What is Sub Classing exactly?

What is Sub Classing exactly?
And where can I get good resource for Sub Classing?

First, a quick intro as to how Windows programs process events. Every window (you may have more than one window per process/thread) has an associated window procedure, which is a function that recieves all messages from the system. Visual Basic creates its own hidden function, and calls your event handlers from it.

Sub-classing is where you replace a window's window procedure with your own, then once you've used the message you forward it to the original function. Inside your program you can use it to catch unsupported messages. Outside of your program you need an external DLL to alter windows not belonging to your program.

Simple example on subclassing:

Code:

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'Create a new project, add a module to it
'Add a command button to Form1
'In the form
Private Sub Form_Load()
    'Subclass this form
    HookForm Me
    'Register this form as a Clipboardviewer
    SetClipboardViewer Me.hwnd
End Sub
Private Sub Form_Unload(Cancel As Integer)
    'Unhook the form
    UnHookForm Me
End Sub
Private Sub Command1_Click()
    'Change the clipboard
    Clipboard.Clear
    Clipboard.SetText "Hello !"
End Sub

'In a module
'These routines are explained in our subclassing tutorial.
'http://www.allapi.net/vbtutor/subclass.htm
Declare Function SetWindowLong Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long) As Long
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 SetClipboardViewer Lib "user32" (ByVal hwnd As Long) As Long
Public Const WM_DRAWCLIPBOARD = &H308
Public Const GWL_WNDPROC = (-4)
Dim PrevProc As Long
Public Sub HookForm(F As Form)
    PrevProc = SetWindowLong(F.hwnd, GWL_WNDPROC, AddressOf WindowProc)
End Sub
Public Sub UnHookForm(F As Form)
    SetWindowLong F.hwnd, GWL_WNDPROC, PrevProc
End Sub
Public Function WindowProc(ByVal hwnd As Long, ByVal uMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
    WindowProc = CallWindowProc(PrevProc, hwnd, uMsg, wParam, lParam)
    If uMsg = WM_DRAWCLIPBOARD Then
        MsgBox "Clipboard changed ..."
    End If
End Function

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It's useful because you can intercept messages before VB has a chance to. Here's a simple example.

Add to a Module.

Code:

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Declare Function SetWindowLong& Lib "user32" Alias "SetWindowLongA" (ByVal hwnd As Long, ByVal nIndex As Long, ByVal dwNewLong As Long)
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
Const GWL_WNDPROC = (-4)
Const WM_LBUTTONDOWN = &H201

Global WndProcOld As Long

Public Function WindProc(ByVal hwnd As Long, ByVal wMsg As Long, ByVal wParam As Long, ByVal lParam As Long) As Long
    If wMsg = WM_LBUTTONDOWN Then Form1.Print "(Subclassed) mouse pressed"
    WindProc = CallWindowProc(WndProcOld&, hwnd&, wMsg&, wParam&, lParam&)
End Function

Sub SubClassWnd(hwnd As Long)
    WndProcOld& = SetWindowLong(hwnd, GWL_WNDPROC, AddressOf WindProc)
End Sub

Sub UnSubclassWnd(hwnd As Long)
    SetWindowLong hwnd, GWL_WNDPROC, WndProcOld&
    WndProcOld& = 0
End Sub

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Add to a Form

Code:

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Private Sub Form_Load()
    SubClassWnd hwnd
End Sub

Private Sub Form_MouseDown(Button As Integer, Shift As Integer, X As Single, Y As Single)
    Print "(VB) Mouse pressed"
End Sub

Private Sub Form_Unload(Cancel As Integer)
    UnSubclassWnd hwnd
End Sub

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Try pressing the left button. As you can see, the code in the subclassed routine fires first.