Delphi GDI - Drawing

Restoring a Drawing

Introduction

If you draw on a canvas using the Mouse events, when the form looses focus, it also looses the image that was drawn. The only way you can keep your drawing is if you draw using the OnPaint event. This has to do with the operating system, not the VCL and you can get it regardless of the programming environment you are using.

This exercise shows how to restore that drawing when the form regains focus.

Start Borland Delphi with the startup form.
On the form, place a PaintBox control.

In the declaration section of the form, you will need a Boolean variable that keeps track whether the user is drawing. You will also need a point variable that finds out the starting point of a drawing
To implement the necessary behavior, you will need to initialize the DrawingBoard bitmap variable to occupy the PaintBox control. Then you will use the OnMouseDown, OnMouseUp, and OnPaint events of the PaintBox to do your thing.
Here is the complete file

unit Unit1;

interface

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

type
  TForm1 = class(TForm)
    PaintBox1: TPaintBox;
    procedure FormCreate(Sender: TObject);
    procedure PaintBox1MouseDown(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;
      Shift: TShiftState; X, Y: Integer);
    procedure PaintBox1Paint(Sender: TObject);
  private
    { Private declarations }
  public
    { Public declarations }
  end;

var
  Form1: TForm1;
  DrawingBoard: TBitmap;
  IsDrawing: Boolean;
  StartX, StartY: Integer;

implementation

{$R *.DFM}

procedure TForm1.FormCreate(Sender: TObject);
begin
    DrawingBoard := TBitmap.Create;
    DrawingBoard.Width := PaintBox1.Width;
    DrawingBoard.Height := PaintBox1.Height;
end;

procedure TForm1.PaintBox1MouseDown(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
     IsDrawing := True;
    StartX := X;
    StartY := Y;
end;

procedure TForm1.PaintBox1MouseUp(Sender: TObject; Button: TMouseButton;
  Shift: TShiftState; X, Y: Integer);
begin
     if IsDrawing then
     begin
        DrawingBoard.Canvas.Rectangle(StartX, StartY, X, Y);
        PaintBox1.Canvas.Draw(0, 0, DrawingBoard);
        IsDrawing := False;
     end;
end;

procedure TForm1.PaintBox1Paint(Sender: TObject);
begin
     PaintBox1.Canvas.Draw(0, 0, DrawingBoard);
end;

end.

Test the application