The Class Module will completely hide
the implementation details. It will expose its functionality and the developer
will never see or edit its source code. Class modules can immensely improve your productivity, help you
solve many common and intricate programming problems, and even permit you to
perform tasks that would be extremely difficult, if not impossible, otherwise.
Even if Visual Basic isn’t a full-fledged object-oriented
programming language, you can still use its classes to better organize your code
into truly reusable modules and design your applications entirely using
concepts derived from the Object-Oriented Design discipline.
Steps to create a
new Class:
1. In
the New Project add a Class Modules folder in the Project Explorer window and a
Class Module under it. The Class Module is named Class1 by default.
2. Change
the Class Module’s Name property to CTimer. The Class Module doesn’t
have a visible interface, so the Code window for the new component will be
displayed.
3. Enter
the lines in the Class Module’s Code window.
Code : The CTimer Class’ Code
Dim totalInterval As Double
Dim T1 As Double
Public Sub StartCounting()
T1 = Time
End Sub
Public Sub StopCounting()
totalInterval =
totalInterval + Time - T1
End Sub
Property Get ElapsedTime() As Double
ElapsedTime =
totalInterval
End Property
Public Sub ResetTimer()
totalInterval = 0
End Sub
The code of the
Class Module is quite similar to the Module’s code, but it doesn’t have a
public variable. The totalInterval variable can’t be accessed directly
from any procedure outside the Class Module. It can only be read through the
ElapsedTime() procedure. Notice that this is a special type procedure called Property
Get. Every time an application attempts to read the value of the totalInterval
variable, the ElapsedTime() procedure is invoked. Its code reads the value of
the totalInterval local variable and assigns it to the ElapsedTime() procedure.
This value is then passed to the calling application.
When an
application attempts to set a property value, a similar procedure is invoked,
only it’s a Property Let procedure. This Class doesn’t have any
properties that can be set, so there are no Property Let procedures in it.
We’ll discuss both Property procedures after looking at this simple example.
The Property Let and Property Get procedures act like buffers between the Class
and the application that uses it. For one thing, the application can’t set the
variable directly.
1. Add
a new project with the File -> Add Project command. Visual Basic will add a
new folder to the Explorer window, the Project1 folder, and it will place a
Form under it.
2. In
order to use the CTimer Class in the test project, we must first add a
reference to the Class. Open the Project menu and select References. In the
References dialog box select the entry TimerProject by checking it. The
References dialog box displays the name of the project, not the name of the
Class.
Then,
open the Form’s Code window and enter the lines in Code:
Dim TMR As New CTimer
Private Sub Command1_Click()
If Command1.Caption = “Start Timing” Then
TMR.StartCounting
Command1.Caption = “Stop Timing”
Else
TMR.StopCounting
Command1.Caption = “Start Timing”
End If
End Sub
Private Sub Command2_Click()
ETime = TMR.ElapsedTime
MsgBox “I’ve been counting for ” & vbCrLf & _
Hour(TMR.ETime) & “ hours” & vbCrLf & _
Minute(TMR.ETime) & “ minutes and ” & vbCrLf & _
Second(TMR.ETime) & “ seconds” & vbCrLf
End Sub
Private Sub Command3_Click()
End
End Sub
Standard module.
1.
When using multiple forms in a project, it is important for you to consider
each sub procedure or function procedure that you create. If the procedure will
be used in only one form, then it should be included in the code for that form
module. If, in fact, you will need to use the procedure in multiple forms,
write the procedure in standard code module.
2.
A standard code module is a Visual Basic file with extension .bas, which is
added to project. Standard code module does not contain a form, only code.
3.
Create a new standard code module by selecting the Add Module command on the
project menu. Select module on the new tab of the add module dialog box and
click open. A new code window titled module1 opens on the screen. Module is
also added to project explorer.
4.
Standard code module has a general declarations section and procedure, just
like form module. You can declare a variable and procedure as you declare in
form module.
