Singleton Pattern

Description

I once saw this pattern summarized as "There is only one Elvis and there will only ever be one Elvis". The Singleton is a pattern that is used when you want one and only one instance of an object.

The usual reasons for this are: the object encapsulates an important resource that is used in many places (for example if you don't want an application to be able to open up several connections to the database make a singleton out of the database connection), or the resource can only be used by one thread at a time (e.g., a class that plays a song file. You only want one song playing at a time.)

Example

The usual way this is implemented is by
1) declare a private static (shared in VB) variable to reference the single instance,
2) create a static method in the singleton class that returns the single instance
3) make the constructor private so the class can only be instantiated by itself.

Here is an example:

Public Class SingletonMP3Player

    '1) the private variable

    Private Shared _instance As SingletonMP3Player

 

    '2) the static method to return the singleton

    Public Shared Function GetInstance() As SingletonMP3Player

        'if it has not already been instantiated then create it

        If _instance Is Nothing Then

            _instance = New SingletonMP3Player

        End If

        Return _instance

    End Function

 

    '3) the private constructor

    Private Sub New()

        'prevents instantiation from outside the class

    End Sub

 

    'Other methods required to make the class useful

    Public Sub Play(filename As String)

        'code to play, stopping self if already playing something else

    End Sub

 

    Public Sub StopPlaying()

        'code here

    End Sub

 

    Public Sub OtherUserfulMethod()

        'etc ...

    End Sub

End Class

Here is how you could use it:

Public Class Singing

    Sub SingTheBlues()

        Dim myPlayer As SingletonMP3Player

        myPlayer = SingletonMP3Player.GetInstance()

        myPlayer.Play("c:\music\blues\Born Under A Bad Sign.mp3")

    End Sub

 

    Sub GrowlDemonically()

        Dim myPlayer As SingletonMP3Player

        myPlayer = SingletonMP3Player.GetInstance()

        myPlayer.Play("c:\music\deathmetal\Blackwater Park.mp3")

    End Sub

 

    'or a show off way to do it

    Sub JustListen()

        SingletonMP3Player.GetInstance().Play("c:\music\prog\3 Min Warning.mp3")

    End Sub

End Class

Resources

Wikipedia - Singleton pattern
Data & Object Factory - Singleton
DotNetExtreme - Singleton pattern