TShape - drawing issue on Paint event - windows

Question

I'm stuck in a class derived from TShape.
In the Paint method, I use the Canvas to draw a rectangle. In the Form, I have a TTrackBar that allows to change Left and Top coordinates of the TShape.
It does not matter what values I set to the Left and Top using the TTrackBar, the rectangle does not move accordingly. Instead, when I set those values via code, the rectangle appears in the correct position.
I'm coding a FireMonkey app with Delphi 10.1 Berlin on Windows 10.
unit frmShapeStudy;
interface
type
tMy_Shape = class (tShape)
protected
procedure Paint; override;
public
constructor Create (aOwner: tComponent); override;
procedure Draw;
end;
tformShapeStudy = class (tForm)
trkBarLeft: TTrackBar;
trkBarTop: TTrackBar;
procedure FormCreate (Sender: tObject);
procedure TrackBarChange (Sender: tObject);
end;
var
formShapeStudy: tformShapeStudy;
implementation
{$R *.fmx}
var
My_Shape : tMy_Shape;
lvShapeRect : tRectF ;
procedure tformShapeStudy.FormCreate (Sender: tObject);
begin
My_Shape := tMy_Shape.Create (Self);
with My_Shape do begin
Parent := Self;
TrackBarChange (Self);
end;
end;
procedure tformShapeStudy.TrackBarChange (Sender: TObject);
begin
My_Shape.Draw;
end;
constructor tMy_Shape.Create (aOwner: tComponent);
begin
inherited;
with lvShapeRect do begin
Left := Self.Left;
Top := Self.Top ;
Height := 20;
Width := 20;
end;
end;
procedure tBS_Shape.Draw;
begin
l := formShapeStudy.trkBarLeft.Value;
t := formShapeStudy.trkBarTop .Value;
{`Left & Top` are set with `l & t` or with `120 & 150`
and tested separately, by commenting the propper code lines}
lvShapeRect.Left := l; // this does no work
lvShapeRect.Top := t; // this does no work
lvShapeRect.Left := 120; // this works
lvShapeRect.Top := 150; // this works
Repaint;
end;
procedure tMy_Shape.Paint;
begin
inherited;
with Canvas do begin
Fill .Color := tAlphaColorRec.Aqua;
Stroke.Color := tAlphaColorRec.Blue;
BeginScene;
FillRect (lvShapeRect, 0, 0, Allcorners, 1, tCornerType.Bevel);
DrawRect (lvShapeRect, 0, 0, Allcorners, 1, tCornerType.Bevel);
EndScene;
end;
end;
end.

Answer 1

Sorry Folks! Is the old habit of using Left & Top instead of Position.X & Position.Y. I agree with the new Position way to set Left & Top, but it doesn't make sense that Embarcadero still makes available this properties, but they do nothing in terms of setting Left & Top of the control. Saying it in other words, since those old properties are still available, they should set the Left & Top properties same as Position.X & Position.Y, otherwise it leads to this kind of mistake, where you sware you are setting Left & Top, but the control does not move.
The correct way to set Left & Top is:
Position.X := aLeft;
Position.Y := aTop;
unless Embarcadero changes the behavior of Left & Top properties (wich is very unlikely).