Make new instance execute methods of the previous

[bourlis]

Dear all,

I am rephrasing if someone didn't understand my previous question.

I have an application that has a collection of objects and I want every time that the user tries to load another instance of the application (clicks on the *.exe file) not to be able to do so, but on the contrary to bring on top the open instance and create a new object and add it to the collection of the previous instance.

In other words I want to terminate the new instance (I know how to do that) and triger to perform actions in the previous instance of the application.

IS IT POSSIBLE?

Thnx a lot!

Chris

[Joacim Andersson]

Well, a simple (?) way of doing this is by subclassing you're form and save the window handle (in the registry for example) and then let the next session send a user defined message to you're previous session before ending.

Hmmm... was this understandable?

[amitabh]

If you know how to subclass a window, I think this might be possible:
First define a special unique message that is not being used by windows. Then every time a new instance is created, find your earlier existing window handle, and use the SendMessage api to send the unique message to the existing window. Now since you have subclassed your window, you can know when your message has been sent. Then you can perform any action you like within the existing window.

[amitabh]

Okay so I was a bit late

[Joacim Andersson]

Originally posted by amitabh
Okay so I was a bit late

Yes, you where

Here's some code example:

Private Sub Form_Load()
    Dim hWindow As Long
    If App.PrevInstance Then
        hWindow = CLng(GetSetting("TheAppName", "SectionName", "hWnd", "0")
        SendMessage hWindow, UM_NEWSESSION, 0, 0
        End
   Else
       SaveSetting "TheAppName", "SectionName", "hWnd", Me.hwnd
       HookForm Me.hwnd
   End If
End Sub

Now add the following code to a module:

Private Declare Function SetWindowLong _
 Lib "user32" Alias "SetWindowLongA" ( _
 ByVal hwnd As Long, _
 ByVal nIndex As Long, _
 ByVal dwNewLong As Long) As Long
 
Private 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
 
Private Const GWL_WNDPROC = (-4)
Private Const WM_DESTROY = &H2&
 
Private nHwnd As Long
Private nPrevWndProc As Long
Public Const UM_NEWSESSION = &H40B& 
 
Public Sub HookForm(frmHandle As Long)
    nHwnd = frmHandle
    nPrevWndProc = SetWindowLong(nHwnd, GWL_WNDPROC, AddressOf WinProc)
End Sub
 
Public Sub UnhookForm()
    SetWindowLong nHwnd, GWL_WNDPROC, nPrevWndProc
End Sub
 
Public Function WinProc(ByVal hw As Long, ByVal uMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
    Dim sFile As String
    Dim nSize As Long
    Select Case uMsg
        Case UM_NEWSESSION
              ' add code to do whatever you like!!!
        Case WM_DESTROY
            'the window has been closed
            UnhookForm
    End Select
    WinProc = CallWindowProc(nPrevWndProc, hw, uMsg, wParam, lParam)
End Function

Be carefull when you subclass a form though.
Never use the End statement (I've used it above but that is before the form is subclassed).
Never click on the End button in the IDE.
Always use the Unload statement otherwise you'll get a GPF.

Best regards

[Joacim Andersson]

Oh I forgot. You'll need to declare the SendMessage API function as well. Add the following to the General Declaration section of your form:

Private Declare Function SendMessage _
 Lib "user32" Alias "SendMessageA" ( _
 ByVal hwnd As Long, _
 ByVal wMsg As Long, _
 ByVal wParam As Long, _
 lParam As Any) As Long

Best regards

[bourlis]

Thanx Joacim, this worked fine!

Best Regards

Chrys