Memento Design Pattern in Visual Basic .NET

Definition

Without violating encapsulation, capture and externalize an object’s internal state so that the object can be restored to this state later.

This pattern is used to save the internal state of an object when it changes  state of over the time. You can use save points when you save the state so that you can retrieve the state of the object

This is the UML diagram to show the Memento Design Pattern

Participant

Now we will see the participant in the UML diagram

Originator

• Hold reference to memento object
• Save state in memnto

Memento

• Stores internal state of the Originator object
• Protects against access by objects other than the originator.

CareTaker

• Is responsible for the memento’s safekeeping.
• Never operates on or examines the contents of a memento.

Memento Design Pattern Example in VB.NET

To understand the memento design pattern completely we will code one example using VB.NET
You have Game class to hold the score of two teams. when they are playing you can store the score with save point so that you can undo using save points.
You can have a look at the UML diagram of the following example

To implement the above example I have used following code
Game Class (Originator)

In this Originator we have the scores of Team A and Team B also we have reference to GameCareTaker object

Public Class Game
    Private m_TeamAScore As Integer
    Private m_GameCareTaker As GameCareTaker
    Public Property TeamAScore() As Integer
        Get
            Return m_TeamAScore
        End Get
        Set(ByVal value As Integer)
            m_TeamAScore = value
        End Set
    End Property

    Private m_TeamBScore As Integer
    Public Property TeamBScore() As Integer
        Get
            Return m_TeamBScore
        End Get
        Set(ByVal value As Integer)
            m_TeamBScore = value
        End Set
    End Property
    Sub New(ByVal teamAScore As String, ByVal teamBScore As String, ByVal ct As GameCareTaker)
        m_TeamAScore = teamAScore
        m_TeamBScore = teamBScore
        m_GameCareTaker = ct
    End Sub

    Public Sub CreateSavePoint(savePoint As String)
        m_GameCareTaker.saveMemento(savePoint, New GameMemento(Me.TeamAScore, Me.TeamBScore))

    End Sub
    Public Sub Undo(savePoint As String)
        Dim memento = m_GameCareTaker.GetMemeto(savePoint)
        Me.TeamAScore = memento.TeamAScore
        Me.TeamBScore = memento.TeamBScore
    End Sub
End Class

GameMemento Class (Memento)

This is our Memento class and it has same properties like originator class

Public Class GameMemento

    Private m_TeamAScore As String
    Public Property TeamAScore() As String
        Get
            Return m_TeamAScore
        End Get
        Set(ByVal value As String)
            m_TeamAScore = value
        End Set
    End Property

    Private m_TeamBScore As String
    Public Property TeamBScore() As String
        Get
            Return m_TeamBScore
        End Get
        Set(ByVal value As String)
            m_TeamBScore = value
        End Set
    End Property
    Sub New(ByVal teamAScore As String, ByVal teamBScore As String)
        m_TeamAScore = teamAScore
        m_TeamBScore = teamBScore

    End Sub



End Class

GameCareTaker Class (CareTaker)

This is our CareTaker class. This will save the Memeto and retrive the Memnto once you give the save point details

Public Class GameCareTaker
    Dim htScore As New Hashtable()

    Public Sub SaveMemento(ByVal savepoint As String, memento As GameMemento)
        htScore.Add(savepoint, memento)
    End Sub

    Public Function GetMemeto(ByVal savepoint As String) As GameMemento
        Return DirectCast(htScore.Item(savepoint), GameMemento)

    End Function

End Class

This is the Client program to test the design pattern

Module Module1

    Sub Main()

        Dim careTaker As New GameCareTaker()
        Dim tennis As New Game(0, 0, careTaker)

        'set score
        tennis.TeamAScore = 0
        tennis.TeamBScore = 1

        tennis.CreateSavePoint("Save Point 1")

        tennis.TeamAScore = 0
        tennis.TeamBScore = 2

        tennis.CreateSavePoint("Save Point 2")

        'Undo 
        tennis.Undo("Save Point 1")
        Console.WriteLine("Team Score :" + tennis.TeamAScore.ToString() + "/" + tennis.TeamBScore.ToString())
        Console.ReadLine()

    End Sub

End Module

Output of the above Example

Team Score :0/1