Project Structure

• MouseOne.dpr
• MouseF.pas
• MouseF.dfm

program MouseOne;

uses
  Forms,
  MouseF in 'MouseF.pas' {MouseForm};

{$R *.RES}

begin
  Application.CreateForm(TMouseForm, MouseForm);
  Application.Run;
end.

unit MouseF;

interface

uses Windows, Classes, Graphics,
  Controls, Forms, SysUtils;

type
  TMouseForm = class(TForm)
    procedure FormMouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormMouseMove(Sender: TObject; Shift: TShiftState; X,
      Y: Integer);
    procedure FormMouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure FormPaint(Sender: TObject);
  private
    fDragging: Boolean;
    fRect: TRect;
  public
    { Public declarations }
  end;

var
  MouseForm: TMouseForm;

implementation

{$R *.DFM}

procedure TMouseForm.FormMouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if Button = mbLeft then
  begin
    fDragging := True;
    Mouse.Capture := Handle;
    fRect.Left := X;
    fRect.Top := Y;
    fRect.BottomRight := fRect.TopLeft;
    Canvas.DrawFocusRect (fRect);
  end;
end;

procedure TMouseForm.FormMouseMove(Sender: TObject;
  Shift: TShiftState; X, Y: Integer);
begin
  // display the position of the mouse in the caption
  Caption := Format ('Mouse in x=%d, y=%d', [X, Y]);
  if fDragging then
  begin
    // remove and redraw the dragging rectangle
    Canvas.DrawFocusRect (fRect);
    fRect.Right := X;
    fRect.Bottom := Y;
    Canvas.DrawFocusRect (fRect);
  end
  else
    if ssShift in Shift then
      // mark points in yellow
      Canvas.Pixels [X, Y] := clYellow;
end;

procedure TMouseForm.FormMouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
  if fDragging then
  begin
    Mouse.Capture := 0; // calls ReleaseCapture
    fDragging := False;
    Invalidate;
  end;
end;

procedure TMouseForm.FormPaint(Sender: TObject);
begin
  Canvas.Rectangle (fRect.Left, fRect.Top,
    fRect.Right, fRect.Bottom);
end;

end.

object MouseForm: TMouseForm
  Left = 223
  Top = 108
  Width = 435
  Height = 300
  Caption = 'Mouse'
  Color = clBtnFace
  Font.Charset = DEFAULT_CHARSET
  Font.Color = clWindowText
  Font.Height = -13
  Font.Name = 'System'
  Font.Style = []
  OldCreateOrder = False
  OnMouseDown = FormMouseDown
  OnMouseMove = FormMouseMove
  OnMouseUp = FormMouseUp
  OnPaint = FormPaint
  PixelsPerInch = 96
  TextHeight = 16
end