Observer in VB.NET

   1:      ' Observer Design Pattern.

   2:      ' See description in //www.vb-net.com/ProgramTheory/Observer.htm

   3:      Class MainApp

   4:          ' Entry point into console application.

   5:          Public Shared Sub Main()

   6:              ' Configure Observer pattern

   7:              Dim s As New ConcreteSubject()

   8:              s.Attach(New ConcreteObserver(s, "X"))

   9:              s.Attach(New ConcreteObserver(s, "Y"))

  10:              s.Attach(New ConcreteObserver(s, "Z"))

  11:              ' Change subject and notify observers

  12:              s.SubjectState = "ABC"

  13:              s.Notify()

  14:              ' Wait for user

  15:              Console.ReadKey()

  16:          End Sub

  17:      End Class

  18:   

  19:      ' The 'Subject' abstract class

  20:      MustInherit Class Subject

  21:          Private _observers As New List(Of Observer)()

  22:          Public Sub Attach(observer As Observer)

  23:              _observers.Add(observer)

  24:          End Sub

  25:          Public Sub Detach(observer As Observer)

  26:              _observers.Remove(observer)

  27:          End Sub

  28:          Public Sub Notify()

  29:              For Each o As Observer In _observers

  30:                  o.Update()

  31:              Next

  32:          End Sub

  33:      End Class

  34:   

  35:      ' The 'ConcreteSubject' class

  36:      Class ConcreteSubject

  37:          Inherits Subject

  38:          Private _subjectState As String

  39:          ' Gets or sets subject state

  40:          Public Property SubjectState() As String

  41:              Get

  42:                  Return _subjectState

  43:              End Get

  44:              Set(value As String)

  45:                  _subjectState = value

  46:              End Set

  47:          End Property

  48:      End Class

  49:   

  50:      ' The 'Observer' abstract class

  51:      MustInherit Class Observer

  52:          Public MustOverride Sub Update()

  53:      End Class

  54:   

  55:      ' The 'ConcreteObserver' class

  56:      Class ConcreteObserver

  57:          Inherits Observer

  58:          Private _name As String

  59:          Private _observerState As String

  60:          Private _subject As ConcreteSubject

  61:          ' Constructor

  62:          Public Sub New(subject As ConcreteSubject, name As String)

  63:              Me._subject = subject

  64:              Me._name = name

  65:          End Sub

  66:          Public Overrides Sub Update()

  67:              _observerState = _subject.SubjectState

  68:              Console.WriteLine("Observer {0}'s new state is {1}", _name, _observerState)

  69:          End Sub

  70:          ' Gets or sets subject

  71:          Public Property Subject() As ConcreteSubject

  72:              Get

  73:                  Return _subject

  74:              End Get

  75:              Set(value As ConcreteSubject)

  76:                  _subject = value

  77:              End Set

  78:          End Property

  79:      End Class