Using an Observer design pattern to keep multiple forms up to date

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Introduction

This article describes how to keep multiple instances of objects up to date using Singleton and Observer Design Patterns.

Singleton Design Pattern

We want to be able to share a variable between different objects. The first step to doing this is to ensure that we store the variable only once. To do this we use a Singleton class, this is a class that can only be instantiated once.

The code below is a VB.NET translation of a C# version of a Singleton that can be downloaded from www.dofactory.com.

Public Class ColorObserver

#Region "Singleton Design Pattern"
    REM Use a Singleton Design pattern to ensure _

            that this class can only be instantiated once
    Public Shared s As ColorObserver
    Public Shared flag As Boolean

    Private Sub New()
        'private constructor disallowing other to create object directly

    End Sub
    Public Shared Function getSingletonObject() As ColorObserver
        If flag = False Then
            'First instance

            'return a new object

            s = New ColorObserver
            flag = True
            Return s
        Else
            'future object request 

            'return an already created object

            Return s
        End If
    End Function
#End Region
End Class

Observer Design Pattern

The next step is to be notified when a change occurs. This can be done using an Observer design pattern. There are many ways of implementing an Observer pattern. An Observer implemented with inheritance can be downloaded from here. I have chosen different approaches by using Delegates and Events as described in MSDN. The idea is to raise an event when data changes. Objects subscribe to the observer by listening for the ColorUpdated event.

Public Class ColorObserver

    Public Shared Event ColorUpdated()
    Private Shared _SelectedColor As Colors
    Public Enum Colors
        Red = 0
        White = 1
        Blue = 2
    End Enum

#Region "Singleton Design Patern"
...
#End Region

#Region "Color Functions"
    Public Property Color() As Colors
        Get
            Return _SelectedColor
        End Get
        Set(ByVal Color As Colors)
            _SelectedColor = Color
            RaiseEvent ColorUpdated()
        End Set
    End Property
#End Region

End Class

To subscribe to this observer, you need:

    Private iColorObserver As ColorObserver

    Private Sub frmChild_Load(ByVal sender As System.Object, _
                ByVal e As System.EventArgs) Handles MyBase.Load
        iColorObserver = iColorObserver.getSingletonObject
        AddHandler iColorObserver.ColorUpdated, AddressOf ChangeColor
    End Sub

    Private Sub ChangeColor()
        Select Case iColorObserver.Color
            Case ColorObserver.Colors.Red
                Me.BackColor = Color.Red
            Case ColorObserver.Colors.White
                Me.BackColor = Color.White
            Case ColorObserver.Colors.Blue
                Me.BackColor = Color.Blue
        End Select
    End Sub

To update the color of all objects subscribing to the Observer:

        iColorObserver.Color = ColorObserver.Colors.Red

Conclusion

This is a good way of propagating updates across multiple windows. I hope you enjoyed the example.