked like a charm but there may be something wrong I'm doing there too. So here are the steps I followed to implement CoreData in my project in Swift.

Step 1: I added the CoreData framework through "Link Binary with Libraries" tab under "Build Phases"

Step 2: I then went to any of my .swift files which would need to implement CoreData (appDelegate.swift and one other file as of now) and added the line:

import CoreData

to the top.

Step 3: I created a data list with an entity called "cData", then made a class for it called "data.swift". Here is data.swift:

import UIKit
import CoreData

@objc( data )
class data: NSManagedObject {
    @NSManaged var something : String
}

Step 4: I wrote the code to actually save the data in another file:

@IBAction func useCoreData(sender: AnyObject)
    {
        let AD : AppDelegate = UIApplication.sharedApplication().delegate as AppDelegate
        let ct : NSManagedObjectContext = AD.managedObjectContext!
        let ent = NSEntityDescription.entityForName( "CData", inManagedObjectContext: ct )
        var dat = data( entity: ent!, insertIntoManagedObjectContext: ct )
        dat.something = someTextField.text
        ct.save( nil )

        println(dat)
    }

Step 5: Here is where I believe I messed up, though I could be wrong. I created a new project on a separate Mac (so I could name it the same thing) and checked the CoreData box. I then copied the entire appDelegate.swift from this project to my current one. Here is my appDelegate.swift:

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 active 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:.
        // Saves changes in the application's managed object context before the application terminates.
        self.saveContext()
    }

    // 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 "y.Simple_Grade" 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("my_app", 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("my_app.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.
            let dict = NSMutableDictionary()
            dict[NSLocalizedDescriptionKey] = "Failed to initialize the application's saved data"
            dict[NSLocalizedFailureReasonErrorKey] = failureReason
            dict[NSUnderlyingErrorKey] = error
            error = NSError.errorWithDomain("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()
            }
        }
    }
}

Once the IBAction method "useCoreData" is called, I get a crash and the exception states "fatal error: unexpectedly found nil while unwrapping an Optional value (lldb)" and states the offending line is in appDelegate.swift:

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

Again, when I try this same CoreData implementation in another project that had the CoreData box checked from the get-go, it works like a charm. I saw a similar question like this (the user was getting the same error), but it doesn't seem like his problem was solved either.

Answer 1

You're getting an nil value there because the file it's looking for doesn't exist in your main bundle.

You need to copy your data model file from the other project you created to your main project. The file would be called something like My_App.xcdatamodeld and should be located in the same folder that your Xcode project file is in.

Note: The URLForResource line is looking for My_App.momd; that file is created by Xcode from My_App.xcdatamodeld when it compiles your project.