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How to: Raise and Consume Events 

The following example program demonstrates raising an event in one class and handling the event in another class. The AlarmClock class defines the public event Alarm, and provides methods to raise the event. The AlarmEventArgs class derives from EventArgs and defines the data specific to an Alarm event. The WakeMeUp class defines the method AlarmRang, which handles an Alarm event. The AlarmDriver class uses the classes together, setting the AlarmRang method of WakeMeUp to handle the Alarm event of the AlarmClock.

This example program uses concepts described in detail in Events and Delegates and Raising an Event.

Example

' EventSample.vb.
'
Option Explicit
Option Strict

Imports System
Imports System.ComponentModel
Imports Microsoft.VisualBasic

Namespace EventSample
   
   ' Class that contains the data for 
   ' the alarm event. Derives from System.EventArgs.
   '
   Public Class AlarmEventArgs
      Inherits EventArgs
      Private _snoozePressed As Boolean
      Private nrings As Integer
      
      'Constructor.
      '
      Public Sub New(snoozePressed As Boolean, nrings As Integer)
         Me._snoozePressed = snoozePressed
         Me.nrings = nrings
      End Sub
      
      ' The NumRings property returns the number of rings
      ' that the alarm clock has sounded when the alarm event 
      ' is generated.
      '
      Public ReadOnly Property NumRings() As Integer
         Get
            Return nrings
         End Get
      End Property 
      
      ' The SnoozePressed property indicates whether the snooze
      ' button is pressed on the alarm when the alarm event is generated.
      '
      Public ReadOnly Property SnoozePressed() As Boolean
         Get
            Return _snoozePressed
         End Get
      End Property 
      
      ' The AlarmText property that contains the wake-up message.
      '
      Public ReadOnly Property AlarmText() As String
         Get
            If _snoozePressed Then
               Return "Wake Up!!! Snooze time is over."
            Else
               Return "Wake Up!"
            End If
         End Get
      End Property
   End Class
   
   
   ' Delegate declaration.
   '
   Public Delegate Sub AlarmEventHandler(sender As Object, _
                                         e As AlarmEventArgs)

   ' The Alarm class that raises the alarm event.
   '
   Public Class AlarmClock
      Private _snoozePressed As Boolean = False
      Private nrings As Integer = 0
      Private stopFlag As Boolean = False
      
      ' The Stop property indicates whether the 
      ' alarm should be turned off.
      '
      Public Property [Stop]() As Boolean
         Get
            Return stopFlag
         End Get
         Set
            stopFlag = value
         End Set
      End Property 
      
      ' The SnoozePressed property indicates whether the snooze
      ' button is pressed on the alarm when the alarm event is generated.
      '
      Public Property SnoozePressed() As Boolean
         Get
            Return _snoozePressed
         End Get
         Set
            _snoozePressed = value
         End Set
      End Property

      ' The event member that is of type AlarmEventHandler.
      '
      Public Event Alarm As AlarmEventHandler
      
      ' The protected OnAlarm method raises the event by invoking 
      ' the delegates. The sender is always this, the current instance 
      ' of the class.
      '
      Protected Overridable Sub OnAlarm(e As AlarmEventArgs)
         RaiseEvent Alarm(Me, e)
      End Sub
      
      
      ' This alarm clock does not have
      ' a user interface. 
      ' To simulate the alarm mechanism it has a loop
      ' that raises the alarm event at every iteration
      ' with a time delay of 300 milliseconds,
      ' if snooze is not pressed. If snooze is pressed,
      ' the time delay is 1000 milliseconds.
      '
      Public Sub Start()
         Do
            nrings += 1
            If stopFlag Then
               Exit Do
            
            Else
               If _snoozePressed Then
                  System.Threading.Thread.Sleep(1000)
                  If (True) Then
                     Dim e As New AlarmEventArgs(_snoozePressed, nrings)
                     OnAlarm(e)
                  End If
               Else
                  System.Threading.Thread.Sleep(300)
                  Dim e As New AlarmEventArgs(_snoozePressed, nrings)
                  OnAlarm(e)
               End If
            End If
         Loop
      End Sub
   End Class
  
   ' The WakeMeUp class has a method AlarmRang that handles the
   ' alarm event.
   '
   Public Class WakeMeUp
      Public Sub AlarmRang(sender As Object, e As AlarmEventArgs)
         
         Console.WriteLine((e.AlarmText + ControlChars.Cr))
         
         If Not e.SnoozePressed Then
            If e.NumRings Mod 10 = 0 Then
               Console.WriteLine(" Let alarm ring? Enter Y")
               Console.WriteLine(" Press Snooze? Enter N")
               Console.WriteLine(" Stop Alarm? Enter Q")
               Dim input As String = Console.ReadLine()
               
               If input.Equals("Y") Or input.Equals("y") Then
                  Return
               
               Else
                  If input.Equals("N") Or input.Equals("n") Then
                     CType(sender, AlarmClock).SnoozePressed = True
                     Return
                  Else
                     CType(sender, AlarmClock).Stop = True
                     Return
                  End If
               End If
            End If
         Else
            Console.WriteLine(" Let alarm ring? Enter Y")
            Console.WriteLine(" Stop Alarm? Enter Q")
            Dim input As String = Console.ReadLine()
            If input.Equals("Y") Or input.Equals("y") Then
               Return
            Else
               CType(sender, AlarmClock).Stop = True
               Return
            End If
         End If
      End Sub
   End Class
   
   ' The driver class that hooks up the event handling method of
   ' WakeMeUp to the alarm event of an Alarm object using a delegate.
   ' In a forms-based application, the driver class is the
   ' form.
   '
   Public Class AlarmDriver
      Public Shared Sub Main()
         ' Instantiates the event receiver.
         Dim w As New WakeMeUp()
         
         ' Instantiates the event source.
         Dim clock As New AlarmClock()
         
         ' Wires the AlarmRang method to the Alarm event.
         AddHandler clock.Alarm, AddressOf w.AlarmRang
         
         clock.Start()
      End Sub
   End Class
End Namespace

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