Visual Basic Code of the Week (COTW)
Issue #48
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Requirements for this Issue

The source code in this issue is designed for Visual Basic 5.0 and above. This is due to the use of the new AddressOf feature of VB 5. AddressOf allows you to pass the address of a subroutine created in a code module.

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In this Issue

This issue introduces a Timer class to be used without any forms or controls. It is designed to be as close to the built-in control Timer that VB includes. It uses two API calls (SetTimer and KillTimer) to accomplish this task.


The methods/properties in this class will be shown below. There are two properties and one method you need to be aware of.

Public Property Let Interval(lInterval As Long)
Public Property Get Interval() As Long

The Interval property sets the timer interval in milliseconds. For example, if you want a the timer to fire every 10 seconds, you would set Interval to 10,000. If a timer is currently active, it will disable the current timer and start a new timer with the updated Interval

Public Sub CallBackProc(lCallBackProc As Long)

The CallBackProc is the subroutine that the timer will call when it ticks off each Interval. We would have liked this to be a property, but the AddressOf keyword does not seem to be supported in a property statement. The subroutine you declare as your callback routine must be in a standard code module and have the following declaration:

Private Sub SampleTimerProc(ByVal hwnd As Long, _
                            ByVal uMsg As Long, _
                            ByVal idEvent As Long, _
                            ByVal dwTime As Long)
    ' Do your stuff here
End Sub

You can use the dwTime to determine exactly how much time has elapsed between each timer tick. This number is also in milliseconds. You should ignore the other parameters for this class module (If you want more information, check out the documentation for SetTimer).

Public Property Let Enabled(bEnabled As Boolean)
Public Property Get Enabled() As Boolean

The enabled property works just like you think it would. If you set Enabled = True the timer will start ticking. If you set it to False, it will stop. If any errors occur here, they will be raised. If an error occurs starting or stopping the timer, an error number of 1 will be raised with an appropriate description.

Sample Usage

This example shows how to use the cTimer class.

In a Code Module we declared this:

Public Sub MyTimerCallback(ByVal hwnd As Long, _
                     ByVal uMsg As Long, _
                     ByVal idEvent As Long, _
                     ByVal dwTime As Long)
    Form1.Label1 = Val(Form1.Label1) + 1
End Sub

On Form1 we created three buttons (cmdStart, cmdStop, cmdChange) and a text box (txtTimer).

Dim oTimer As New cTimer

Private Sub cmdStart_Click()
    ' Start up the timer
    oTimer.CallBackProc AddressOf MyTimerCallback
    oTimer.Interval = Val(txtTimer.Text)
    oTimer.Enabled = True
End Sub

Private Sub cmdStop_Click()
    ' Stop the timer
    oTimer.Enabled = False
End Sub

Private Sub cmdChange_Click()
    ' Change the interval on the timer
    oTimer.Interval = Val(txtTimer.Text)
End Sub

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