Other Controls

A Timer control is invisible at run time, and its purpose is to send a periodic pulse to the current application. You can trap this pulse by writing code in the Timer's Timer event procedure and take advantage of it to execute a task in the background or to monitor a user's actions. This control exposes only two meaningful properties: Interval and Enabled. Interval stands for the number of milliseconds between subsequent pulses (Timer events), while Enabled lets you activate or deactivate events. When you place the Timer control on a form, its Interval is 0, which means no events. Therefore, remember to set this property to a suitable value in the Properties window or in the Form_Load event procedure:

Private Sub Form_Load()
    Timer1.Interval = 500    ' Fire two Timer events per second.
End Sub

Timer controls let you write interesting programs with just a few lines of code. The typical (and abused) example is a digital clock. Just to make things a bit more compelling, I added flashing colons:

Private Sub Timer1_Timer()
    Dim strTime As String
    strTime = Time$
    If Mid$(lblClock.Caption, 3, 1) = ":" Then
        Mid$(strTime, 3, 1)= " "
        Mid$(strTime, 6, 1) = " "
    End If
    lblClock.Caption = strTime
End Sub

CAUTION
You must be careful not to write a lot of code in the Timer event procedure because this code will be executed at every pulse and therefore can easily degrade your application's performance. Just as important, never execute a DoEvents statement inside a Timer event procedure because you might cause the procedure to be reentered, especially if the Interval property is set to a small value and there's a lot of code inside the procedure.

Timer controls are often useful for updating status information on a regular basis. For example, you might want to display on a status bar a short description of the control that currently has the input focus. You can achieve that by writing some code in the GotFocus event for all the controls on the form, but when you have dozens of controls this will require a lot of code (and time). Instead, at design time load a short description for each control in its Tag property, and then place a Timer control on the form with an Interval setting of 500. This isn't a time-critical task, so you can use an even larger value. Finally add two lines of code to the control's Timer event:

Private Sub Timer1_Timer()
    On Error Resume Next
    lblStatusBar.Caption = ActiveControl.Tag
End Sub

The Line control is a decorative control whose only purpose is let you draw one or more straight lines at design time, instead of displaying them using a Line graphical method at run time. This control exposes a few properties whose meaning should sound familiar to you by now: BorderColor (the color of the line), BorderStyle (the same as a form's DrawStyle property), BorderWidth (the same as a form's DrawWidth property), and DrawMode. While the Line control is handy, remember that using a Line method at run time is usually better in terms of performance.

In a sense, the Shape control is an extension of the Line control. It can display six basic shapes: Rectangle, Square, Oval, Circle, Rounded Rectangle, and Rounded Square. It supports all the Line control's properties and a few more: BorderStyle (0-Transparent, 1-Solid), FillColor, and FillStyle (the same as a form's properties with the same names). The same performance considerations I pointed out for the Line control apply to the Shape control.

When OLE first made its appearance, the concept of Object Linking and Embedding seemed to most developers nothing short of magic. The ability to embed a Microsoft Word Document or a Microsoft Excel worksheet (see Figure 3-17) within another Windows application seemed an exciting one, and Microsoft promptly released the OLE control—then called the OLE Container control—to help Visual Basic support this capability.

In the long run, however, the Embedding term in OLE has lost much of its appeal and importance, and nowadays programmers are more concerned and thrilled about Automation, a subset of OLE that lets them control other Windows applications from the outside, manipulating their object hierarchies through OLE. For this reason, I won't describe the OLE control: It's a rather complex object, and a thorough description of its many properties, methods, and events (and quirks) would take too much space.

Figure 3-17. Your Visual Basic application can host an Excel worksheet—and its menus too.

Other Controls

The Timer Control

The Line Control

The Shape Control

The OLE Control