How to make list view scroll while dragging an item up or down?

Question

I'm working with a TListView which has drag/drop capabilities to drag one or multiple items into one other item, as a form of merging. When you drag an item to the top or bottom of the control, I need it to automatically scroll up or down but it doesn't. The same applies for scrolling right or left in certain view styles. How can I make it automatically scroll in the direction the user's dragging the item?

PS: I have VCL Themes enabled as well

Answer 1

Did not test much, but the below try enables a timer when an item is dragged outside the control over its parent (in the case of the example, the form), and the timer event tests the cursor position to find out if a scroll message should be send to the listview.

procedure TForm1.FormCreate(Sender: TObject);
begin
  Timer1.Enabled := False;
  Timer1.Interval := 500;
end;

procedure TForm1.FormDragOver(Sender, Source: TObject; X, Y: Integer;
  State: TDragState; var Accept: Boolean);
begin
  if Source = ListView1 then 
    Timer1.Enabled := True
  else
    Timer1.Enabled := False;
end;

procedure TForm1.Timer1Timer(Sender: TObject);
var
  Pt: TPoint;
begin
  // Stop timer and exit if not dragging any more
  if not ListView1.Dragging then begin
    Timer1.Enabled := False;
    Exit;
  end;

  Pt := ListView1.ScreenToClient(Mouse.CursorPos);
  if Pt.Y < 0 then
    ListView1.Perform(WM_VSCROLL, SB_LINEUP, 0)
  else
    if Pt.Y > ListView1.ClientHeight then
      ListView1.Perform(WM_VSCROLL, SB_LINEDOWN, 0)
    else
      Timer1.Enabled := False;
end;

procedure TForm1.FormDragDrop(Sender, Source: TObject; X, Y: Integer);
begin
  Timer1.Enabled := False;
end;

If it works Ok, you can incorporate horizontal scrolling too.