An asynchronous Swift 2.0 Core Data stack loosely based on Marcus Zarra's (http://martiancraft.com/blog/2015/03/core-data-stack/) with asynchronous configuration

SGAsyncSetupCoreDataStore.swift

//
//  SGAsyncSetupCoreDataStore.swift
//  Core Data Example
//
//  Created by Groom, Stephen on 10/07/2015.
//  Copyright © 2015 Stephen Groom. All rights reserved.
//

import CoreData

public enum Fallible<T> {
    case Success(T)
    case Failure(ErrorType)
}

public enum SGCoreDataError: ErrorType {
    case InvalidModelURL
}

public class SGAsyncSetupCoreDataStore {
    
    /**
    Used internally so that saving to disk is done asynchronously
    */
    private var privateContext: NSManagedObjectContext!
    /**
    For reading on the main thread only
    */
    public var mainContext: NSManagedObjectContext!
    /**
    For all write operations and fetches which can be performed asynchronously
    */
    public var backgroundContext: NSManagedObjectContext!

    /**
    Attempts to create a core data store in the background and calls a completion when done

    :param: modelURL The URL of the .mom or .momd file
    
    :param: storeURL The url used to create or read the SQLite store
    
    :param: completion The completion closure is called with either a fully configured instance of the store
                        or an ErrorType
                        
    :warning: The closure is called on an arbitrary queue

    */
    class func setupDataStore(modelURL modelURL: NSURL, storeURL: NSURL, completion: (Fallible<SGAsyncSetupCoreDataStore>) -> ())
    {
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0)) { () -> Void in
            do {
                let instance = SGAsyncSetupCoreDataStore()
                guard let managedObjectModel = NSManagedObjectModel(contentsOfURL: modelURL) else { throw SGCoreDataError.InvalidModelURL }
                let persistentStoreCoordinator = NSPersistentStoreCoordinator(managedObjectModel: managedObjectModel)
                try persistentStoreCoordinator.addPersistentStoreWithType(NSSQLiteStoreType,
                    configuration: nil,
                    URL: storeURL,
                    options: [NSInferMappingModelAutomaticallyOption : true, NSMigratePersistentStoresAutomaticallyOption : true])
                
                instance.privateContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
                instance.privateContext.persistentStoreCoordinator = persistentStoreCoordinator
                
                instance.mainContext = NSManagedObjectContext(concurrencyType: .MainQueueConcurrencyType)
                instance.mainContext.parentContext = instance.privateContext
                
                instance.backgroundContext = NSManagedObjectContext(concurrencyType: .PrivateQueueConcurrencyType)
                instance.backgroundContext.parentContext = instance.mainContext
                
                completion(Fallible.Success(instance))
            } catch let error {
                completion(Fallible.Failure(error))
            }
        }
    }
}