Memento

May 03, 2025

The Memento pattern is a type of design pattern that helps you save and restore an object’s earlier state without needing to understand how the object works internally.

Scheme Memento

The first thing that comes to mind is the undo (Ctrl+z) operation in any text or graphics editor.

Usually, memento is implemented with three classes: Memento, Originator and Caretaker, but these classes can be combined into one class or use a function.

Memento class

Memento itself is a very simple class, with a constructor and a field.

---@class Memento
---@field public value any
local Memento = {}
Memento.__index = Memento

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

return Memento

Originator class

This class is responsible for creating a memento, storing it, and returning it to the caretaker. This also can be omitted and moved to the Memento class as “static” methods. Or use the functions.

---@class Originator
local Originator = {
    ---@param value any
    ---@return Memento
    save = function(value)
        return Memento:new(value)
    end,
    ---@param memento Memento
    ---@return any
    restore = function(memento)
        return memento.value
    end,
}

return Originator

Caretaker class

The Caretaker is responsible for storing the mementos and returning them to the originator. The logic behind the caretaker is to know “when” and “why” to capture the originator’s state, but also when the state should be restored.

---@class Caretaker
---@field private mementoes Memento[]
local Caretaker = {}
Caretaker.__index = Caretaker

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

---@param memento Memento
function Caretaker:addMemento(memento)
    table.insert(self.mementoes, memento)
end

---@param index number
---@return Memento
function Caretaker:getMemento(index)
    return self.mementoes[index]
end

return Caretaker

Usage of Memento

local Caretaker = require("Caretaker")
local Originator = require("Originator")

local caretaker = Caretaker:new()
caretaker:addMemento(Originator.save("hi"))
caretaker:addMemento(Originator.save("hello"))
caretaker:addMemento(Originator.save("hello, world"))

print(Originator.restore(caretaker:getMemento(1))) -- hi
print(Originator.restore(caretaker:getMemento(2))) -- hello
print(Originator.restore(caretaker:getMemento(3))) -- hello, world!
print(Originator.restore(caretaker:getMemento(4))) -- error!

Feedback

For feedback, please check the contacts section. Before writing, please specify where you came from and who you are. Sometimes spammers go insane. Thank you in advance for your understanding.