Swift builder pattern

BuilerPatternExample.swift

import Foundation

//sources :
//https://blog.xebia.fr/2016/12/28/design-pattern-builder-et-builder-sont-dans-un-bateau/
//https://jlordiales.me/2012/12/13/the-builder-pattern-in-practice/
//https://pastebin.com/CXyxh658

class Window { }
class Door { }
class Wall { }
class Roof { }

class House {
    var windows: [Window]?
    var doors: [Door]?
    var walls: [Wall]?
    var roof: Roof?

    init(builder: HouseBuilder) {
        self.walls = builder.walls
        self.roof = builder.roof
        self.doors = builder.optionalDoors
        self.windows = builder.optionalWindows
    }
}

public class HouseBuilder {

    var walls: [Wall]
    var roof: Roof
    var optionalWindows: [Window]?
    var optionalDoors: [Door]?

    public init() {
        //have to initialize all fields ...
        walls = [Wall]()
        roof = Roof()
    }

    func getWallsRequiredBuilder(roof: Roof) -> WallsRequiredBuilder{
        self.roof = roof
        return WallsRequiredBuilder(builder: self)
    }

    public class WallsRequiredBuilder {
        private let builder: HouseBuilder

        init(builder: HouseBuilder) {
            self.builder = builder
        }

        func setWalls(walls: [Wall]) -> OptionalWindowsBuilder {
            builder.walls = walls
            return OptionalWindowsBuilder(builder: builder)
        }
    }

    public class OptionalWindowsBuilder {
        private let builder: HouseBuilder

        init(builder: HouseBuilder) {
            self.builder = builder
        }

        func build() -> House {
            return House(builder: builder)
        }
    }
}

main.swift

import Foundation

let walls = [Wall(), Wall(), Wall(), Wall()]
let windows = [Window(), Window()]
let roof = Roof()

let house = HouseBuilder()
        .getWallsRequiredBuilder(roof: roof)
        .setWalls(walls: walls)
        .build()