IMPLEMENTING OBJECT COLLABORATION in Font

Maker Data Matrix ECC200 in Font IMPLEMENTING OBJECT COLLABORATION

7. In the Solution Explorer, select the project node. Right-click the project node and select Properties. In the Property Pages dialog box, change the startup object to frmLogin. 8. Select Build Build Solution. Make sure there are no build errors in the Error List window. If there are, fix them, and then rebuild. 9. Select Debug Start to run the project. 10. To test to make sure the LogLogin event message is raised, enter a login name of Smith and a password of js. This should trigger a login status of true. 11. After testing the LogLogin event, close the form, which will stop the debugger.

To receive events in the client class using the AddHandler method, follow these steps: 1. Open the frmLogin class code in the code editor. 2. Comment out the oEmployee declaration added previously. 'Private WithEvents oEmployee As Employee 3. Delete the following code from the oEmployee_LogLogin method declaration: Handles oEmployee.LogLogin 4. Comment out the following line of code in the btnLogin_Click method: 'oEmployee = New Employee() 5. Add the following code after the line of code commented out in step 4: Dim oEmployee As Employee = New Employee() AddHandler oEmployee.LogLogin, AddressOf oEmployee_LogLogin 6. Select Build Build Solution. Make sure there are no build errors in the Error List window. If there are, fix them, and then rebuild. 7. Select Debug Start to run the project. Test to make sure the LogLogin event message is raised when you log in. 8. After testing the LogLogin event, close the form. 9. Select File Save All.

To handle multiple events with one method, follow these steps: 1. Open frmLogin in the form designer by right-clicking the frmLogin node in the Solution Explorer and choosing View Designer. 2. From the Toolbox, add a MenuStrip control to the form. Click where it says Type Here and enter &File for the top-level menu and E&xit for its submenu. (See Figure 8-2.)

3. Open the frmLogin class code in the code editor and locate the btnClose_Click method. 4. Change the name of the method to FormClose and add a handler for the ExitToolStripMenuItem click event: Private Sub FormClose(ByVal sender As Object, _ ByVal e As System.EventArgs) _ Handles btnClose.Click, ExitToolStripMenuItem.Click Me.Close() End Sub 5. Select Build Build Solution. Make sure there are no build errors in the Error List window. If there are, fix them, and then rebuild. 6. Select Debug Start to run the project. Test mnuExit and btnClose. 7. After testing, close the form. 8. Save the project, and then exit VS.

Delegation is when you request a service from a server by making a method call. The server then reroutes this service request to another method, which services the request. The delegate class can examine the service request and dynamically determine at runtime where to route the request. Returning to the company analogy, when a manager receives a service request, she often delegates it to a member of her department. (In fact, many would argue that a common trait among successful managers is the ability to know when and how to delegate responsibilities.)

When creating a delegated method, you define the delegated method s signature. Because the delegate function does not actually service the request, it does not contain any implementation code. The following code shows a delegated method used to compare integer values: Delegate Function CompareInt(ByVal I1 As Integer, _ ByVal I2 As Integer) As Boolean

Once the delegated method s signature is defined, you can then create the methods that will be delegated to. These methods must have the same parameters and return types as the delegated method. The following code shows two methods, which the delegated method will delegate to: Private Function AscendOrder(ByVal I1 As Integer, _ ByVal I2 As Integer) As Boolean If I1 < I2 Then Return True End If End Function Private Function DescendOrder(ByVal I1 As Integer, _ ByVal I2 As Integer) As Boolean If I1 > I2 Then Return True End If End Function Once the delegate and its delegating methods have been defined, you are ready to use the delegate. The following code shows a portion of a sorting routine that determines which delegated method to call depending on a SortType passed in as a parameter: Public Sub SortIntegers(ByVal SortDirection As SortType, _ ByVal intArray() As Integer) Dim CheckOrder As CompareInt If SortDirection = SortType.Ascending Then CheckOrder = New CompareInt(AddressOf AscendOrder) Else CheckOrder = New CompareInt(AddressOf DescendOrder) End If 'Code contines . . . End Sub Using delegating techniques, the same sorting routine can be called by clients to implement descending and ascending sorting of integers.