BENEDIKT TERHECHTE

This is my research notebook. I'm an OSX / iOS indie developer. After 8 years of Objective-C I really enjoy Swift nowadays. Trying to publish all my research on Development, Swift & other technologies here.

Sat, 23 Apr 2016 #

Raw value initializers for enums with associated types

Enums are a beautiful way of structuring information in Swift. Sometimes you find yourself initializing enums from raw values, maybe because the values were intermittendly stored somewhere else, say in the NSUserDefaults:

enum Device: String {
  case phone, tablet, watch
}
let aDevice = Device(rawValue: "phone")
print(aDevice)
Prints Optional(main.Device.phone)

1 The Problem

As soon as you're using associated values in your enums, this doesn't work anymore:

enum Example {
   case Factory(workers: Int)
   case House(street: String)
}

Swift can't create an instance of Example because the two cases, Factory and House have different associated types (the workers integer and the street string. Each invocation of Example requires different parameters, so this can't be generalized.

However, that's not the case when your associated types all match up:

enum Device {
    case phone(name: String, screenSize: CGSize)
    case watch(name: String, screenSize: CGSize)
    case tablet(name: String, screenSize: CGSize)
}

In thise case, all the associated types are the same. There're a myriad of other ways to model this, but I found the device enum to be a concise example for what I'm about to explain. Even though every Device invocation is the same now, you still can't just call it with some sort of raw value and expect the correct type. Instead, what you have to do is perform a match in order to create the correct instance:

import Foundation

enum Device {
    case phone(name: String, screenSize: CGSize)
    case watch(name: String, screenSize: CGSize)
    case tablet(name: String, screenSize: CGSize)

    static func fromDefaults(rawValue: String, name: String, screenSize: CGSize) -> Device? {
	switch rawValue {
	case "phone": return Device.phone(name: name, screenSize: screenSize)
	case "watch": return Device.watch(name: name, screenSize: screenSize)
	case "tablet": return Device.tablet(name: name, screenSize: screenSize)
	default: return nil
	}
    }
}
let b = Device.fromDefaults("phone", name: "iPhone SE", screenSize: CGSize(width: 640, height: 1136))
print(b)
prints Optional(main.Device.phone("iPhone SE", (640.0, 1136.0)))

This looks ok, but it is already a bit of repetitive code. Once you develop more than just three enum cases / two associated types, this will quickly get out of hand.

enum Vehicle {
  case .car(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .ship(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .yacht(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .truck(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .motorbike(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .helicopter(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  case .train(wheels: Int, capacity: Int, weight: Int, length: Int, height: Int, width: Int, color: Int, name: Int, producer: Int, creation: NSDate, amountOfProducedUnits: Int)
  ...
}

I think you get my point.

2 The Solution

So.. how do we solve this? Interestingly, there's a quirky similarity between the initializer of an associated type and a closure. Take this code:

enum Example {
  case test(x: Int)
}
let exampleClosure = Example.test

What is the type of exampleClosure here? The type is (Int) -> Example. That's right, calling an associated value enum case without any parameters will yield a closure that, when called with the correct types, will return an instance of said type.

This means that, the following is valid, working Swift:

enum Fruit {
  case apple(amount: Int)
  case orange(amount: Int)
}
let appleMaker = Fruit.apple
let firstApple = appleMaker(amount: 10)
let secondApple = appleMaker(amount: 12)
print(firstApple, secondApple)
apple(10) apple(12)

So, how would that help us simplify the gross code duplication problem above? Have a look:

import Foundation

enum Device {
    case phone(name: String, screenSize: CGSize)
    case watch(name: String, screenSize: CGSize)
    case tablet(name: String, screenSize: CGSize)

    private static var initializers: [String: (name: String, screenSize: CGSize) -> Device] = {
	return ["phone": Device.phone, "watch": Device.watch, "tablet": Device.tablet]
    }()

    static func fromDefaults(rawValue: String, name: String, screenSize: CGSize) -> Device? {
	return Device.initializers[rawValue]?(name: name, screenSize: screenSize)
    }
}

let iPhone = Device.fromDefaults("phone", name: "iPhone SE", screenSize: CGSize(width: 640, height: 1134))
print(iPhone)
Optional(main.Device.phone("iPhone SE", (640.0, 1134.0)))

So, let's try to figure out what happened here. We have a new property initializers on our Device. It's a Dictionary of type [String: (name: String, screenSize: CGSize) -> Device]. I.e. something that maps from a String key to a closure with the same type as our Device cases. The dictionary contains the initializers of each of our distinct cases, simply by using the same trick as above, just handing in the closure: phone:Device.phone

The fromDefaults function, then, only has to know the key of the device we'd like to create, and it can call the appropriate closure. This leads to a much shorter implementation, especially for bigger enums (like our Vehicle example above). As you can see, creating a Device instance is then as simple as:

Device.initializers["phone"]?(name: "iPhone 5", screenSize: CGSize(width: 640, height: 1134)))

Just as with raw values, in case there is no enum case phone we'd just get an empty optional back.

This solution isn't perfect of course. You still have to have the initializers dictionary, however it will be much less repetitve than having to match over all cases manually.

Finally, I suppose it goes without saying that the code above ignores an important best practice to be concise and to be able to concentrate on the task at hand; Nevertheless: having stringified code like Device.initializers["phone"] is not the best way to write this. Instead, those keys should be properly defined somewhere else.

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