How To Determine When Your Application Gains or Loses Focus

MORE INFORMATION

WARNING: ANY USE BY YOU OF THE SAMPLE CODE PROVIDED IN THIS ARTICLE IS AT YOUR OWN RISK. Microsoft provides this sample code "as is" without warranty of any kind, either express or implied, including but not limited to the implied warranties of merchantability and/or fitness for a particular purpose.

WARNING: The following steps demonstrate how to create a sample application that is capable of determining when an application as a whole looses or gains focus. To achieve this functionality, you must use subclassing to detect when the WM_ACTIVATEAPP message occurs. For more information on subclassing, please see the following article in the Microsoft Knowledge Base:

NOTE: Failure to unhook a window before its destruction results in application errors, Invalid Page Faults, or data loss. This occurs because the new WindowProc function that is being pointed to no longer exists, but the window has not been notified of the change. Always unhook the subclassed window upon unloading the subclassed form or exiting the application.

This is especially important when you use Visual Basic to debug an application that includes subclassing. Pressing the End button or selecting End from the Run menu without unhooking may cause an Invalid Page Fault and close Microsoft Visual Basic.

Steps to Create Sample Application

1. Create a new Standard EXE project. Form1 is created by default.
2. Paste the following code into the code window for Form1:

         Sub Form_Load()
            'Store handle to this form's window
            gHW = Me.hWnd
   
            'Call procedure to begin capturing messages for this window
            Hook
         End Sub
   
         Private Sub Form_Unload(Cancel As Integer)
            'Call procedure to stop intercepting the messages for this window
            Unhook
         End Sub
   
   						

3. Add a standard module to the project.
4. Paste the following code into the module:

         Option Explicit
   
         Declare Function CallWindowProc Lib "user32" Alias _
           "CallWindowProcA" (ByVal lpPrevWndFunc As Long, _
           ByVal hwnd As Long, ByVal Msg As Long, _
           ByVal wParam As Long, ByVal lParam As Long) As Long
   
         Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" _
           (ByVal hwnd As Long, ByVal nIndex As Long, _
           ByVal dwNewLong As Long) As Long
   
         Public Const WM_ACTIVATEAPP = &H1C
         Public Const GWL_WNDPROC = -4
   
         Global lpPrevWndProc As Long
         Global gHW As Long
   
         Public Sub Hook()
            'Establish a hook to capture messages to this window
            lpPrevWndProc = SetWindowLong(gHW, GWL_WNDPROC, _
              AddressOf WindowProc)
         End Sub
   
         Public Sub Unhook()
            Dim temp As Long
   
            'Reset the message handler for this window
            temp = SetWindowLong(gHW, GWL_WNDPROC, lpPrevWndProc)
         End Sub
   
         Function WindowProc(ByVal hw As Long, ByVal uMsg As Long, _
           ByVal wParam As Long, ByVal lParam As Long) As Long
            'Check for the ActivateApp message
   
            If uMsg = WM_ACTIVATEAPP Then
                'Check to see if Activating the application
                If wParam <> 0 Then
                    'Application Received Focus
                    Form1.Caption = "Focus Restored"
                Else
                    'Application Lost Focus
                    Form1.Caption = "Focus Lost"
                End If
            End If
   
            'Pass message on to the original window message handler
            WindowProc = CallWindowProc(lpPrevWndProc, hw, uMsg, wParam, _
              lParam)
         End Function
   
   						

5. Save and run the sample.
6. Click on another application (such as the Windows Desktop), and note that the caption of Form1 indicates that focus was lost. (If the Form is not visible, refer to the Form caption in the Windows taskbar.)
7. Click on the sample application, and note that the caption of Form1 indicates that focus was received.