Memento Design Pattern in Ruby

memento.rb

class Memento
  attr_accessor :state

  def initialize(state)
    @state = state
  end
end

class Caretaker
  attr_reader :memento

  def save(memento)
    @memento = memento
  end
end

# Usage:

class VideoGame
  attr_accessor :state

  def initialize(state={})
    @state = state
  end

  def save
    Memento.new(@state)
  end

  def restore(memento)
    @state = memento.state
  end
end

RSpec.describe 'Memento' do
  before(:all) do
    @memory_card = Array.new(8, Caretaker.new)
    @session = VideoGame.new(name: 'Joe', level: 0)
  end

  it 'performs initial game save' do
    @memory_card[0].save(@session.save)
    expect(@memory_card[0].memento.state).to eq(@session.state)
  end

  it 'overwrites game saves' do
    original_state = @memory_card[0].memento.state
    @session.state[:level] = 1
    @memory_card[0].save(@session.save)
    expect(@memory_card[0].memento).not_to eq(original_state)
  end

  it 'can restore game saves' do
    original_state = @memory_card[0].memento.state
    @session.state[:level] = nil
    @session.restore(@memory_card[0].memento)
    expect(@session.state).to eq(original_state)
  end

  it 'cannot corrupt game saves' do
    expect { @memory_card[0].memento = nil }.to raise_error(NoMethodError)
  end
end

The original Ruby implementation I saw was created by D.L. Jerome
One notable improvement I have made in the implementation is that the Caretaker is only able to overwrite/return a memento but is not permitted to access it directly. To me this is an important characteristic of the Memento design pattern as it ensures encapsulation is not broken throughout the life of the memento.

Additionally I have included the usage as if it were being added to a video game which personally makes patterns easier for me to relate to. For simpler usage, background information, or to compare implementations please visit D.L. Jerome's write up on the pattern here: https://medium.com/@dljerome/design-patterns-in-ruby-memento-1287f926fd0a