Swift Dependency Injection Types (https://riptutorial.com/swift/example/26345/dependency-injection-types)

DI.swift

import Foundation 

protocol Engine {
    func startEngine()
    func stopEngine()
}

class TrainEngine : Engine {
    func startEngine() {
        print("Engine started")
    }
    func stopEngine() {
        print("Engine stopped")
    }
}

protocol TrainCar {
    var numOfSeats: Int { get }
    func attachCar(attach: Bool)
}

class RestaurantCar : TrainCar {
    var numOfSeats: Int {
        get { return 30 }
    }
    func attachCar(attach: Bool) {
        print("Attach car")
    }
}

class PassengerCar: TrainCar {
    var numOfSeats: Int {
        get { return 50 }
    }
    func attachCar(attach: Bool) {
        print("Attach car")
    }
}

// Non DI
class NonDITrain {
    let engine: Engine = TrainEngine() as Engine
    var mainCar: TrainCar = RestaurantCar() as TrainCar
}

let nonDITrain = NonDITrain()

// Initializer DI
class InitDITrain {
    let engine: Engine?
    var mainCar: TrainCar?
    
    init(engine: Engine) {
        self.engine = engine
    }
}

let initDITrain = InitDITrain(engine: TrainEngine())

// Properties DI
class PropertiesDITrain {
    let engine: Engine?
    var mainCar: TrainCar?
    
    init(engine: Engine) {
        self.engine = engine
    }
}
let propertiesDITrain = PropertiesDITrain()
// same as non DI until above code
propertiesDITrain.engine = TrainEngine()
propertiesDITrain.mainCar = PassengerCar()

// Method DI
class MethodDITrain {
    let engine: Engine?
    
    func repackCar(trainCar: TrainCar) {
        trainCar.attachCar(attach: true)
        engine?.startEngine()
        engine?.stopEngine()
        trainCar.attachCar(attach: false)
    }
}

let methodDITrain = MethodDiTrain()
methodDITrain.repackCar(trainCar: RestaurantCar())