Delphi实例之橡皮筋画图的实现

在《Delphi7基础教程》这本书的练习中提到过一个橡皮筋画图的例子，书上的源码是错误的！不知道是打印的错误还是本身源码就有问题，我将它改了过来。

在Form1上放置一个Image组件，Image1的Align设为Client。

1 unit Unit1; 2  3 interface 4  5  6 uses 7   Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms, 8   Dialogs, ExtCtrls; 9 10 type11   TForm1 = class(TForm)12     Image1: TImage;13     procedure Image1MouseDown(Sender: TObject; Button: TMouseButton;14       Shift: TShiftState; X, Y: Integer);15     procedure Image1MouseMove(Sender: TObject; Shift: TShiftState; X,16       Y: Integer);17     procedure Image1MouseUp(Sender: TObject; Button: TMouseButton;18       Shift: TShiftState; X, Y: Integer);19   private20     {
     Private declarations }21   public22     {
     Public declarations }23   end;24 25 var26   Form1: TForm1;27   Prior,Origin:TPoint;{Oringin用于记录起始位置也就是鼠标按下的位置，Prior用于记录最后的位置也就是放开鼠标的位置}28   IsDown:boolean=false;{用于判断鼠标是否仍处于按下的状态}29 30 implementation31 32 {
    $R *.dfm}33 34 procedure TForm1.Image1MouseDown(Sender: TObject; Button: TMouseButton;35   Shift: TShiftState; X, Y: Integer);36 begin37   IsDown:=true;38   Image1.Canvas.Moveto(x,y);39   Origin:=Point(x,y);40   Prior:=Point(x,y){记录下按下时的位置，此时Origin和Prior重合}41 end;42 43 procedure TForm1.Image1MouseMove(Sender: TObject; Shift: TShiftState; X,44   Y: Integer);45 46 var47   pX,pY:integer;48 begin49   Form1.DoubleBuffered:=true;50   if isDown then51   begin52     pX:=Prior.X;53     pY:=prior.Y;{着两个值用于保存上一次的位置}54     Prior:=Point(x,y);55     Image1.Canvas.MoveTo(Origin.x,origin.Y);56     Image1.Canvas.Lineto(Prior.X,Prior.Y);{显示现在的线}57     Image1.Canvas.Pen.Mode:=pmNotXor;{通过异或操作删掉上一条线}58     Image1.Canvas.MoveTo(Origin.x,origin.Y); 59     Image1.Canvas.Lineto(pX,pY);60 61 62   end;63 64 end;65 66 procedure TForm1.Image1MouseUp(Sender: TObject; Button: TMouseButton;67   Shift: TShiftState; X, Y: Integer);68 begin69   IsDown:=false ;70   Image1.Canvas.Pen.Mode:=pmCopy;71    72     Image1.Canvas.MoveTo(Origin.x,origin.Y);73     Image1.Canvas.Lineto(Prior.X,Prior.Y); {放开鼠标，画出最后的线}74 75 end;76 77 end.