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-----------------------------------------------------------------------      Copyright © 1997 Microsoft Corporation. All rights reserved. You have a royalty-free right to use, modify, reproduce and distribute the Sample Application Files (and/or any modified version) in any way you find useful, if you agree that Microsoft has no warranty,      obligations or liability for any Sample Application Files. ----------------------------------------------------------------------- Visual Basic Dbgwproc.dll (Debug Object for AddressOf Subclassing) June 13, 1997 1. Introduction: Subclassing is a technique that enables you to intercept Windows messages being sent to a form or control. By intercepting these messages, you can then write your own code to change or extend the behavior of the object. VB5 provides the AddressOf keyword, which can be used to reroute Windows messages to your own message processing procedure. Subclassing using the AddressOf is very efficient, but makes debugging a project difficult. If the window you are subclassing receives a message when you are in break mode, VB will crash. The DbgWProc.Dll (Debug Object for AddressOf Subclassing) enables you to debug normally while a subclass is active without adding any unnecessary overhead to your finished product or distributing an extra component. For a more detailed discussion of the risks and rewards of subclassing, and for additional sample code, please refer to the Books Online topic "Passing Function Pointers to DLL Procedures and Type Libraries" NOTE - Before jumping into subclassing with AddressOf ===================================================== This is an advanced programming technique. It is very easy to crash VB using these methods. In particular, the Stop button and End statement are completely off limits for projects with an active subclass. It is your responsibility as a programmer to correctly turn off the subclass before VB shuts down. Advantages ========== Rolling your own subclass has the performance and distribution advantages mentioned earlier, as well as the ability to process messages at any time. Traditional, event based subclassing techniques required with earlier versions of VB have the disadvantage of being unable to process messages while a modal form is showing over the subclassed window. For example, if you're relying on receiving WM_PAINT or WM_DRAWITEM messages to properly display a custom window, then you'll have to use AddressOf directly, or a subclassing component which relies on an Implements based direct callback model instead of the more common Event model. Using DbgWProc.dll: =================== To use the DbgWProc.Dll to enable debugging: 1) Copy DbgWProc.Dll to your Windows\System[32] or VB5 directory. 2) regsvr32 DbgWProc.Dll 3) Add a reference to 'Debug Object for AddressOf Subclassing' 4) In the Make tab of the Project Properties dialog, add the DEBUGWINDOWPROC=-1 : USEGETPROP=-1 conditional compilation arguments 5) Now include the following code in your class, control, or form module (this is only the parts of the subclassing code which actually change): 'At the top of the file Private m_wndprcNext As Long #If DEBUGWINDOWPROC Then Private m_SCHook As WindowProcHook #End If 'In the SubClass procedure. For the finished version, this code is a 'single call to SetWindowLong #If DEBUGWINDOWPROC Then On Error Resume Next Set m_SCHook = CreateWindowProcHook If Err Then MsgBox Err.Description Err.Clear UnSubClass Exit Sub End If On Error GoTo 0 With m_SCHook .SetMainProc AddressOf Form1Proc m_wndprcNext = SetWindowLong(hWnd, GWL_WNDPROC, .ProcAddress) .SetDebugProc m_wndprcNext End With #Else m_wndprcNext = SetWindowLong(hWnd, GWL_WNDPROC, AddressOf_ Form1Proc) #End If Each WindowProcHook object provides a ProcAddress value to use in SetWindowLong. You call SetMainProc with AddressOf - your subclassing procedure - and SetDebugProc with the original window procedure. The WindowProcHook will call the debug procedure directly if you're in break mode, skipping the subclassing code and the invalid zero return produced by the IDE. You can create up to 100 simulateous WindowProcHook objects (hence the error trapping code), so be sure to release them when they're not in use. Samples ======= SubClass.vbp contains two samples: The AboutBox (simple sample which puts an AboutBox item on the system menu) and DrawItem (advanced ownerdraw listbox OCX sample). Both give the option of associating a subclassed hWnd with a class instance using either SetWindowLong(GWL_USERDATA) or SetProp/GetProp. USEGETPROP=-1 uses the more robust SetProp/GetProp method, which also requires the Atomizer.cls file. To view these samples, load them (AboutBox\SubClass.vbp or DrawItem\Group1.vbg) and press F5 to run them. To exit, close the forms - do not press the Stop button. To view DbgWProc in action, load the samples and begin single-stepping through the code with the F8 key. As an alternative, put a breakpoint in Friend Function WindowProc (in Form1 of the AboutBox sample and the OwnerDrawListBox usercontrol of the DrawItem sample). Both of these samples initiate subclassing with a routine called SubClass and end it with a routine called UnSubClass. If you press the Stop button while subclassing is active (between SubClass and UnSubClass), VB will GPF. Note on Compiling after using DbgWproc.dll: =========================================== Before you compile your final OCX or EXE file, change the conditional compilation value for DEBUGWINDOWPROC to 0. If you run a project which uses DbgWProc when the IDE isn't active, it will give you a warning error message when the Dll loads. This is meant to strongly discourage you from actually shipping the Dll with your project (your users will see the same message). Also be sure to remove the Dll from the list distributed files in SetupWizard. For more information and samples with AddressOf subclass, see the Black-Belt column in the February and June '97 issues of the Visual Basic Programmer's Journal magazine (www.windx.com).