Simple DI Container in Swift (Demo at iOSCon 2017 in London)

SimpleDIContainer.swift

// Demo at iOSCon 2017 in London
// https://speakerdeck.com/yoichitgy/dependency-injection-in-practice-ioscon
//
// How to Use: Paste this program to a Playground.
// Tested with: Xcode 8.2.1

protocol Animal: class { }
class Cat: Animal { }
class Dog: Animal { }

class PetOwner {
    let pet: Animal // Depends on protocol
    init(pet: Animal) {
        self.pet = pet
    }
}

class Container {
    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()
do {
    let cat = Cat()
    container.register(Animal.self) { cat }
    container.register(PetOwner.self) {
        PetOwner(pet: container.resolve(Animal.self)!)
    }
}

// Anywhere later
let pet = container.resolve(Animal.self)
pet is Cat

let yoichi = container.resolve(PetOwner.self)!
yoichi.pet is Cat

let anya = container.resolve(PetOwner.self)!
anya !== yoichi
anya.pet === yoichi.pet