One of the simplest ways to implement multi-tasking in Visual Basic is to create a seperate executable program to do each task and simply use the Shell command to run them as neccessary. The only problem with this is that once a program is running you need to communicate with it in order to control its operation.

One way of doing this is using the RegisterWindowMessage and SendMessage API calls to create your own particluar windows messages and to send them between windows thus allowing you to create two or more programs that communicate with each other.

In this example the server has the job of watching a printer queue and sending a message to every interested client whenever an event (job added, driver changed, job printed etc.) occurs.

1. Specifying your own unique messages

Windows communicate with each other by sending each other standard windows messages such as WM_CLOSE to close and terminate the window.
There are a large number of standard messages which cover most of the standard operations that can be performed by and to different windows. However if you want to implement your own custom communication you need to create your own custom messages. This is done with the RegisterWindowMessage API call:

'\ Declaration to register custom messages

Private Declare Function RegisterWindowMessage Lib "user32" Alias
"RegisterWindowMessageA" (ByVal lpString As String) As Long

This API call takes an unique string and registers it as a defined windows message, returning a system wide unique identifier for that message as a result. Thereafter any call to RegisterWindowMessage in any application that specifies the same string will return the same unique message id.
Because this value is constant during each session it is safe to store it in a global variable to speed up execution thus:

Public Const MSG_CHANGENOTIFY = "MCL_PRINT_NOTIFY"


Public Function WM_MCL_CHANGENOTIFY() As Long

Static msg As Long


If msg = 0 Then

msg = RegisterWindowMessage(MSG_CHANGENOTIFY)

End If


WM_MCL_CHANGENOTIFY = msg


End Function

Since this message needs to be known to every application that is using it to communicate, it is a good idea to put this into a shared code module common to all projects.

2. Creating windows to listen for these messages

To create a window in Visual Basic you usually use the form designer and add a new form to your project. However, since our communications window has no visible component nor interaction with the user, this is a bit excessive.
Instead we can use the CreateWindowEx API call to create a window solely for our communication:

Private Declare Function CreateWindowEx Lib "user32" Alias "CreateWindowExA"

  (ByVal dwExStyle As Long,

  ByVal lpClassName As String, '\ The window class, e.g. "STATIC","BUTTON" etc.

  ByVal lpWindowName As String, '\ The window's name (and caption if it has one)

  ByVal dwStyle As Long,

  ByVal x As Long,

  ByVal y As Long,

  ByVal nWidth As Long,

  ByVal nHeight As Long,

  ByVal hWndParent As Long,

  ByVal hMenu As Long,

  ByVal hInstance As Long,

  lpParam As Any) As Long

If this call is successful, it returns an unique window handle which can be used to refer to that window. This can be used in SendMessage calls to send a message to it.

In a typical client/server communication you need to create one window for the client(s) and one window for the server. Again this can be done with a bit of code common to each application:

Public Const WINDOWTITLE_CLIENT = "Merrion Computing IPC - Client"

Public Const WINDOWTITLE_SERVER = "Merrion Computing IPC - Server"


Public Function CreateCommunicationWindow(ByVal client As Boolean) As Long


Dim hwndThis As Long

Dim sWindowTitle As String


  If client Then

    sWindowTitle = WINDOWTITLE_CLIENT

  Else

    sWindowTitle = WINDOWTITLE_SERVER

  End If


  hwndThis = CreateWindowEx(0, "STATIC", sWindowTitle, 0, 0, 0, 0, 0, 0, 0, App.hInstance, ByVal 0&)


  CreateCommunicationWindow = hwndThis


End Function

Obviously for your own applications you should use different text for the WINDOWTITLE_CLIENT and WINDOWTITLE_SERVER than above to ensure that your window names are unique.

3. Processing the custom messages

As it stands you have a custom message and have created a window to which you can send that message. However, as this message is entirely new to windows it does not do anything when it recieves it. To actually process the message you need to subclass the window to intercept and react to the message yourself.

To subclass the window you create a procedure that processes windows messages and substitute this for the default message handling procedure of that window. Your procedure must have the same parameters and return type as the default window procedure:

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


'\ --[VB_WindowProc]---------------------------------------------------------------

'\ 'typedef LRESULT (CALLBACK* WNDPROC)(HWND, UINT, WPARAM, LPARAM);

'\ Parameters:

'\ hwnd - window handle receiving message

'\ wMsg - The window message (WM_..etc.)

'\ wParam - First message parameter

'\ lParam - Second message parameter

Public Function VB_WindowProc(ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long


If wMsg = WM_MCL_CHANGENOTIFY Then

  '\Respond to the custom message here


Else

  '\Pass the message to the previous window procedure to handle it

  VB_WindowProc = CallWindowProc(hOldProc, hwnd, wMsg, wParam, lParam)

End If


End Function

You then need to inform Windows to substitute this procedure for the existing window procedure. To do this you call SetWindowLong to change the address of the procedure as stored in the GWL_WINDPROC index.

Public Const GWL_WNDPROC = (-4)

Public Declare Function SetWindowLongApi Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long


'\ Use (after creating the window...)

hOldProc = SetWindowLongApi(hwndThis, GWL_WNDPROC, AddressOf VB_WindowProc)

You keep the address of the previous window procedure address in hOldProc in order to pass on all the messages that you don't deal with for default processing. It is a good idea to set the window procedure back to this address before closing the window.

4. Sending the custom messages

There are two steps to sending the custom message to your server window: First you need to find the window handle of that window using the FindWindowEx API call then you need to send the message using the SendMessage API call.

'\ Declarations

Public Declare Function SendMessageLong Lib "user32" Alias "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long

Public Declare Function FindWindow Lib "user32" Alias "FindWindowA" ( ByVal lpClassName As String, ByVal lpWindowName As String) As Long


'\ use....

Dim hwndTarget As Long


hwndTarget = FindWindow(vbNullString, WINDOWTITLE_SERVER)


If hwndTarget <> 0 Then

  Call SendMessageLong(hwnd_Server, WM_MCL_CHANGENOTIFY, 0,0)

End If

This will send the WM_MCL_CHANGENOTIFY message to the server window and return when it has been processed.