WndProc - Lazarus vs. Delphi

Question

I am moving some code for a Windows message handler/intercepter from Lazarus to Delphi.

In Lazarus, I had this code:

function WndCallback(Ahwnd: HWND; uMsg: UINT; wParam: WParam; lParam: LParam):LRESULT; stdcall;
(Handle WM_ messages)
result:=CallWindowProc(PrevWndProc,Ahwnd, uMsg, WParam, LParam);
end;

(with this invoked in the FormCreate)
PrevWndProc:=Windows.WNDPROC(SetWindowLongPtr(Self.Handle,GWL_WNDPROC,PtrInt(@WndCallback))); 

Notice that WndCallback can return LRESULT to the invoking SendMessage on the non-Pascal application.

In Delphi, Windows.WNDPROC does not exist, so I created my own WndProc() that overrides the WndProc() in the VCL. However, WndProc() is a procedure, not a function, so it cannot return a result to the invoking SendMessage. Is there a way to return a result from a Delphi WndProc() ?

Thanks !

Answer 1

Inside your overridden WndProc()¹ method, you can send a result value back to the caller by setting the TMessage.Result field. Or calling the inherited method for default handling.

procedure TMyForm.WndProc(var Message: TMessage);
begin
  (Handle WM_ messages)
  if (want to return a value) then
    Message.Result := ...
  else
    inherited;
end;

¹: BTW, the virtual WndProc() method also exists in FreePascal/Lazarus. There is also a virtual MainWndProc() method, too. So, you don't need to use SetWindowLongPtr() on your TForm at all, in either compiler.