How to move a TImage on a scrollbox

Problem/Question/Abstract:

I placed some TImage components on a TScrollBox. Now I would like my users to be able to scroll these images by clicking on them and moving the cursor with the mouse button down.

Answer:

Attach handlers to the OnMouseDown, Move, Up events of the image. Modify as below. The key here is to not use the X and Y mouse positions the handlers get. Each time the image is scrolled the origin for this position moves and that screws up the calculation. The code below uses the screen-relative mouse position.

unit Unit1;

interface

uses
  Windows, Messages, SysUtils, Classes, Graphics, Controls, Forms, Dialogs,
  ExtCtrls, JPEG, StdCtrls;

type
  TForm1 = class(TForm)
    ScrollBox1: TScrollBox;
    Image1: TImage;
    Label1: TLabel;
    procedure Image1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure Image1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
  private
    { Private declarations }
    FLastDown: TPoint;
  public
    { Public declarations }
  end;

var
  Form1: TForm1;

implementation

{$R *.DFM}

procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  GetCursorPos(FLastDown);
end;

procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  FLastDown := Point(-1, -1);
end;

procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X, Y: Integer);
var
  pt: TPoint;
begin
  if (ssLeft in Shift) and (FLastDown.X >= 0) then
  begin
    GetCursorPos(pt);
    Scrollbox1.VertScrollBar.Position := Scrollbox1.VertScrollBar.Position + FLastDown.Y - pt.Y;
    Scrollbox1.HorzScrollBar.POsition := Scrollbox1.HorzScrollBar.Position + FLastDown.X - pt.X;
    FLastDown := pt;
    label1.caption := format('%d:%d', [pt.x, pt.y]);
  end;
end;

end.