The builder pattern is an object creation software design pattern. Unlike the abstract factory pattern and the factory method pattern whose intention is to enable polymorphism, the intention of the builder pattern is to find a solution to the telescoping constructor anti-pattern. The telescoping constructor anti-pattern occurs when the increase of object constructor parameter combination leads to an exponential list of constructors. Instead of using numerous constructors, the builder pattern uses another object, a builder, that receives each initialization parameter step by step and then returns the resulting constructed object at once.
The main goal of the builder pattern is to setup a default configuration for an object from its creation. It is an intermediary between the object will be built and all other objects related to building it.
To make it more clear, let’s take a look at a Car Builder example.
Consider that we have a Car class contains many options to create an object, such as:
import UIKit
enum CarType {
case
sportage,
saloon
}
enum GearType {
case
manual,
automatic
}
struct Motor {
var id: String
var name: String
var model: String
var numberOfCylinders: UInt8
}
class Car: CustomStringConvertible {
var color: UIColor
var numberOfSeats: UInt8
var numberOfWheels: UInt8
var type: CarType
var gearType: GearType
var motor: Motor
var shouldHasAirbags: Bool
var description: String {
return "color: \\(color)\\nNumber of seats: \\(numberOfSeats)\\nNumber of Wheels: \\(numberOfWheels)\\n Type: \\(gearType)\\nMotor: \\(motor)\\nAirbag Availability: \\(shouldHasAirbags)"
}
init(color: UIColor, numberOfSeats: UInt8, numberOfWheels: UInt8, type: CarType, gearType: GearType, motor: Motor, shouldHasAirbags: Bool) {
self.color = color
self.numberOfSeats = numberOfSeats
self.numberOfWheels = numberOfWheels
self.type = type
self.gearType = gearType
self.motor = motor
self.shouldHasAirbags = shouldHasAirbags
}
}
Creating a car object:
let aCar = Car(color: UIColor.black,
numberOfSeats: 4,
numberOfWheels: 4,
type: .saloon,
gearType: .automatic,
motor: Motor(id: "101", name: "Super Motor",
model: "c4", numberOfCylinders: 6),
shouldHasAirbags: true)
print(aCar)
/* Printing
color: UIExtendedGrayColorSpace 0 1
Number of seats: 4
Number of Wheels: 4
Type: automatic
Motor: Motor(id: "101", name: "Super Motor", model: "c4", numberOfCylinders: 6)
Airbag Availability: true
*/
The problem arises when creating a car object is that the car requires many configuration data to be created.
For applying the Builder Pattern, the initializer parameters should have default values which are changeable if needed.
CarBuilder class:
class CarBuilder {
var color: UIColor = UIColor.black
var numberOfSeats: UInt8 = 5
var numberOfWheels: UInt8 = 4
var type: CarType = .saloon
var gearType: GearType = .automatic
var motor: Motor = Motor(id: "111", name: "Default Motor",
model: "T9", numberOfCylinders: 4)
var shouldHasAirbags: Bool = false
func buildCar() -> Car {
return Car(color: color, numberOfSeats: numberOfSeats, numberOfWheels: numberOfWheels, type: type, gearType: gearType, motor: motor, shouldHasAirbags: shouldHasAirbags)
}
}