Command Pattern

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Oct 11, 2013

CPOL

1 min read

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Command Pattern"Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and

Command Pattern

"Encapsulate a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations. "

A VB example of the Command Pattern

A lot of the new multimedia keyboards have extra buttons on them for opening up a web browser, an mp3 player etc. we can use the Command pattern to program them so that the keybaord need not know what application it is executing.

' This is the Invoker
Public Class MultiMediaKeyBoard

     ' This array holds commands for the buttons
     ' of our keyboard
     Private _ButtonSlots As New ArrayList(2)      

     Public Sub Main()
         ' Set up the commands to corresponding keyboard
         ' buttons.
         Dim aWebBrowser As New WebBrowser
         _ButtonSlots(0) = New WebBrowserCommand(aWebBrowser) 
         Dim aWordProcessor As New WordProcessor
         _ButtonSlots(1) = New WordProcessorCommand(aWordProcessor)

      End Sub
      Public Sub MediaButtonPushed(ByVal buttonNumber As Integer)

          ' When a button is clicked on the keyboard its API can send us the
          ' buttons ID. We can then call the execute method of the class in the 
          ' corresponding 
          CType(_ButtonSlots(buttonNumber), ICommand).execute()

      End Sub 

End Class

Public Interface ICommand
   Sub execute()
End Interface

' The receiver
Public Class WebBrowser

      Public Sub OpenWebBrowser()
          ' Code to open a web browser
      End Sub
End Class

' The concrete command class
Public Class WebBrowserCommand
      Implements ICommand

      Private _WebBrowser As WebBrowser
    
      Public Sub New(ByVal WebBrowser As WebBrowser)
            _WebBrowser = WebBrowser
      End Sub      

      Public Sub execute() Implements ICommand.execute
           _WebBrowser.OpenWebBrowser()
      End Sub

End Class

' The receiver
Public Class WordProcessor

      Public Sub OpenWordProcessor()
           ' Code to open a word processor
      End Sub
End Class

' The concrete command class
Public Class WordProcessorCommand
      Implements ICommand 
      Private _WordProcessor As WordProcessor

      Public Sub New(ByVal WordProcessor As WordProcessor)
           _WordProcessor = WordProcessor
      End Sub
      Public Sub execute() Implements ICommand.execute
           _WordProcessor.OpenWordProcessor()
      End Sub

End Class