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subclassing in vb

Posted on 2003-02-26
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Last Modified: 2010-04-07
Hi
i want to know about subclassing and their requirement?
Thanks
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Question by:mmalwan
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5 Comments
 
LVL 1

Expert Comment

by:Belfry
ID: 8023868
not much requirement,
first you declare the following functions and constants:
SetWindowLong()
CallWindowProc()
const GWL_WNDPROC = -4

then you need a public .bas module for subclass code

'in a .bas module:
'OldProc restores the original procedure for the window,should be public
public OldProc as long
'the subclassing windowproc for you to handle
public function myWndProc(byval hWnd as long, byval uMsg as long, byval wParam as long, byval lParam as long)as long

'do your handling here

'after your handling,you need to call original windowproc to process the unhandled messages
 myWndProc=CallWindowProc(OldProc,hWnd,uMsg,wParam,lParam)
End Function


in form code where you want to subclass(typically Form_Load), type in:
OldProc=SetWindowLong(me.hWnd,GWL_WNDPROC,addressof myWndProc)

and in form code where you want to unsubclass it(typically Form_Unload), type:
SetWindowLong me.hWnd, GWL_WNDPROC, OldProc


Good luck!
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LVL 5

Expert Comment

by:Julian_K
ID: 8024576
And after all that you pray not to have bugs, couse if the program is interrupted while in subclassing part, the VB environment says good-bye.
:)))
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Author Comment

by:mmalwan
ID: 8024831
thanks all
these are good knowlege but i want to know some theory about subclassing and why to use it
MMAlwan
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LVL 5

Accepted Solution

by:
Julian_K earned 210 total points
ID: 8024976
Okay.
All user controls, forms, etc. has a specific structure, which we call window. It is used by the system (Windows, etc.) to send them events. For example when a window needs to be redrawn, or when the mouse is clicked over a button - in these (and hell of a lot more) events, the system sends a message to the window which must do something regarding to the event. In most cases you don't have access to these messages. (In fact you have access to some of them - they are implemented as events in VB - like "Button_Click" or "Button_MouseMove" and so on)

Subclassing is process of intercepting these messages which the system sends, when you need to use them, but VB does not allows you to do so directly.
All these messages are sent to a function, which memory address is stored in the window structure. You replace this address with the address of your own function and recieve the events instead of the owner window. Don't forget to call the previous window function if you recieve events that are not of interest to you.

If you need more specifics, drop a comment.

Regards,
Julian
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Author Comment

by:mmalwan
ID: 8025363
Good Code and good Explanation
Thanks for all of you

MMAlwan
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