How can I update the tooltip text of a Windows trackbar control?

Question

The TTrackBar component in Delphi is a wrapper around the standard Windows trackbar control. When the PositionToolTip property is not None a tooltip is displayed while dragging the thumb of the trackbar control.

By default, that tooltip displays a plain integer number between Min and Max of the control. Is there a way to manually update that position tooltip text so that the number value can be formatted like e.g., "80 %" in case of a volume control?

When looking at the Windows trackbar control documentation there seems to be a TBM_GETTOOLTIPS message which allows to retrieve a handle for the tooltip. Not sure how to update the tooltip text using that handle, though.

Answer 1

I believe the best approach is to use the TTN_NEEDTEXT notification and fill the NMTTDISPINFO structure:

unit Unit1;

interface

uses
  Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants, System.Classes, Vcl.Graphics,
  Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.ComCtrls, CommCtrl;

type
  TTrackBar = class(Vcl.ComCtrls.TTrackBar)
    procedure WndProc(var Message: TMessage); override;
  end;

  TForm1 = class(TForm)
    TrackBar1: TTrackBar;
  private
  public
  end;

implementation

{$R *.dfm}

procedure TTrackBar.WndProc(var Message: TMessage);
begin
  inherited;
  case Message.Msg of
    WM_NOTIFY:
      if PNMHdr(Message.LParam).code = TTN_NEEDTEXT then
      begin
        var S := Round(100 * Position / Max).ToString + '%';
        FillChar(
          PNMTTDispInfo(Message.LParam).szText,
          Length(PNMTTDispInfo(Message.LParam).szText) * SizeOf(Char),
          0);
        if S.Length <= Length(PNMTTDispInfo(Message.LParam).szText) then
          CopyMemory(
            @PNMTTDispInfo(Message.LParam).szText,
            Pointer(S),
            S.Length * SizeOf(Char)
          );
      end;
  end;
end;

end.

(Most often, you don't use a space before a percent sign in English. In many other languages, however, there should be a space. For instance, in Swedish, you always have a non-breaking space before the percent sign. So 50% in English, but 50 % in Swedish.)