README.md

Decorator

Attach additional responsibilities to an object dynamically keeping the same interface. Decorators provide a flexible alternative to subclassing for extending functionality.

Source: wikipedia.org

Swift Example

protocol Coffee {
    func getCost() -> Double
    func getIngredients() -> String
}

class SimpleCoffee: Coffee {
    func getCost() -> Double {
        return 1.0
    }

    func getIngredients() -> String {
        return "Coffee"
    }
}

class CoffeeDecorator: Coffee {
    private let decoratedCoffee: Coffee
    fileprivate let ingredientSeparator: String = ", "

    required init(decoratedCoffee: Coffee) {
        self.decoratedCoffee = decoratedCoffee
    }

    func getCost() -> Double {
        return decoratedCoffee.getCost()
    }

    func getIngredients() -> String {
        return decoratedCoffee.getIngredients()
    }
}

final class Milk: CoffeeDecorator {
    required init(decoratedCoffee: Coffee) {
        super.init(decoratedCoffee: decoratedCoffee)
    }

    override func getCost() -> Double {
        return super.getCost() + 0.5
    }

    override func getIngredients() -> String {
        return super.getIngredients() + ingredientSeparator + "Milk"
    }
}

final class WhipCoffee: CoffeeDecorator {
    required init(decoratedCoffee: Coffee) {
        super.init(decoratedCoffee: decoratedCoffee)
    }

    override func getCost() -> Double {
        return super.getCost() + 0.7
    }

    override func getIngredients() -> String {
        return super.getIngredients() + ingredientSeparator + "Whip"
    }
}

Swift Usage

var someCoffee: Coffee = SimpleCoffee()
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")
someCoffee = Milk(decoratedCoffee: someCoffee)
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")
someCoffee = WhipCoffee(decoratedCoffee: someCoffee)
print("Cost : \(someCoffee.getCost()); Ingredients: \(someCoffee.getIngredients())")

Kotlin Example

interface CoffeeMachine {
    fun makeSmallCoffee()
    fun makeLargeCoffee()
}

class NormalCoffeeMachine : CoffeeMachine {
    override fun makeSmallCoffee() = println("Normal: Making small coffee")

    override fun makeLargeCoffee() = println("Normal: Making large coffee")
}

//Decorator:
class EnhancedCoffeeMachine(val coffeeMachine: CoffeeMachine) : CoffeeMachine by coffeeMachine {

    // overriding behaviour
    override fun makeLargeCoffee() {
        println("Enhanced: Making large coffee")
        coffeeMachine.makeLargeCoffee()
    }

    // extended behaviour
    fun makeCoffeeWithMilk() {
        println("Enhanced: Making coffee with milk")
        coffeeMachine.makeSmallCoffee()
        println("Enhanced: Adding milk")
    }
}

Kotlin Usage

fun main(args: Array<String>) {
    val normalMachine = NormalCoffeeMachine()
    val enhancedMachine = EnhancedCoffeeMachine(normalMachine)

    // non-overridden behaviour
    enhancedMachine.makeSmallCoffee()
    // overriding behaviour
    enhancedMachine.makeLargeCoffee()
    // extended behaviour
    enhancedMachine.makeCoffeeWithMilk()
}