Decorator

May 01, 2025

Decorator is a design pattern that lets you add new features or behaviors to an object without changing its original code. You do this by wrapping the object in another object (called a decorator) that adds the new behavior.

Decorator became so popular pattern that some programming languages like Java, TypeScript, and JavaScript added the decorators to their syntax with @Decorator syntax.

Let’s imagine we have the RPG game where our hero can be blessed, which increases the hero attributes by 10%. And we do not want to change our hero’s original attributes.

Decorator Scheme

Hero class

Hero class is just straight to keep simplicity in demonstration it has only getters.

---@class Hero
---@field private name string
---@field private hp number
---@field private damage number
---@field private xp number
local Hero = {}
Hero.__index = Hero

---@return Hero
function Hero:new(name, hp, damage, xp)
    local t = {
        name = name,
        hp = hp,
        damage = damage,
        xp = xp,
    }
    return setmetatable(t, self)
end

---@return string
function Hero:getName()
    return self.name
end

---@return number
function Hero:getDamage()
    return self.damage
end

---@return number
function Hero:getHP()
    return self.hp
end

---@return number
function Hero:getXP()
    return self.xp
end

return Hero

BlessSpellDecorator class

Here we have same interface as Hero class, but decorator can decorate not all methods and/or properties, but only which are needed to be decorated. As you can see we do not change xp field.

---@class BlessSpellDecorator
---@field private hero Hero
local BlessSpellDecorator = {}
BlessSpellDecorator.__index = BlessSpellDecorator

---@return Hero
---@param hero Hero
function BlessSpellDecorator:new(hero)
    local t = {
        hero = hero
    }
    return setmetatable(t, self)
end

---@return string
function BlessSpellDecorator:getName()
    return self.hero:getName() .. " (blessed)"
end

---@return number
function BlessSpellDecorator:getDamage()
    return self.hero:getDamage() + self.hero:getDamage() * 0.1
end

---@return number
function BlessSpellDecorator:getHP()
    return self.hero:getHP() + self.hero:getHP() * 0.1
end

return BlessSpellDecorator

BlessSpellDecorator class usage

Here we decorate our hero with BlessSpellDecorator and get updated attributes.

local Hero = require("Hero")
local BlessSpellDecorator = require("BlessSpellDecorator")

local hero = Hero:new("Max", 100, 10, 0)
local blessedHero = BlessSpellDecorator:new(hero)
print(hero:getName(), hero:getDamage(), hero:getHP()) --> Max     10      100
print(blessedHero:getName(), blessedHero:getDamage(), blessedHero:getHP()) --> Max (blessed)    11.0    110.0