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备忘录

备忘录在 Ruby 中的实现

Memento is a behavioral design pattern that allows making snapshots of an object's state and restoring it in future.

The Memento doesn't compromise the internal structure of the object it works with, as well as data kept inside the snapshots.

进一步了解备忘录模式

在 Ruby 中使用模式

复杂度:

流行度:

使用示例: The Memento's principle can be achieved using the serialization, which is quite common in Ruby. While it's not the only and the most efficient way to make snapshots of an object's state, it still allows storing state backups while protecting the originator's structure from other objects.

概念示例

本例说明了备忘录设计模式的结构并重点回答了下面的问题:

  • 它由哪些类组成?
  • 这些类扮演了哪些角色?
  • 模式中的各个元素会以何种方式相互关联?

main.rb: 概念示例

# 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: 执行结果

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: BFDECxFORrlPIMDCDfQuRwHcuvpbTv

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: xjyrOYzoBYIPYHhGKlvbQrsRvKSRzY

Caretaker: Saving Originator's state...
Originator: I'm doing something important.
Originator: and my state has changed to: xHfPozLWyhsamFVUUPfhIaGBhaBBvK

Caretaker: Here's the list of mementos:
2019-03-06 23:04:13 / (Super-dup...)
2019-03-06 23:04:13 / (BFDECxFOR...)
2019-03-06 23:04:13 / (xjyrOYzoB...)

Client: Now, let's rollback!
Caretaker: Restoring state to: 2019-03-06 23:04:13 / (xjyrOYzoB...)
Originator: My state has changed to: xjyrOYzoBYIPYHhGKlvbQrsRvKSRzY

Client: Once more!
Caretaker: Restoring state to: 2019-03-06 23:04:13 / (BFDECxFOR...)
Originator: My state has changed to: BFDECxFORrlPIMDCDfQuRwHcuvpbTv

备忘录在其他编程语言中的实现

备忘录 在 Java 中的实现 备忘录 在 C# 中的实现 备忘录 在 PHP 中的实现 备忘录 在 Python 中的实现 备忘录 在 Swift 中的实现 备忘录 在 TypeScript 中的实现