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Mémento

Mémento en C#

Le Mémento est un patron de conception comportemental qui permet de prendre des instantanés de l’état d’un objet et de les restaurer plus tard.

Le mémento ne compromet pas l’intégrité de la structure interne de l’objet avec lequel il interagit, ni celle des données contenues dans l’instantané.

Utilisation du patron de conception en C#

Complexité :

Popularité :

Exemples d’utilisation : Le principe du mémento peut être imité en utilisant la sérialisation, ce qui est assez commun en C#. Ce n’est pas forcément la manière la plus efficace de prendre des instantanés de l’état d’un objet, mais il permet de stocker des sauvegardes de l’état tout en protégeant la structure du créateur des autres objets.

Exemple conceptuel

Dans cet exemple, nous allons voir la structure du Mémento. Nous allons répondre aux questions suivantes :

  • Que contiennent les classes ?
  • Quels rôles jouent-elles ?
  • Comment les éléments du patron sont-ils reliés ?

Program.cs: Exemple conceptuel

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;

namespace RefactoringGuru.DesignPatterns.Memento.Conceptual
{
    // 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.
        private string _state;

        public Originator(string state)
        {
            this._state = state;
            Console.WriteLine("Originator: My initial state is: " + state);
        }

        // 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.
        public void DoSomething()
        {
            Console.WriteLine("Originator: I'm doing something important.");
            this._state = this.GenerateRandomString(30);
            Console.WriteLine($"Originator: and my state has changed to: {_state}");
        }

        private string GenerateRandomString(int length = 10)
        {
            string allowedSymbols = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string result = string.Empty;

            while (length > 0)
            {
                result += allowedSymbols[new Random().Next(0, allowedSymbols.Length)];

                Thread.Sleep(12);

                length--;
            }

            return result;
        }

        // Saves the current state inside a memento.
        public IMemento Save()
        {
            return new ConcreteMemento(this._state);
        }

        // Restores the Originator's state from a memento object.
        public void Restore(IMemento memento)
        {
            if (!(memento is ConcreteMemento))
            {
                throw new Exception("Unknown memento class " + memento.ToString());
            }

            this._state = memento.GetState();
            Console.Write($"Originator: My state has changed to: {_state}");
        }
    }

    // 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.
    public interface IMemento
    {
        string GetName();

        string GetState();

        DateTime GetDate();
    }

    // The Concrete Memento contains the infrastructure for storing the
    // Originator's state.
    class ConcreteMemento : IMemento
    {
        private string _state;

        private DateTime _date;

        public ConcreteMemento(string state)
        {
            this._state = state;
            this._date = DateTime.Now;
        }

        // The Originator uses this method when restoring its state.
        public string GetState()
        {
            return this._state;
        }
        
        // The rest of the methods are used by the Caretaker to display
        // metadata.
        public string GetName()
        {
            return $"{this._date} / ({this._state.Substring(0, 9)})...";
        }

        public DateTime GetDate()
        {
            return this._date;
        }
    }

    // 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
    {
        private List<IMemento> _mementos = new List<IMemento>();

        private Originator _originator = null;

        public Caretaker(Originator originator)
        {
            this._originator = originator;
        }

        public void Backup()
        {
            Console.WriteLine("\nCaretaker: Saving Originator's state...");
            this._mementos.Add(this._originator.Save());
        }

        public void Undo()
        {
            if (this._mementos.Count == 0)
            {
                return;
            }

            var memento = this._mementos.Last();
            this._mementos.Remove(memento);

            Console.WriteLine("Caretaker: Restoring state to: " + memento.GetName());

            try
            {
                this._originator.Restore(memento);
            }
            catch (Exception)
            {
                this.Undo();
            }
        }

        public void ShowHistory()
        {
            Console.WriteLine("Caretaker: Here's the list of mementos:");

            foreach (var memento in this._mementos)
            {
                Console.WriteLine(memento.GetName());
            }
        }
    }
    
    class Program
    {
        static void Main(string[] args)
        {
            // Client code.
            Originator originator = new Originator("Super-duper-super-puper-super.");
            Caretaker caretaker = new Caretaker(originator);

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            caretaker.Backup();
            originator.DoSomething();

            Console.WriteLine();
            caretaker.ShowHistory();

            Console.WriteLine("\nClient: Now, let's rollback!\n");
            caretaker.Undo();

            Console.WriteLine("\n\nClient: Once more!\n");
            caretaker.Undo();

            Console.WriteLine();
        }
    }
}

Output.txt: Résultat de l’exécution

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: oGyQIIatlDDWNgYYqJATTmdwnnGZQj

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

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

Caretaker: Here's the list of mementos:
12.06.2018 15:52:45 / (Super-dup...)
12.06.2018 15:52:46 / (oGyQIIatl...)
12.06.2018 15:52:46 / (jBtMDDWog...)

Client: Now, let's rollback!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (jBtMDDWog...)
Originator: My state has changed to: jBtMDDWogzzRJbTTmEwOOhZrjjBULe

Client: Once more!

Caretaker: Restoring state to: 12.06.2018 15:52:46 / (oGyQIIatl...)
Originator: My state has changed to: oGyQIIatlDDWNgYYqJATTmdwnnGZQj

Mémento dans les autres langues

Patrons de conception : Mémento en Java Patrons de conception : Mémento en C++ Patrons de conception : Mémento en PHP Patrons de conception : Mémento en Python Patrons de conception : Mémento en Ruby Patrons de conception : Mémento en Swift Patrons de conception : Mémento en TypeScript Patrons de conception : Mémento en Go