Memento を Ruby で
Memento は、 振る舞いに関するデザインパターンの一つで、 オブジェクトの状態のスナップショットを作成し、 それを将来復元します。
Memento は、 その対象オブジェクトの内部構造やスナップショットの内部に保存されるデータの機密を守ります。
複雑度：
人気度：
使用例： Memento の原則は、 シリアライゼーションを使って達成することが可能で、 Ruby ではよく見かけます。 これは、 オブジェクトの状態のスナップショットを作る上で唯一の方法でも、 最も効率のいい方法でもありませんが、 オリジネーターの構造を他のオブジェクトから隠蔽しつつ状態のバックアップを取ることはできます。
概念的な例
この例は、 Memento デザインパターンの構造を説明するためのものです。 以下の質問に答えることを目的としています：
- どういうクラスからできているか？
- それぞれのクラスの役割は？
- パターンの要素同士はどう関係しているのか？
main.rb: 概念的な例
# The Originator holds some important state that may change over time. It also
# defines a method for saving the state inside a memento and another method for
# restoring the state from it.
class Originator
# For the sake of simplicity, the originator's state is stored inside a single
# variable.
attr_accessor :state
private :state
# @param [String] state
def initialize(state)
@state = state
puts "Originator: My initial state is: #{@state}"
end
# The Originator's business logic may affect its internal state. Therefore,
# the client should backup the state before launching methods of the business
# logic via the save() method.
def do_something
puts 'Originator: I\'m doing something important.'
@state = generate_random_string(30)
puts "Originator: and my state has changed to: #{@state}"
end
private def generate_random_string(length = 10)
ascii_letters = [*'a'..'z', *'A'..'Z']
(0...length).map { ascii_letters.sample }.join
end
# Saves the current state inside a memento.
def save
ConcreteMemento.new(@state)
end
# Restores the Originator's state from a memento object.
def restore(memento)
@state = memento.state
puts "Originator: My state has changed to: #{@state}"
end
end
# The Memento interface provides a way to retrieve the memento's metadata, such
# as creation date or name. However, it doesn't expose the Originator's state.
class Memento
# @abstract
#
# @return [String]
def name
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
# @abstract
#
# @return [String]
def date
raise NotImplementedError, "#{self.class} has not implemented method '#{__method__}'"
end
end
class ConcreteMemento < Memento
# @param [String] state
def initialize(state)
@state = state
@date = Time.now.strftime('%F %T')
end
# The Originator uses this method when restoring its state.
attr_reader :state
# The rest of the methods are used by the Caretaker to display metadata.
def name
"#{@date} / (#{@state[0, 9]}...)"
end
# @return [String]
attr_reader :date
end
# The Caretaker doesn't depend on the Concrete Memento class. Therefore, it
# doesn't have access to the originator's state, stored inside the memento. It
# works with all mementos via the base Memento interface.
class Caretaker
# @param [Originator] originator
def initialize(originator)
@mementos = []
@originator = originator
end
def backup
puts "\nCaretaker: Saving Originator's state..."
@mementos << @originator.save
end
def undo
return if @mementos.empty?
memento = @mementos.pop
puts "Caretaker: Restoring state to: #{memento.name}"
begin
@originator.restore(memento)
rescue StandardError
undo
end
end
def show_history
puts 'Caretaker: Here\'s the list of mementos:'
@mementos.each { |memento| puts memento.name }
end
end
originator = Originator.new('Super-duper-super-puper-super.')
caretaker = Caretaker.new(originator)
caretaker.backup
originator.do_something
caretaker.backup
originator.do_something
caretaker.backup
originator.do_something
puts "\n"
caretaker.show_history
puts "\nClient: Now, let's rollback!\n"
caretaker.undo
puts "\nClient: Once more!\n"
caretaker.undo
output.txt: 実行結果
Originator: My initial state is: Super-duper-super-puper-super.
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: CHYzYSIWbqvWkCzIHOqTyEJWfQlFMn
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: vbkhwCeAEQBpLwQLlhmpcvUnwzxVnT
Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: SBWlQnAEPLsitiOQAZbGlXHZAeWBoW
Caretaker: Here's the list of mementos:
2023-08-11 15:02:35 / (Super-dup...)
2023-08-11 15:02:35 / (CHYzYSIWb...)
2023-08-11 15:02:35 / (vbkhwCeAE...)
Client: Now, let's rollback!
Caretaker: Restoring state to: 2023-08-11 15:02:35 / (vbkhwCeAE...)
Originator: My state has changed to: vbkhwCeAEQBpLwQLlhmpcvUnwzxVnT
Client: Once more!
Caretaker: Restoring state to: 2023-08-11 15:02:35 / (CHYzYSIWb...)
Originator: My state has changed to: CHYzYSIWbqvWkCzIHOqTyEJWfQlFMn
