Gate to Delegates in VB.NET
Gate to Delegates in VB.NET
| Author: | Arun Nair |
| Submission Date: | 2006-04-24 |
| Website: | |
| Email: |
Visual Basic .NET has not only extended the versatility of the Visual Basic language but also harnessed the power of the .NET framework, the new "Delegate" feature is one of them. So now you don't get the sneer of the self proclaimed C++ or Java programmers looking down on VB programmers as mere mortals!
More about Delegates, what is the closest you can think of a Delegate? Well a Delegate is an entity that is entrusted with the task of representation, assign or passing on information. In code sense, it means a Delegate is entrusted with a Method to report information back to it when a certain task(which the Method expects) is accomplished outside the Method's class. Now does that sound familiar to you? Well, think Event Handlers and you got Delegates into the picture. Let me demystify, when a Mouse move event occurs its the Form_MouseMove method that is invoked. But who does the work of passing the invokation from the event to the method? Its the MouseEventHandler Delegate that is entrusted with representing the Form_MouseMove method and whose job is report back to Form_MouseMove when a MouseMove event occurs. Now delegates are Typesafe which means the Delegate can work with a Method or "callback" only if it has the same signature as the Delegate. Delegates form an integral part of the .NET framework and the more you know the better! Now that I've hopefully clarified what a Delegate is, lets see an example of a Printer to give you a clearer picture. We'll create a Printer class which has basic attributes like the Name of the Printer and the Ink level in the cartridge. Now the Printer class has a delegate CartridgeEventHandler which is invoked by the InkLevel property when the level of ink in the cartridge passes through certain intervals. The delegate in turn invokes the Method it represents or is entrusted with dutifully passing on information to it.
Public Class Printer
\' ** The attributes of the Printer
Private m_sName As String \' ** Name of the Printer
Private m_nInkLevelinCartridge As Short \' ** Ink Level in the cartridge
\' ** Enumeration of the various ink states of the cartridge
Public Enum CartridgeState
EMPTY
LOW
HALF
FULL
End Enum
\' *** IMPORTANT
\' Declare the Delegate CartridgeEventHandler which is capable of invoking a
\' Method with the same signature, i.e any Method which accepts a CartridgeState
\' enumeration as parameter and that returns a Void
Public Delegate Sub CartridgeEventHandler(ByVal nState As CartridgeState)
Private m_dlgCEH As CartridgeEventHandler
\' ** This method will be used by the Caller to pass the Delegate of the
\' function,
\' i.e the Delegate will carry the Address of the Method
Public Sub SetDlgRef(ByVal dlgCartridge As CartridgeEventHandler)
m_dlgCEH = dlgCartridge
End Sub
Public Property Name() As String
Get
Return m_sName
End Get
Set(ByVal Value As String)
If Value.Length = 0 Or Value.Length > 20 Then
m_sName = \"Not Specified\"
Else
m_sName = Value
End If
End Set
End Property
\'** Read or set the level of ink in the cartridge
\' the range of the ink level is 0-100.
Public Property InkLevel() As Short
Get
Return m_nInkLevelinCartridge
End Get
Set(ByVal Value As Short)
If Value < 0 Or Value > 100 Then
m_nInkLevelinCartridge = 0
Else
m_nInkLevelinCartridge = Value
\' ** Invoke the delegate and pass the state according to the ink level
Select Case m_nInkLevelinCartridge
Case 100
m_dlgCEH.Invoke(CartridgeState.FULL)
Case 45 To 55
m_dlgCEH.Invoke(CartridgeState.HALF)
Case 1 To 20
m_dlgCEH.Invoke(CartridgeState.LOW)
Case 0
m_dlgCEH.Invoke(CartridgeState.EMPTY)
End Select
End If
End Set
End Property
End Class
Now lets create a Windows form which interfaces with the Printer class. The Windows form has a Status panel which displays the Ink status of the cartridge(Full, Half, Low and Empty) and a Trackbar. We can decrease/increase the level of ink in the cartridge using the Trackbar.
\' ** Create an instance of the Printer class
Dim oPrinter As New Printer
Private Sub Form1_Load(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles MyBase.Load
\' ** register the Delegate of CartridgeStatus Method with the Printer class
\' Notice that when we pass the Address of the Method along with a new
\' instance of the Delegate, New Printer.CartridgeEventHandler
oPrinter.SetDlgRef(New Printer.CartridgeEventHandler(AddressOf _
CartridgeStatus))
oPrinter.Name = \"HP DeskJet 640C\"
\' ** Set the level of ink in the cartridge
oPrinter.InkLevel = 100
\' ** set the position of the Trackbar based on the value of the Ink level
Me.trInkLevel.Value = (oPrinter.InkLevel / 100) * trInkLevel.Maximum
End Sub
\' ** The Method that will invoked by the Delegate, Notice the Method signature
\' is the same as that of the Delegate
\' ** The status panel will be updated with values based on the values that was
\' passed by the delegate
Private Sub CartridgeStatus(ByVal nCartridgeState As Printer.CartridgeState)
Dim sState As String
Select Case nCartridgeState
Case Printer.CartridgeState.FULL
sState = \"FULL\"
Case Printer.CartridgeState.HALF
sState = \"HALF\"
Case Printer.CartridgeState.LOW
sState = \"LOW\"
Case Printer.CartridgeState.EMPTY
sState = \"EMPTY\"
End Select
pnCartridgeInk.Text = sState
End Sub
\' ** As the Trackbar is scrolled, the InkLevel property of the Printer is also
\' updated
Private Sub trInkLevel_Scroll(ByVal sender As System.Object, _
ByVal e As System.EventArgs) Handles trInkLevel.Scroll
oPrinter.InkLevel = (trInkLevel.Value / trInkLevel.Maximum) * 100
End Sub
Imagine the number of applications you can have with the power of Delegates woven into your programming! To be crystal clear with the usage of Delegates, I suggest you get your hands dirty and try examples on your own and don't delegate it to somebody!
The above example is that of Singlecast Delegates. Now what if you want the Delegate to represent Multiple Methods ...sounds like fun,thats where Multicast Delegates come in. But that is in the next section later on this week. Ciao!
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