// updated for Swift 3
class MigrationManager: NSObject {

    weak var delegate: MigrationManagerDelegate?

    func migrate(from sourceURL: URL, to destinationURL: URL) throws {

        guard let sourceMetadata = try? NSPersistentStoreCoordinator.metadataForPersistentStore(ofType: NSSQLiteStoreType, at: sourceURL) else {
            throw MigrationError.noMetadata
        }

        guard let sourceModel = NSManagedObjectModel.mergedModel(from: [Bundle.main], forStoreMetadata: sourceMetadata) else {
            throw MigrationError.noSourceModel
        }

        // ...
        // get destination model
        // for more code surrounding this portion, you should reference the great objc.io article here:
        // https://www.objc.io/issues/4-core-data/core-data-migration/
        // ...

        let manager = NSMigrationManager(sourceModel: sourceModel, destinationModel: destinationModel)

        manager.addObserver(self, forKeyPath: "migrationProgress", options: .new, context: nil)

        // start migrating...
    }

    // ...

    override func observeValue(forKeyPath keyPath: String?, of object: Any?, change: [NSKeyValueChangeKey : Any]?, context: UnsafeMutableRawPointer?) {
        if keyPath == "migrationProgress" {
            guard let manager = object as? NSMigrationManager else {
                return
            }
            delegate?.migrationManager(manager: self, progress: manager.migrationProgress, userInfo: nil)
        }
    }
}

class Policy: NSEntityMigrationPolicy {
    override func createDestinationInstances(forSource sInstance: NSManagedObject, in mapping: NSEntityMapping, manager: NSMigrationManager) throws {

        if (manager.userInfo == nil) {
            let fetchRequest = NSFetchRequest<NSFetchRequestResult>(entityName: "MyObject")
            fetchRequest.includesSubentities = false

            // get the count and save it
            if let count = try? manager.sourceContext.count(for: fetchRequest),
                count != NSNotFound
            {
                manager.userInfo = ["totalCount": count]
            }
        }

        // ... set up our destination object

        // when we've migrated the object, let's give the current count
        if let userInfo = manager.userInfo {
            var mutableUserInfo = userInfo
            let currentCount = mutableUserInfo["currentCount"] as? Int ?? 0
            mutableUserInfo["currentCount"] = currentCount + 1
            manager.userInfo = mutableUserInfo
        }
    }
}