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

Question

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?

Answer 1

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;