Making someone else’s class conform to Codable

Paul Hudson    @twostraws    December 4th 2019

Any app that asks the user to enter data usually works better when it stores the data it was given, but this is easier said than done when working with Apple’s frameworks.

In our app we’re using MKPointAnnotation to store interesting places the user wants to visit, and we’d like to use the iOS storage to save it permanently. Create a new Swift file called MKPointAnnotation-Codable.swift, add an import for MapKit, then give it this code:

extension MKPointAnnotation: Codable {
    public required init(from decoder: Decoder) throws {

    }

    public func encode(to encoder: Encoder) throws {

    }
}

That’s a custom conformance to Codable, but it doesn’t do anything. However, it already doesn’t work: if you try building you’ll see the error “'required' initializer must be declared directly in class 'MKPointAnnotation' (not in an extension)”.

Let me get right to the point: there is no way of making this work in Swift.

It’s not required that you understand why this is impossible, but I do think it sheds some light on how Swift works.

MKPointAnnotation isn’t a final class, which means other classes can inherit from it. We might be able to implement Codable conformance for this one class, but in doing so we’re also saying that all subclasses should also be Codable and that’s not a promise we can keep.

There are a few solutions to this:

• MKPointAnnotation is a class that implements the MKAnnotation protocol, so we could just create our own class that conforms to the same protocol.
• We could create a subclass of MKPointAnnotation and implement Codable there, effectively shielding the MKPointAnnotation from any knowledge that Codable is being used. This is now our class so we can force subclasses to conform to Codable.
• We could create a wrapper struct around the class, making the struct conform to Codable and store an MKPointAnnotation internally.

All three of those are good options, and you can easily make a case that any of them are the right option here. However, the easiest option is the subclass, because we can implement it in a single file, then change only two instances of MKPointAnnotation to make it work with the rest of our code.

First, the code. We’re going to create a new class called CodableMKPointAnnotation that inherits from MKPointAnnotation and conforms to Codable. We do need to provide a custom Codable implementation so that all our data gets saved, and that’s mostly straightforward – the only wrinkle is that CLLocationCoordinate2D doesn’t already conform to Codable, so we’ll save it as latitude and longitude.

Other than that there’s nothing special here, so replace whatever you have in MKPointAnnotation-Codable.swift with this:

class CodableMKPointAnnotation: MKPointAnnotation, Codable {
    enum CodingKeys: CodingKey {
        case title, subtitle, latitude, longitude
    }

    override init() {
        super.init()
    }

    public required init(from decoder: Decoder) throws {
        super.init()

        let container = try decoder.container(keyedBy: CodingKeys.self)
        title = try container.decode(String.self, forKey: .title)
        subtitle = try container.decode(String.self, forKey: .subtitle)

        let latitude = try container.decode(CLLocationDegrees.self, forKey: .latitude)
        let longitude = try container.decode(CLLocationDegrees.self, forKey: .longitude)
        coordinate = CLLocationCoordinate2D(latitude: latitude, longitude: longitude)
    }

    public func encode(to encoder: Encoder) throws {
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(title, forKey: .title)
        try container.encode(subtitle, forKey: .subtitle)
        try container.encode(coordinate.latitude, forKey: .latitude)
        try container.encode(coordinate.longitude, forKey: .longitude)
    }
}

The MKPointAnnotation class is used in several places around our project, but we only need to change it in two places. First, change the location property in ContentView to this:

@State private var locations = [CodableMKPointAnnotation]()

And now change the action of the + button in ContentView so that newLocation also uses our new subclass:

let newLocation = CodableMKPointAnnotation()

We don’t need to change the other places because CodableMKPointAnnotation is a subclass of MKPointAnnotation, which means any place we use an MKPointAnnotation we can send in a CodableMKPointAnnotation. This is technically known as behavioral subtyping, but you’ll more commonly hear it called the Liskov Substitution Principle after its creator, Barbara Liskov. If you’ve ever heard the term “SOLID”, this is the “L”!

Anyway, where things get interesting is how we load and save the data, because this time we’re not going to use UserDefaults. Instead, we’re going to write our JSON to the iOS filesystem, so we can write as much data as we need.

Previously I showed you how to find our app’s documents directory, so start by adding this method to ContentView:

func getDocumentsDirectory() -> URL {
    let paths = FileManager.default.urls(for: .documentDirectory, in: .userDomainMask)
    return paths[0]
}

With that in place, we can now using getDocumentsDirectory().appendingPathComponent() to create new URLs that point to a specific file in the documents directory. Once we have that, it’s as simple as using Data(contentsOf:) and JSONDecoder() to load our data – both things we’ve used before.

So, add this loadData() method to ContentView:

func loadData() {
    let filename = getDocumentsDirectory().appendingPathComponent("SavedPlaces")

    do {
        let data = try Data(contentsOf: filename)
        locations = try JSONDecoder().decode([CodableMKPointAnnotation].self, from: data)
    } catch {
        print("Unable to load saved data.")
    }
}

Using this approach we can write any amount of data in any number of files – it’s much more flexible than UserDefaults, and if we need it also allows us to load and save data as needed rather than immediately when the app launches as with UserDefaults.

However, another benefit of this approach is the way we write stuff. Sure, we’re going to use the same getDocumentsDirectory() and JSONEncoder dance to get our data ready, but this time we’re going to use the write(to:) method to save the data to disk, writing to a particular URL.

Previously I showed you this method with strings, but the Data version is even better because it lets us do something quite amazing in just one line of code: we can ask iOS to ensure the file is written with encryption so that it can only be read once the user has unlocked their device. This is in addition to requesting atomic writes – iOS does almost all the work for us.

Add this method to ContentView now:

func saveData() {
    do {
        let filename = getDocumentsDirectory().appendingPathComponent("SavedPlaces")
        let data = try JSONEncoder().encode(self.locations)
        try data.write(to: filename, options: [.atomicWrite, .completeFileProtection])
    } catch {
        print("Unable to save data.")
    }
}

Yes, all it takes to ensure that the file is stored with strong encryption is to add .completeFileProtection to the data writing options.

After all that work, the last thing we need to do is actually connect those methods up to SwiftUI, so that everything gets automatically loaded and saved.

For loading data, we just need to add an onAppear() modifier to the ZStack in ContentView:

.onAppear(perform: loadData)

For saving, we can use the same onDismiss parameter for sheet() that was introduced back in project 13. This means we save the data every time EditView is dismissed, which means we save new items as well as edited items.

So, change the sheet() modifier in ContentView to this:

.sheet(isPresented: $showingEditScreen, onDismiss: saveData) {

Go ahead and run the app now, and you should find that you can add items freely, then relaunch the app to see them restored just as they were.

That took quite a bit of code in total, but the end result is that we have loading and saving done really well:

• The Codable conformance is all isolated in one file, so SwiftUI doesn’t have to care about it.
• When we write data we’re making iOS encrypt it so the file can’t be read or written until the user unlocks their device.
• The load and save process is almost transparent – we added one modifier and changed another, and that’s all it took.

Of course, our app isn’t truly secure yet: we’ve ensured our data file is saved out using encryption so that it can only be read once the device has been unlocked, but there’s nothing stopping someone else from reading the data afterwards.