Flyweight

The Flyweight pattern is a structural design pattern that helps conserve memory by sharing common parts of an object’s state across multiple instances, rather than storing all data separately in each object. Flyweight is easier to create and work with than the factory design pattern. Factory can produce flyweight with all internal state values. The advantage of the factory is that it can be realized as a singleton to avoid extra memory allocation.

Flyweight has one important requirement: it must be immutable; this means that it can’t be changed after creation (contain neither public fields nor setters).

Monster class

Here is a simple monster class with some fields: name, damage, HP and special. Nothing special here just a common class.

---@class Monster
---@field public name string
---@field public damage number
---@field public hp number
---@field public special string
local Monster = {}
Monster.__index = Monster

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

return Monster

MonsterFactory class

Factory of the monsters is also pretty straightforward. It has a table of monsters and a method to create a new monster. If the monster with name already exists, it will return it. Otherwise, it will create a new monster and store it in the table. It works like a singleton. Maybe this example is not the best, but it’s enough to show the idea of how to save memory.

---@class MonsterFactory
---@field private monsters Monster[]
local MonsterFactory = {}
MonsterFactory.__index = MonsterFactory

---@return MonsterFactory
function MonsterFactory:new()
    local t = {
        monsters = {},
    }
    return setmetatable(t, self)
end

---@return Monster
function MonsterFactory:create(name, damage, hp, special)
    local monster = self.monsters[name]
    if monster then
        return monster
    end
    self.monsters[name] = Monster:new(name, damage, hp, special)
    return self.monsters[name]
end

return MonsterFactory

Usage of Flyweight

Notice that variables goblin01 and goblin02 are references to the same object in memory.

local MonsterFactory = require("MonsterFactory")

local factory = MonsterFactory:new()
local goblin01 = factory:create("Goblin", 10, 100, "None")
local goblin02 = factory:create("Goblin", 10, 100, "None")
local skelly = factory:create("Skeleton", 12, 120, "Reform")
print(goblin01, goblin02, skelly)
--[[
table: 0x561795c73af0   table: 0x561795c73af0   table: 0x561795c74090 
--]]

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