Detecting when another application is activated

Download the Deactivation detection sample project (13kb)

In a form, there is a Deactivate method. Exactly what this method is for is hard to determine, because it hardly ever seems to fire. Ok, that's perhaps a little unfair. But one thing you can't detect without a bit of additional work is when the user Alt-Tabs to another application. Detecting this can be useful, for example, when you are showing a pop-up tool window

When a form in your application is deactivated, Windows fires a WM_ACTIVATE message to the form. The wParam of this message tells you the reason the message has been fired:

The code to make this work is very simple with SSUBTMR:

' Subclassing object to catch Alt-Tab
Implements ISubclass
Private Const WM_ACTIVATE = &H6

Private Sub Form_Load()
    ' Start subclassing for WM_ACTIVATE
    AttachMessage Me, Me.hwnd, WM_ACTIVATE
End Sub

Private Sub Form_QueryUnload(Cancel As Integer, UnloadMode As Integer)
    ' Clear up:
    DetachMessage Me, Me.hwnd, WM_ACTIVATE
End Sub

Private Property Let ISubclass_MsgResponse(ByVal RHS As SSubTimer.EMsgResponse)
    ' NR
End Property

Private Property Get ISubclass_MsgResponse() As SSubTimer.EMsgResponse
    ' Respond to the message after windows has done its stuff:
    ISubclass_MsgResponse = emrPreprocess
End Property

Private Function ISubclass_WindowProc(ByVal hwnd As Long, ByVal iMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long

    Select Case wParam
    Case 0
        Me.Caption = "Deactivate"
    Case 1
        Me.Caption = "Activate"
    Case 2
        Me.Caption = "Mouse Activate"
    End Select

End Function