Create a Custom Application Block That Decouples Your Code, Page 2

Turning on the Radio

Next, you need a class that represents the radio station (or broadcaster). I used the Singleton pattern to ensure there was only one broadcaster. Although you could implement multiple radio stations, using just one keeps things simple.

The Broadcaster has only shared public methods and an internal reference to the typed ListenerCollection. To begin receiving messages, implement IListener and invoke Broadcaster.Add(me). To stop listening, invoke Broadcaster.Remove(me). Anyone can send messages by calling Broadcaster.Broadcast.

Broadcasting works simply by iterating through the list of listeners. If a listener's Listening property returns True, then that listener's Listen method is called. (This is pretty much how multicast delegates work too.) Listing 3 contains the complete implementation of the Broadcaster class.

Listing 3: The Broadcaster Class

Public Class Broadcaster

    Private list As ListenerCollection
    Private Shared this As Broadcaster = Nothing

    Protected Sub New()
        list = New ListenerCollection
    End Sub

    Protected Shared ReadOnly Property Instance() As Broadcaster
        Get
            If (this Is Nothing) Then this = New Broadcaster
            Return this
        End Get
    End Property

    Public Shared Function Add(ByVal value As IListener) As Integer
        Return Instance.list.Add(value)
    End Function

    Public Shared Sub Remove(ByVal value As IListener)
        Instance.list.Remove(value)
    End Sub

    Public Shared Sub Broadcast(ByVal message As String)
        Dim listener As IListener
        For Each listener In Instance.list
            Try
                If (listener.Listening) Then
                    listener.Listen(message)
                End If
            Catch ex As Exception
                Instance.list.Remove(listener)
            End Try
        Next
    End Sub
End Class

To complete the application block, use the sn.exe utility (sn –k) to create a strong name if you want the class library to be installed in the GAC.

Finally, use the setup wizard to create a .msi file. (I won't explain how to use the wizard; I know you can figure that out.)

That's it! Distribute the .msi file and you have implemented your first application block.

Testing the Radio Application Block

To test the radio application block, use the following steps:

1. Create a new Windows Forms application.
2. Indicate that the main form implements Ilistener.
3. Make Listening return True.
4. Add a statusbar to the main form and define Listen to write to the StatusBar.Text property.
5. In Form_Load, call Broadcaster.Add(Me) and then call Broadcaster.Broadcast("My first application block");.

That's all there is to it. When you run the Windows Forms application, the text should appear in the StatusBar. The key here is that the Radio Application Block is reusable and has no specific knowledge of any of the listeners' types; that is, the RAB can go anywhere without dragging your applications along with it. Listing 4 contains an example Windows Form (without the automatically generated code).

Listing 4: A Sample Test Windows Forms Form

Imports ApplicationBlock.Radio

Public Class Form1
    Inherits System.Windows.Forms.Form
    Implements IListener


#Region [ Windows Form Designer generated code ]

    Private Sub Form1_Load(ByVal sender As System.Object, _
      ByVal e As System.EventArgs) Handles MyBase.Load
        Broadcaster.Add(Me)
        Broadcaster.Broadcast("This is my first application block")
    End Sub

    Public ReadOnly Property LIstening() As Boolean _
      Implements IListener.Listening
        Get
            Return True
        End Get
    End Property

    Public Sub Listen(ByVal message As String) _
      Implements IListener.Listen
        StatusBar1.Text = message
    End Sub
End Class

Loosely Coupled Code

The Observer pattern and the Radio Application Block, which represents an implementation of the Observer pattern, are perfect examples of loosely coupled code. If you don't get it, that's okay. It's important that you try to get it, however.

If someone you are working with poo-poos it as overly complex, then go ahead and feel superior. At least in this instance, you are.

About the Author

Paul Kimmel is the VB Today columnist for www.codeguru.com and has written several books on object-oriented programming and .NET. Check out his upcoming book UML DeMystified from McGraw-Hill/Osborne (Spring 2005) and Expert One-on-One Visual Studio 2005 from Wrox (Fall 2005). Paul is also the founder and chief architect for Software Conceptions, Inc, founded 1990. He is available to help design and build software worldwide. You may contact him for consulting opportunities or technology questions at pkimmel@softconcepts.com.

If you are interested in joining, sponsoring a meeting, or posting a job, check out www.glugnet.org, the Web page of the Greater Lansing area Users Group for .NET.

Copyright © 2005 by Paul T. Kimmel. All Rights Reserved.