Mercari Tech Conf 2017 - Demo implementing a simple dependency injection container

Playground-MercariTechConf2017-DIContainer.swift

protocol Animal {
    func sound() -> String
}
class Cat: Animal {
    func sound() -> String { return "😺Meow" }
}
class Dog: Animal {
    func sound() -> String { return "🐶Bow wow" }
}

class PetOwner {
    let pet: Animal // Depends on protocol
    init(pet: Animal) {
        self.pet = pet
    }
    func play() -> String {
        return "😃👐 \(pet.sound())"
    }
}

// Manual dependency injection
let catOwner = PetOwner(pet: Dog())
catOwner.play()

// Container
class Container {
    private var factories = [String: Any]()

    func register<T>(_ type: T.Type, factory: @escaping () -> T) {
        let key = String(reflecting: type)
        factories[key] = factory as Any
    }
    func resolve<T>(_ type: T.Type) -> T? {
        let key = String(reflecting: type)
        guard let factory = factories[key] as? () -> T else {
            return nil
        }
        return factory()
    }
}

// At program entry point
let container = Container()
container.register(Animal.self) { Dog() }
container.register(PetOwner.self) {
    PetOwner(pet: container.resolve(Animal.self)!)
}

// Anywhere later
let pet = container.resolve(Animal.self)!
pet.sound()
let petOwner = container.resolve(PetOwner.self)!
petOwner.play()

Mercari Tech Conf 2017: https://techconf.mercari.com/2017/en/