How to Trap VB Form Lost Focus with GetActiveWindow API

- Microsoft Visual Basic programming system for Windows,

  versions 1.0, 2.0, and 3.0

SUMMARY

The LostFocus event in Microsoft Visual Basic is useful when transferring control within an application, and you can use the form deactivate and activate events in versions 2.0 and 3.0 to see if the entire form has lost the focus. However, in version 1.0, no global routine exists to check for the entire form losing the focus. To check whether your version 1.0 application has lost the focus, periodically check the Windows API function GetActiveWindow in a Visual Basic timer event, as explained below.

MORE INFORMATION

The only way that version 1.0 provides a check for loss of focus on a form or control is by triggering the LostFocus event. A form does support a LostFocus event; however, a form will only get focus if there are no controls on that form. Focus goes to the controls on a form, and when you click any other visible form, the control's LostFocus procedure will be called. A control's LostFocus procedure will also be called when another control on the form is activated. To perform a routine that occurs only when the form loses focus requires careful management of what generated a LostFocus event on each control (such as setting a flag if another control's Click event was called).

For a simpler method to check if a whole form has lost the focus, you can call the Windows API function GetActiveWindow, located in USER.EXE (a DLL provided with Windows 3.0). The GetActiveWindow API call returns the window handle of the currently active window, which is the new window that you last clicked anywhere in Microsoft Windows. In a timer event procedure for the form, call GetActiveWindow and compare the handle of the currently active Window with the handle of the form window (Form1.hWND). If the handle differs, you know the form has lost the focus. The following program example demonstrates this technique:

Program Example

This single-form example will print "Lost Focus" on the form when you click a different window (such as when you click another program running in Windows).

In Visual Basic, draw one timer control (Timer1) and one command button (Command1) on a single form (Form1).

From the VB.EXE Code menu, choose View Code, and enter the following code for Form1, using (general) from the Object box, and (declarations) from the Procedure box:

   Declare Function GetActiveWindow Lib "User" () As Integer
   Dim FOCUS As Integer
   Const TRUE = -1
   Const FALSE = 0

Sub Timer1_Timer ()
   If FOCUS = TRUE Then
    ' Compare the handle of the currently active Window with the handle
    ' of the Form1 window:
    If GetActiveWindow() <> Form1.hWND Then
       'Do form's lost-focus routines here.
       Print "Lost Focus"
       FOCUS = FALSE
    End If
   End If
End Sub

From the Object box, choose Command1, and from the Procedure box, choose Click, then put the following code in the Command1_Click procedure:

   Sub Command1_Click ()
      FOCUS = TRUE
   End Sub

   Sub Form_Click ()
      FOCUS = TRUE
      Timer1.Interval = 10
   End Sub

REFERENCES

"Programming Windows: the Microsoft Guide to Writing Applications for Windows 3," Charles Petzold. Microsoft Press, 1990.

"Microsoft Windows Software Development Kit: Reference Volume 1," version 3.0.

WINSDK.HLP file shipped with Microsoft Windows 3.0 Software Development Kit.