Events in the .NET Framework are based on the delegate model. Delegates are type-safe Function Pointers or Callbacks. A delegate can reference both static and instance methods.
Implementation of an event is a three-step procedure.
- Declare a delegate, if definition is not provided the .Net Framework would provide a default delegate implementation.
- Declare the event signature using the Event keyword and Raise the event using the RaiseEvent statement.
- Handle the Event by declaring an event receiver, often called an event handler. Which is a subroutine that responds to an event.
Lets now take each step on by one.
Declare a delegate.
A delegate is a class that can hold a reference to a method. A delegate class has a signature, and it can hold references only to methods that match its signature. A delegate can be defined as
Delegate Sub sampleDel(Cancel as Boolean).
Any method that has same signature can be attached to this delegate; a procedure that would have the same signature can handle this event.
Declare an Event and Raise It.
An even can be declared in mainly three ways.
evtSample As sampleDel-The mechanism to register the event handler, for this type of declaration is to be provided by the class declaring the event. The event is implemented by using the explicitly declared delegate.
The event is raised by making a call to evtSample.
Public Event evtSample as sampleDel-The event handler can be registered by using AddHandler method in the Class that would provide a Handler. The event is implemented by using the above-declared delegate.
The event is raised by making a call to RaiseEvent.
Public Event evtSample(Cancel as Boolean)-The event handler procedure would be registered by using the Handles keyword in the declaration itself. The event is implemented by using implicitly declared delegate by the framework.
The event is raised by making a call to RaiseEvent.
Handle the Event.
Declaring a sub and either attaching it to the delegate or registering with the event declaring class can handle the event.
Example.
The example shown declares a delegate in class CTimer and also declares three events in the class using all the aforementioned declarations. The class also provides a mechanism to register an event. And raises events at intervals simulating 5, 10 and 30 units of interval. Another class CClock is defined that has members to handle the RaisedEvents from CTimer class.
To run the sample from command line use-"vbc /out:Event.exe Event.vb".
Imports
System
Public Class CTimer
Delegate Sub SecondDel(ByVal xintTime As Integer)
Private evtSecond As SecondDel
Public Event evtMinute As SecondDel
Public Event evtHour(ByVal xHour As Integer)
Public Shared lngSeconds As Long
Public Sub Register(ByVal objSecond As SecondDel)
evtSecond = evtSecond.Combine(evtSecond, objSecond)
End Sub
Public Sub OnTimer()
lngSeconds = lngSeconds + 1
If lngSeconds Mod 5 = 0 Then
evtSecond(lngSeconds)
End If
If lngSeconds Mod 10 = 0 Then
RaiseEvent evtMinute(lngSeconds)
End If
If lngSeconds Mod 30 = 0 Then
RaiseEvent evtHour(lngSeconds)
End If
End Sub
End Class
Public Class CClock
Private WithEvents mobjTimer As CTimer
Sub New()
mobjTimer = New CTimer
mobjTimer.Register(New CTimer.SecondDel(AddressOf SecondEvent))
AddHandler mobjTimer.evtMinute, AddressOf MinuteEvent
While (mobjTimer.lngSeconds < 60)
mobjTimer.OnTimer()
System.Threading.Thread.Sleep(100)
End While
End Sub
Private Sub SecondEvent(ByVal xintTime As Integer)
Console.WriteLine("Second's Event")
End Sub
Private Sub MinuteEvent(ByVal xintTime As Integer)
Console.WriteLine("Minute's Event")
End Sub
Private Sub mobjTimer_evtHour(ByVal xintTime As Integer) Handles
objTimer.evtHour
Console.WriteLine("Hour's Event")
End Sub
Public Shared Sub Main()
Dim cc1 = New CClock
End Sub
End Class