Export (0) Print
Expand All

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.



Imports System

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)
                    Else
                        System.Threading.Thread.Sleep(300)
                    End If
                    Dim e As New AlarmEventArgs(_snoozePressed, nrings)
                    OnAlarm(e)
                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



Community Additions

ADD
Show:
© 2015 Microsoft