Migrating to NSPersistentContainer

Update - 2017/07/12

if let oldUrl = oldUrl {
        let description = NSPersistentStoreDescription(url: oldUrl)
        persistentContainer.persistentStoreDescriptions = [description]
    }
    persistentContainer.loadPersistentStores { (description, error) in
        if let oldUrl = self.oldUrl {
        do {
            let psc = persistentContainer.persistentStoreCoordinator
            let store = psc.persistentStores[0]
            try psc.migratePersistentStore(store, to: url, options: nil, withType: NSSQLiteStoreType)                                                   self.deleteDatabase(url: oldUrl)
            self.cleanDb()
        } catch {
            block(description, error)
            return
        }
    }

Small update - instead of replacing, I use migratePersistentStore instead. This seems to work a little better with the current setup. So I have an oldUrl which is returned if the old store exists. If it's not nil I create the database against this and then migrate it to the correct location under Application Support. The old database still needs to be deleted in a separate step (assuming you no longer need it).


iOS 10 brought some welcome changes to Core Data and I need to migrate an existing database into this. Here’s how I did it.

First off, my existing DB is in the Documents folder. That’s not really good practice and NSPersistentContainer will create one in Application Support. That seems more sensible so I decided to move my Database. The default SQLite backed Core Data store includes SHM and WAL files (Write-Ahead Logging). To ensure you don’t lose data you need to move all three files at the same time. Fortunately, NSPersistentStoreCoordinator has built-in support for moving a database around.

First, I set up the NSPersistentStoreContainer:

persistentContainer = NSPersistentContainer(name: "GameModel")
persistentContainer.loadPersistentStores { (description, error) in
    if let error = error {
        fatalError("Could not create CoreData store: \(error)")
    }
    print(description)
}

This gives me something to work against. NSPersistentStoreContainer will create a NSPersistentStoreCoordinator with an sqlite DB at ApplicationSupport/GameModel.sqlite upon calling loadPersistentStores(completionHandler:). I want to replace that with the old Documents/Core_Data.sqlite. Fortunately, there’s a replace function right there.

let psc = persistentContainer.persistentStoreCoordinator
guard let storeUrl = psc.persistentStores.first?.url else {
    return
}
do {
    try psc.replacePersistentStore(at: storeUrl,
                                   destinationOptions: nil,
                                   withPersistentStoreFrom: oldUrl,
                                   sourceOptions: nil,
                                   ofType: NSSQLiteStoreType)
    persistentContainer.loadPersistentStores(completionHandler: { (description, error) in
        if let error = error {
            fatalError("Could not create CoreData store: \(error)")
        }
        print(description)
    })
} catch {
    print("Could not replace store: \(error)")
}

The newly created GameModel sqlite DB is replaced with the contents of the existing database by the replace call. I then need to call loadPersistentStores again to set up the NSPersistentContainer against the updated DB. Once it’s successful, I can delete the old DB files. There might be a better way, but this worked for me:

private func deleteOld(url: URL) {
        let parent = url.deletingLastPathComponent()
        let name = url.lastPathComponent
        do {
            try FileManager.default.contentsOfDirectory(at: parent, includingPropertiesForKeys: nil, options: [])
                .filter {
                    $0.lastPathComponent.hasPrefix(name)
                }
                .forEach {
                    try FileManager.default.removeItem(at: $0)
                }
        } catch {
            print("Failed to clear old DB: \(error)")
        }
    }

Where the passed URL is the old store URL.

Functional Programming - Getting Started

As, principally, an iOS developer, Swift has made for interesting times. Whilst the first code I wrote in Swift was heavily influenced by Obj-C patterns I (like many before me) quickly discovered that this was not the best way. A core part of Reactive Programming is Functional Programming, so that seems like a good place to start.

Following some best practices, and advice from others (including Apple’s WWDC sessions) I found I was moving to writing code that adhered more closely to the Functional Programming patterns. That’s not to say it was intentional, or that I was accidentally discovering Functional Programming on my own. Just that adopting it more formally is a smaller step than I was expecting.

In summary (and this is simplified) functional programming emphasises immutability and minimises state. The output of a function is dependent only on its input. An inherent requirement is that functions do not have side effects. A common description states:

[C]omputation as the evaluation of mathematical functions
- Functional programming - Wikipedia

I’ll be honest, it took me longer than it really should have for that statement to click with me. In the hope that I’m not the only one, here’s an example showing a simple mathematical function using imperative and functional approaches. We’ll evaluate the mathematical function 3 + 2 + 6 + -1:

class ImperativeNumber {
    var value: Int

   func add(value: Int) {
        self.value += value
    }

    init(value: Int) {
        self.value = value
    }
}

let imperativeThree = ImperativeNumber(value: 3)
var imperativeNumber = imperativeThree
imperativeNumber.add(value: 2)
imperativeNumber.add(value: 6)
imperativeNumber.add(value: -1)
print(imperativeNumber.value)      // 10
print(imperativeThree.value)   // 10

struct FunctionalNumber {
    let value: Int

    func add(value: Int) -> FunctionalNumber {
        return FunctionalNumber(value: self.value + value)
    }
}

let functionalThree = FunctionalNumber(value: 3)
let functionalNumber = functionalThree
    .add(value: 2)
    .add(value: 6)
    .add(value: -1)
print(functionalNumber.value)      // 10
print(functionalThree.value)   // 3

In functional programming, like the mathematical function, the value of “3” does not change because we added “2” to it. Instead, we have a new number that we can perform a new function on. If “3” did change, as in the imperative case, then anything else that used “3” in its calculations would be affected as “3” could now be “5”, or “11”, or “10”. It’s quite common to see this chaining pattern in functional programming, and reactive programming.

A note on naming. According to Swift guidelines, a function without side-effects should be a noun, and a function that has side-effects should be a verb. It’s the difference between getting an object, or performing an action on it. See also, Array.sort and Array.sorted. However, whilst some Swift types have imperative and functional equivalents, the versions which have their roots in functional programming typically only have the functional equivalent (e.g. filter, map, flatMap) regardless of if they could be implemented in an imperative fashion. If we have a purely functional type, it should not be unexpected, to use the verb form. However, when mixing functional and imperative code, it’s likely best to adhere to the guidelines:

struct ScoreBoard {
    private(set) var counter = 0

    mutating func increment() {
        counter += 1
    }

    func incremented() -> ScoreBoard {
        return ScoreBoard(counter: counter + 1)
    }
}

What functional programming means for your code is that it is safer. If an object can’t be mutated, you aren’t at risk of an object being modified by another thread whilst you read from it. It’s also easier to test when you are guaranteed the same result based on a consistent input.

Nonoku - Composition

Since I’m using the same background in a bunch of places I want to make it really easy to re-use. I could make a sub-class of UIViewController which implements the functionality. I’m not a fan of that, if it can be avoided. These things have tendency to grow. I could also create a static class method to return the background SKNode. That’s a little better but that’s just a form of composition and Swift actually gives us a nice way to implement this with Protcols and Extensions.

I start by moving declaring the function used for creating the background in a protocol. I’ll call it BackgroundProtocol because I’m awful at names. That’s easy to change later if I decide to add more functionality anyway.

protocol BackgroundProtocol {
    func createBackground(size: CGSize) -> SKNode
}

I can add that protocol to other classes and they’ll have the createBackground(size:) function available to them. Using an extension I can then create a default implementation (by striking coincidence, that’s the code I already had for this).

extension BackgroundProtocol {
    func createBackground(size: CGSize) -> SKNode {         
        let node = SKEffectNode()
        // snip ...
        return node
    }
}

Now I can add that to all the views in my app and they can just call it to get the functionality. And, of course, it can be overridden if needed.

class MainMenuViewController: UIViewController, BackgroundProtocol {
    override func viewDidLoad() {
        super.viewDidLoad()
        let spriteView = (view as? SKView)!
        let scene = SKScene(size: view.bounds.size)
        scene.addChild(createBackground(size: scene.size))
        spriteView.presentScene(scene)
    }
}

Nonoku - Creating a Background

So, I was getting bored of looking at a grey background and, as a break from cleaning up, refactoring, and adding the fiddly (nut necessary) UI code, I decided to run up a quick background.

The original version of Nonoku had a vertical gradient fill, light to dark, with subtle diagonal striping. I liked this so decided to reproduce it. However, it made sense to me to implement this in code. Since I’m using SpriteKit already, rendering to an SKEffectNode which rasterises the nodes made sense (the background is not animated so I can basically update once and be done).

There’s no native way, as far as I’m aware, to draw a gradient on a node, so the first thing would be to create an SKTexture. Again, since this is a one time deal, I’m not too worried. I’ve done something similar before so I already knew I could use a CIFilter to generate this. From that, I can get a CIImage. That can be passed to a CIContext to create a CGImage which can finally be passed to the SKTexture which I’ll pass into an SKSprite.

From reviewing the CIFilter documentation, I can get a list of available filters as follows:

class func filterNames(inCategory category: String?) -> [String]

Even better, under the list of category constants, there is kCICategoryGradient. For this kind of quick exploratory code, I usually start a Swift Playground to spike out the things I don’t know.

So, skipping a step, I have this:

print("\(CIFilter.filterNames(inCategory: kCICategoryGradient))")
let filter = CIFilter(name: "CILinearGradient")
print("\(filter?.attributes)")

The list of filterNames has a couple of likely contenders, CILinearGradient and CISmoothLinearGradient. They take the same attributes (two colours, and two vectors) so I ended up trying both. In my use case I couldn’t see any difference between the two so decided to stick with CILinearGradient. If it ever comes up as an issue, it’s a very simple change to make.

CIFilter.attributes() gives me a list of the supported attributes, and a brief description of them. That's enough to define how I'm going to use it so I can leave the playground and come back to my code.

Since I want to create a new SKTexture with this gradient it makes sense to do it as an extension on SKTexture. I need the size, the start colour, and the end colour. I could add additional logic here but, following YAGNI (You Ain’t Gonna Need It), I am only interested in a vertical gradient so that’s all I’ll support.

extension SKTexture {
    convenience init?(size: CGSize, color1: SKColor, color2: SKColor) {
        guard let filter = CIFilter(name: "CILinearGradient") else {
            return nil
        }
        let startVector = CIVector(x: 0, y: size.height)
        let endVector = CIVector(x: 0, y: 0)
        filter.setValue(startVector, forKey: "inputPoint0")
        filter.setValue(endVector, forKey: "inputPoint1")
        filter.setValue(CIColor(color: color1), forKey: "inputColor0")
        filter.setValue(CIColor(color: color2), forKey: "inputColor1")

        let context = CIContext(options: nil)
        let rect = CGRect(x: 0, y: 0, width: size.width, height: size.height)
        guard let filterImage = filter.outputImage,
            let image = context.createCGImage(filterImage, from: rect) else {
            return nil
        }

        self.init(cgImage: image)
    }
}

Interestingly, whilst I can just get a ciColor from a UIColor (SKColor aliases UIColor), I can’t use it in the CIFilter due to the following issue.

*** -CIColor not defined for the UIColor UIExtendedSRGBColorSpace 0.666667 0.223529 0.223529 1; need to first convert colorspace.

Instead, I had to create a new instance of CIColor with the UIColor. I’m not sure if there are any issues associated with this, but it’s working fine for me so far.

From there, I just created an SKSpriteNode with the generated texture. The lines were just stroked SKShapeNodes with a CGPath which just have a defined start and end point.

So here’s how it looks:

It’s still early so I’m not sure if this is the final version but it does make the game easier to play as it’s much easier to tell where the empty spaces are. Also, with this approach I can easily generate backgrounds with different colours so I have a bunch of options.

It’s still early so I’m not sure if this is the final version but it does make the game easier to play as it’s much easier to tell where the empty spaces are. Also, with this approach I can easily generate backgrounds with different colours so I have a bunch of options.

Unit Testing - Mocking In Swift

Mocking is a useful tool when writing unit tests. Due to limitations in the current swift version, there aren’t any real mocking frameworks like the ones you see for Java and Obj-C. That said, there are work arounds. Here’s a quick one when you need a one-off:

Method to test:

func crossDissolve(toIdentifier identifier: StoryboardIdentifier) {
    let nextViewController = viewController(forIdentifier: identifier)
    nextViewController.modalPresentationStyle = .fullScreen
    nextViewController.modalTransitionStyle = .crossDissolve
    show(nextViewController, sender: self)
}

This just performs a simple cross-dissolve between two view controllers (the one it’s on to the new one).There are four things to validate:

  1. The UIViewController passed to show(_:sender:) is the one expect.
  2. The sender is correct
  3. That the presentation style is fullScreen
  4. The transition style is crossDissolve

Since it doesn’t return any values I’m going to have to capture them instead. The method under test is crossDissolve(…) so I don’t want to change that behaviour. Everything else is fair game though. In this case, if I intercept the call to show(…) I can capture the parameters passed and validate them.

Since this is a one-off I can nest a class inside my test and capture the values appropriately. Then I can fill in the test.

func testCrossDissolve() {
    class MockSut: UIViewController {
        var showViewController: UIViewController?
        var showSender: Any?
        override func show(_ vc: UIViewController, sender: Any?) {
            showViewController = vc
            showSender = sender
        }
    }
    let mockSut = MockSut()
    mockSut.crossDissolve(toIdentifier: .gameViewController)
    XCTAssertNotNil(mockSut.showViewController as? GameViewController)
    XCTAssertEqual(mockSut.showSender as? UIViewController, mockSut)
    XCTAssertEqual(mockSut.showViewController?.modalPresentationStyle, .fullScreen)
    XCTAssertEqual(mockSut.showViewController?.modalTransitionStyle, .crossDissolve)
}

So, we’re creating a subclass of UIViewController and overriding a method that is called by the method we are interested in testing. Then we can use assertions to complete our test.

Of course, this could get messy if we had a bunch of test cases which needed to handle overrides. In that case I’d move the MockSut class out of the function and into the parent class. If I needed it outside of this specific set of tests, I’d move it into its own class so it could be used in multiple places.

Cleaning Up The Code

So, looking back over the last code, I realised that I was overthinking it massively. The original reasoning behind it didn’t hold up, so I fixed it.

protocol NavigationProtocol {
    func viewController(forIdentifier identifier: StoryboardIdentifier) -> UIViewController
}

extension NavigationProtocol {
    private var storyboard: UIStoryboard {
        return UIStoryboard(name: "Main", bundle: nil)
    }

    func viewController(forIdentifier identifier: StoryboardIdentifier) -> UIViewController {
        return storyboard.instantiate(withIdentifier: identifier)
    }
}

Yep, that’s a bit smaller. Hooray.

Bonus - time for unit tests ;)

func testInstantiateViewController_Splash() {
        let viewController = sut.viewController(forIdentifier: .splashViewController)
        XCTAssertTrue(viewController is SplashViewController)
    }

    func testInstantiateViewController_MainMenu() {
        let viewController = sut.viewController(forIdentifier: .mainMenuViewController)
        XCTAssertTrue(viewController is MainMenuViewController)
    }

Swift Generics

After installing SwiftLint I was going through resolving the issues. Fortunately, I hadn’t too many problems, mostly because it’s still a small project.

Some longish lines did catch my attention though.

enum StoryboardIdentifier: String {
    case splashViewController
    case mainMenuViewController
    case levelSelectViewController
    case optionsViewController
    case gameViewController
}

fileprivate extension UIStoryboard {
    func instantiateAppViewController(withIdentifier identifier: StoryboardIdentifier) -> UIViewController {
        return instantiateViewController(withIdentifier: identifier.rawValue)
    }
}

fileprivate class NavigationAssistant {
    private static let mainStoryboard = UIStoryboard(name: "Main", bundle: nil)

    private static var splashViewController: SplashViewController {
        return (mainStoryboard.instantiateAppViewController(withIdentifier: .splashViewController) as? SplashViewController)!
    }

    private static var mainMenuViewController: MainMenuViewController {
        return (mainStoryboard.instantiateAppViewController(withIdentifier: .mainMenuViewController) as? MainMenuViewController)!
    }

    private static var levelSelectViewController: LevelSelectViewController {
        return (mainStoryboard.instantiateAppViewController(withIdentifier: .levelSelectViewController) as? LevelSelectViewController)!
    }

    private static var optionsViewController: OptionsViewController {
        return (mainStoryboard.instantiateAppViewController(withIdentifier: .optionsViewController) as? OptionsViewController)!
    }

    private static var gameViewController: GameViewController {
        return (mainStoryboard.instantiateAppViewController(withIdentifier: .gameViewController) as? GameViewController)!
    }

As often seems to happen, on revisiting I can see a way to improve it. First off, it's a little bit verbose in the naming. Changing instantiateAppViewController to just instantiate shortens everything up to get it well under the limit. AppViewController is superfluous, named only to stop a naming collision between the extension and the actual class, and ViewController for the thing it is returning. instantiate could possibly be improved upon but it's clear enough in context for the moment.

That’s minor stuff though. What really caught my eye is that I’m repeating the same code over and over. That cast is kind of messy since I’m kind of cheating SwiftLint there. Without that I had return mainStoryboard.instantiateAppViewController(withIdentifier: .splashViewController) as! SplashViewController which is marginally shorter but it’s not really the problem. What I don’t like is that it’s the same call each time with the only change being the Type I’m casting to. If, instead of casting back as a UIViewController, I was to cast back as the type I was expecting then that would save a bunch of repeated code.

I’ve used generics a bunch of times but I keep forgetting about them - at least when I’m first writing out the code. That’s not necessarily a bad thing since the current way works and I’ve come back to it eventually to clean things up. That’s partly my reasoning for writing this post though, to remember to use these things (appropriately).

So, modify the call which returns the UIViewController to return a generic. Easy:

fileprivate extension UIStoryboard {
    func instantiate<T>(withIdentifier identifier: StoryboardIdentifier) -> T {
        return (instantiateViewController(withIdentifier: identifier.rawValue) as? T)!
    }
}

Fairly straight forward, still have that ugly ‘cast to optional, force unwrap’ to appease SwiftLint. Wait a second, SwiftLint also has a rule about force unwrapping! Optional, but maybe there’s a better way.

guard let viewController = instantiateViewController(withIdentifier: identifier.rawValue) as? T else {
    fatalError("Could not instantiate viewController")
 }
return viewController

There we go. Now it’s explicit what the behaviour is meant to be. It’s a little more code but no-one scanning the code should be confused by the intent.

So the whole thing looks like this now. Much better. The repeated logic is now in a single place. I'm happier with how it looks and, whilst there might be more lines, it feels more compact. I think there are some other improvements that can be made but that goes outside the scope of this post.

enum StoryboardIdentifier: String {
    case splashViewController
    case mainMenuViewController
    case levelSelectViewController
    case optionsViewController
    case gameViewController
}

fileprivate extension UIStoryboard {
    func instantiate<T>(withIdentifier identifier: StoryboardIdentifier) -> T {
        guard let viewController = instantiateViewController(withIdentifier: identifier.rawValue) as? T else {
            fatalError("Could not instantiate viewController")
        }
        return viewController
    }
}

fileprivate class NavigationAssistant {
    private static let mainStoryboard = UIStoryboard(name: "Main", bundle: nil)

    private static var splashViewController: SplashViewController {
        return mainStoryboard.instantiate(withIdentifier: .splashViewController)
    }

    private static var mainMenuViewController: MainMenuViewController {
        return mainStoryboard.instantiate(withIdentifier: .mainMenuViewController)
    }

    private static var levelSelectViewController: LevelSelectViewController {
        return mainStoryboard.instantiate(withIdentifier: .levelSelectViewController)
    }

    private static var optionsViewController: OptionsViewController {
        return mainStoryboard.instantiate(withIdentifier: .optionsViewController)
    }

    private static var gameViewController: GameViewController {
        return mainStoryboard.instantiatZ(withIdentifier: .gameViewController)
    }