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In my extended TComboBox class, I overrided ComboWndProc() procedure handler, but I was not able to detect neither CN_VSCROLL nor WM_VSCROLL messages from the scroll bar of the List (FListHandle).

I basically want to implement an infinite scroll using winapi.
I imagine that, to do what I want, I basically would need to know the track bar position of the scroll so when the track bar touch the line down button I would add more data to strings.

The idea is simple and maybe naive, but I could start from there and see what problems I would have.

Is it possible to do such a thing?

Is there a way to track scroll bar messages from TComboBox?

More importantly:

  • If yes, How?
  • If no, Why?
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  • Alternatives are welcome too, just saying Commented Mar 5, 2015 at 13:08
  • Surely you need ListWndProc. And I cannot see you getting any CN_XXX messages. I'd expect this to be a pure Win32 window. Commented Mar 5, 2015 at 13:10
  • @DavidHeffernan ListWndProc just calls ComboWndProc. I wondered that, I think those messages never reaches the list handle Commented Mar 5, 2015 at 13:10
  • And what Style is your combo? In case that matters Commented Mar 5, 2015 at 13:14
  • @DavidHeffernan csDropDown Commented Mar 5, 2015 at 13:15

1 Answer 1

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You can use WM_VSCROLL, to do so you have to subclass the listbox control of the combobox. CN_VSCROLL will not work because the listbox part of the combobox is not a VCL control.

Below example is essentially from this answer of Kobik, included here for the sake of completeness.

type TForm1 = class(TForm) ComboBox1: TComboBox; procedure FormCreate(Sender: TObject); procedure FormDestroy(Sender: TObject); private FComboListWnd: HWND; FComboListWndProc, FSaveComboListWndProc: Pointer; procedure ComboListWndProc(var Message: TMessage); end; var Form1: TForm1; implementation {$R *.dfm} procedure TForm1.FormCreate(Sender: TObject); var Info: TComboBoxInfo; begin ZeroMemory(@Info, SizeOf(Info)); Info.cbSize := SizeOf(Info); GetComboBoxInfo(ComboBox1.Handle, Info); FComboListWnd := Info.hwndList; FComboListWndProc := classes.MakeObjectInstance(ComboListWndProc); FSaveComboListWndProc := Pointer(GetWindowLong(FComboListWnd, GWL_WNDPROC)); SetWindowLong(FComboListWnd, GWL_WNDPROC, Longint(FComboListWndProc)); end; procedure TForm1.FormDestroy(Sender: TObject); begin SetWindowLong(FComboListWnd, GWL_WNDPROC, Longint(FSaveComboListWndProc)); classes.FreeObjectInstance(FComboListWndProc); end; procedure TForm1.ComboListWndProc(var Message: TMessage); begin case Message.Msg of WM_VSCROLL: OutputDebugString('scrolling'); end; Message.Result := CallWindowProc(FSaveComboListWndProc, FComboListWnd, Message.Msg, Message.WParam, Message.LParam); end;
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3 Comments

Thank you so much. I believe I can manage to make it work now
By the way, how did you knew the list is child of the of the desktop window. How can I learn more about it? This kind of knowledge only comes with practice? May I read about it somewhere?
@epro - I guess the most you can find documented is that the listbox control is some 'ComboLBox'. Other than that you may find pieces here and there. What I do to quickly examine volatile popups like this (menu, etc..) is to construct an owner-drawn one and put a breakpoint in drawing code. Then you can freeze the window and try spy++ on it. It's kind of delicate though, when I also include message processing, this setup sometimes manages to freeze the entire box. Otherwise, if you can subclass the window then you can do whatever you want.. You're welcome!

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