How to move a TPanel controlled by mouse at runtime? - delphi

Question

I want to move a TPanel on another TPanel by mouse at runtime with Delphi 10.4.2 and FMX. I tried OnMouseDown, OnMouseMove and OnMouseUp events. But it is not clear what the contents of X and Y values are in the events. The documentation says that they are screen coordinates. Relative to the screen, form, parent control or the control itself? How can I solve the movement of the TPanel?

Answer 1

The documentation says that they are screen coordinates.
No, it doesn't. The FMX.Types.TMouseEvent and FMX.Types.TMouseMoveEvent
documentation both say:
X and Y--the pixel coordinates of the mouse pointer within the client area of the control.
How can I solve the movement of the TPanel?
Like this:
var
LastPt: TPointF;
Dragging: Boolean = False;
procedure TMyForm.PanelToDragMouseDown(Sender: TObject;
Button: TMouseButton; Shift: TShiftState; X, Y: Single);
begin
if Button = TMouseButton.mbLeft then
begin
LastPt := TPointF.Create(X, Y);
Dragging := True;
end;
end;
procedure TMyForm.PanelToDragMouseUp(Sender: TObject;
Button: TMouseButton; Shift: TShiftState; X, Y: Single);
begin
if Button = TMouseButton.mbLeft then
Dragging := False;
end;
procedure TMyForm.PanelToDragMouseMove(Sender: TObject;
Shift: TShiftState; X, Y: Single);
var
CurrPt: TPointF;
begin
if Dragging then
begin
CurrPt := TPointF.Create(X, Y);
PanelToDrag.Position.Point := PanelToDrag.Position.Point + (CurrPt - LastPt);
LastPt := CurrPt;
end;
end;
Basically, while the mouse is moving around the Panel, the code is simply calculating the offset the mouse has moved from the last known position to the current position, and then applying that offset to the Panel's current Position within its Parent.