Subclassing descendant of VCL TWinControl

Question

Using pseudo funcs for subclassing:

CreateSpecialHandle(TWinControl *Control, const TCreateParams &Params, const AnsiString SubClass)  
{  
......;  
    set Control DefWndProc to SubClass.lpfnWndProc  
    set Control WindowHandle from CreateWindowEx 
......;  
subclass(TWinControl *Control);  
}

subclass(TWinControl *Control)
{
  ......;
  oldWProc = (void*)GetWindowLong(Control->Handle, GWL_WNDPROC);
  oldDefWProc = (void*)(Control->DefWndProc);
  oldWindowProc = Control->WindowProc;
  MakeObjectInstance(newWProc) for SetWindowLong
  MakeObjectInstance(newDefWProc) for Control->DefWndProc
  Control->WindowProc = newWindowProc;
  ......;
}

Now, we have unexpected behavior of subclassed control. WM_NCHITTEST result 0, etc...
For example when newWProc intercepts WM_NCHITTEST and sets Result to HTCLIENT we have mouse response, but, is that not responding without setting msg.result to 1 for msg.msg WM_NCHITTEST consequence of my mistake and wrong subclassing, what else we need to handle manually?

• newWProc make callback of oldWProc
• newDefWProc make callback of oldDefWProc
• newWindowProc calls oldWindowProc

Do we have to subclass parent control of subclassed control as well?
Also, sending WM_GETTEXT results with empty buffer.
Obviously, we are doing something wrong here. We need explanation,
Thank You all in advance
Update:

   in TDCEdit:public TCustomEdit overriding CreateWindowHandle
   void __fastcal CreateWindowHandle(const TCreateParams &Params)
      {
       CreateSpecialHandle(this,Params,TEXT("EDIT"));
      }
     void CreateSpecialHandle(TWinControl *Control,const TCreateParams &Params, AnsiString SubClass)  
     {
     ... 
     Control->WindowHandle = CreateWindowEx(...,"EDIT",....);
     ....
     subclass(Control);
     }
     subclass(TWinControl* Control)
     {
     ......;
     oldWProc = (void*)GetWindowLong(Control->Handle, GWL_WNDPROC);
     oldDefWProc = (void*)(Control->DefWndProc);
     oldWindowProc = Control->WindowProc;
     MakeObjectInstance(newWProc) for SetWindowLong
     MakeObjectInstance(newDefWProc) for Control->DefWndProc
     Control->WindowProc = newWindowProc;
     ......;
     }

Now, when I use TDCEdit and intercept Message.Msg == WM_NCHITTEST
inside newWProc Message.Result is 0 and stay 0 through all message process chain.
Note that subclassing TCustomEdit is one among other controls we need to subclass
in project and we try to use same subclass(TWinControl*) function for all.

Here is part of newWProc with few more lines to focus on problem

void __fastcall TControlWrapper::newWProc(Messages::TMessage &Message)
 {
   if(Message.Msg == WM_NCHITTEST ) // TEST
            if(Message.Result == 0) 
                  Message.Result=1;//<- WHY I NEED TO DO THIS
    if( Message.Msg == WM_DESTROY) {
      HandleWMDestroy(Message);
      return; 
    }
      CallWindowProcW( (int(__stdcall*)())oldWProc, 
              Handle, Message.Msg, Message.WParam, 
              Message.LParam);

     if(Message.Msg == WM_NCHITTEST )
            if(Message.Result == 0)Message.Result=1;//<- OR THIS


 }

Answer 1

This is a confusing question - it doesn't help that your code samples are not C++.

set Control DefWndProc to SubClass.lpfnWndProc

is not a line in a C++ function, for example. Can you show your actual code please?

I can make a guess at what you're trying to do: are you trying to subclass a window (perhaps a form?) so that it moves when the mouse is clicked on it? If so, you don't need to do any raw Windows API-style subclassing, the way you appear to be doing with GetWindowLong. In C++ Builder, the VCL is an object-oriented wrapper around the Windows API, and you can do this in one of two much cleaner ways:

1. Create a new WindowProc and set it; this is a property pointing to a new window procedure, and you simply call the old one too;
2. Create a descendant class of your TWinControl (if you're using a form, you already have one) and implement the virtual method WndProc.

An example of #1, in Delphi (but you should be easily able to convert it to C++) is in the Embarcadero documentation on subclassing WndProc.

An example of #2, the cleanest OO version, is here, and this actually shows how to do what you're trying to do, too: C++Builder: Create a TForm with BorderStyle bsNone that is nevertheless movable and resizable

Given what you appear to want to do, I would suggest going with #2.