本文属于「设计模式」系列文章之一。这一系列开始时间为2021/10/27，着重于「设计模式的学习与实践」。由于文章内容随时可能发生更新变动，欢迎关注和收藏设计模式系列文章汇总目录一文以作备忘。需要特别说明的是，为了透彻理解和全面掌握设计模式，本系列文章中参考了诸多博客、教程、文档、书籍等资料，限于时间精力有限，这里无法一一列出。部分重要资料的不完全参考目录如下所示，在后续学习整理中还会逐渐补充：

• 大话设计模式（C# Design Patterns），程杰著，清华大学出版社，
• （国外经典书籍）设计模式：可复用面向对象软件的基础，Erich Gamm, Richard Helm, Ralph, Johnson, John Vlissides 著，李英军、马晓星、蔡敏、刘建中译，机械工业出版社
• Head First Design Patterns，Eric Freeman & Elisabeth Freeman with Kathy Sierra & Bert Bates ，东南大学出版社
• 设计模式解析 第2版，Allan Shalloway, James R. Trott 著，徐言声译，人民邮电出版社
• 敏捷软件开发：原则、模式与实践，Robert C. Martin 著，邓辉译，清华大学出版社
• 重构——改善既有代码的设计，Martin Fowler 著，侯捷、熊杰译，中国电力出版社
• 企业应用架构模式，Martin Fowler 著，王怀明、周斌译，机械工业出版社
• Java与模式，阎宏著，电子工业出版社

文章目录

• Memento
• UML class diagram
• Participants
• Structural code in C++/C#/Java
• Real-world code in C++/C#/Java

Memento

The Memento design pattern captures and externalizes an object‘s internal state without violating encapsulation, so that the object can be restored to this state later.

Frequency of use: Low(level 1)

UML class diagram

A visualization of the classes and objects participating in this pattern.

Participants

The classes and objects participating in this pattern include:

• Memento (Memento)
  • stores internal state of the Originator object . The Memento may store as much or as little of the Originator's internal state as necessary at its Originator's discretion.
  • protect against access by objects of other than the Originator . Mementos have effectively two interfaces.
    • Caretaker sees a narrow interface to the Memento – it can only pass the Memento to the other objects.
    • Originator , in contrast, sees a wide interface, one that lets it access all the data necessary to restore itself to its previous state.
      Ideally, only the Originator that produces the Memento would be permitted to access the memento’s internal state.
• Originator (SalesProspect)
  • creates a Memento containing a snapshot of its current internal state.
  • uses the Memento to restore its internal state
• Caretaker (Caretaker)
  • is responsible for the Memento's safekeeping
  • never operates on or examines the contents of a Memento .

Structural code in C++/C#/Java

This structural code demonstrates the Memento pattern which temporarily saves and restores another object’s internal state.

using System;

namespace Memento.Structural
{
    /// <summary>
    /// Memento Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            Originator o = new Originator();
            o.State = "On";

            // Store internal state

            Caretaker c = new Caretaker();
            c.Memento = o.CreateMemento();

            // Continue changing originator

            o.State = "Off";

            // Restore saved state

            o.SetMemento(c.Memento);

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Originator' class
    /// </summary>

    public class Originator
    {
        string state;

        public string State
        {
            get { return state; }
            set
            {
                state = value;
                Console.WriteLine("State = " + state);
            }
        }

        // Creates memento 

        public Memento CreateMemento()
        {
            return (new Memento(state));
        }

        // Restores original state

        public void SetMemento(Memento memento)
        {
            Console.WriteLine("Restoring state...");
            State = memento.State;
        }
    }

    /// <summary>
    /// The 'Memento' class
    /// </summary>

    public class Memento
    {
        string state;

        // Constructor

        public Memento(string state)
        {
            this.state = state;
        }

        public string State
        {
            get { return state; }
        }
    }

    /// <summary>
    /// The 'Caretaker' class
    /// </summary>

    public class Caretaker
    {
        Memento memento;

        public Memento Memento
        {
            set { memento = value; }
            get { return memento; }
        }
    }
}

Output

State = On
State = Off
Restoring state:
State = On

Real-world code in C++/C#/Java

This real-world code demonstrates the Memento pattern which temporarily saves and then restores the SalesProspect's internal state.

using System;

namespace Memento.RealWorld
{
    /// <summary>
    /// Memento Design Pattern
    /// </summary>

    public class Program
    {
        public static void Main(string[] args)
        {
            SalesProspect s = new SalesProspect();
            s.Name = "Noel van Halen";
            s.Phone = "(412) 256-0990";
            s.Budget = 25000.0;

            // Store internal state

            ProspectMemory m = new ProspectMemory();
            m.Memento = s.SaveMemento();

            // Continue changing originator

            s.Name = "Leo Welch";
            s.Phone = "(310) 209-7111";
            s.Budget = 1000000.0;

            // Restore saved state

            s.RestoreMemento(m.Memento);

            // Wait for user

            Console.ReadKey();
        }
    }

    /// <summary>
    /// The 'Originator' class
    /// </summary>

    public class SalesProspect
    {
        string name;
        string phone;
        double budget;

        // Gets or sets name

        public string Name
        {
            get { return name; }
            set
            {
                name = value;
                Console.WriteLine("Name:   " + name);
            }
        }

        // Gets or sets phone

        public string Phone
        {
            get { return phone; }
            set
            {
                phone = value;
                Console.WriteLine("Phone:  " + phone);
            }
        }

        // Gets or sets budget

        public double Budget
        {
            get { return budget; }
            set
            {
                budget = value;
                Console.WriteLine("Budget: " + budget);
            }
        }

        // Stores memento

        public Memento SaveMemento()
        {
            Console.WriteLine("\nSaving state --\n");
            return new Memento(name, phone, budget);
        }

        // Restores memento

        public void RestoreMemento(Memento memento)
        {
            Console.WriteLine("\nRestoring state --\n");
            Name = memento.Name;
            Phone = memento.Phone;
            Budget = memento.Budget;
        }
    }

    /// <summary>
    /// The 'Memento' class
    /// </summary>

    public class Memento
    {
        string name;
        string phone;
        double budget;

        // Constructor

        public Memento(string name, string phone, double budget)
        {
            this.name = name;
            this.phone = phone;
            this.budget = budget;
        }

        public string Name
        {
            get { return name; }
            set { name = value; }
        }

        public string Phone
        {
            get { return phone; }
            set { phone = value; }
        }

        public double Budget
        {
            get { return budget; }
            set { budget = value; }
        }
    }

    /// <summary>
    /// The 'Caretaker' class
    /// </summary>

    public class ProspectMemory
    {
        Memento memento;

        public Memento Memento
        {
            set { memento = value; }
            get { return memento; }
        }
    }
}