Bouncing Ball

unit BallU;

interface

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

type
TForm1 = class(TForm)
Timer1: TTimer;
procedure FormCreate(Sender: TObject);
procedure FormResize(Sender: TObject);
procedure FormDestroy(Sender: TObject);
procedure Timer1Timer(Sender: TObject);
procedure FormPaint(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;

var
Form1: TForm1;
screen1: tbitmap; // a bitmap used to buffer the drawing of the ball
ballPos: tpoint; // the coordinates of the top, left corner of the ball
ballMotion: tpoint; // the change in position each interval of the timer

const BallDiameter=20; // this is obvious

implementation

{$R *.DFM}

procedure DrawScreen; // output the ball drawing to the screen
begin
with screen1.canvas do // in the followind block it is assumed that screen1.canvas is typed before each line
begin
brush.color:=clBlack; // background colour
fillrect(form1.clientrect); // fill a rectangle over the entire background
brush.color:=clwhite; // set the ball's colour
ellipse(ballpos.x,ballpos.y,ballpos.x+BallDiameter,ballpos.y+BallDiameter);
// draw the ball as a circle with a top left corner of ballpos and a bottom right corner of
// the ball's position plus the diameter of the ball
end;
form1.canvas.draw(0,0,screen1);
// draw the screen bitmap on the form
end;

procedure TForm1.FormCreate(Sender: TObject);
begin // called once when the program is first run
screen1:=tbitmap.create; // create a bitmap to use for buffering and prevent flicker
BallPos:=Point(100,100); // the top, left point of the ball
BallMotion:=Point(10,10); // initial motion of the ball
end;

procedure TForm1.FormResize(Sender: TObject);
begin // this is called whenever the window is resized
screen1.height:=clientheight;
screen1.width:=clientwidth;
// adjust the dimensions of the bitmap to the dimensions of the form
end;

procedure TForm1.FormDestroy(Sender: TObject);
begin // this is called once when the program is closing
screen1.free; // free the bitmap so it doesn't continue occupying memory after the program closes
end;

procedure TForm1.Timer1Timer(Sender: TObject);
begin // this is called every 1/30 seconds or so
BallPos.x:=BallPos.x+BallMotion.x;
BallPos.y:=BallPos.y+BallMotion.y;
// update the position of the ball
if (BallPos.x>ClientWidth-BallDiameter) then // the ball is going off the left side of the window
BallMotion.x:=-Abs(BallMotion.x)
// ensure that the next moving is going to move the ball left
else if BallPos.x<0 then // the ball is going off the right side of the window
BallMotion.x:=Abs(BallMotion.x);
if (BallPos.y>ClientHeight-BallDiameter) then // ... off the bottom of the screen
BallMotion.y:=-Abs(BallMotion.y)
// ensure that the next moving is going to move the ball left
else if BallPos.y<0 then
BallMotion.y:=Abs(BallMotion.y); // ... off the bottom of the screen

// The abs function always returns a positive value.
// if the sign is negative, it returns positive.
// if the sign is positive, nothing changes.
// The rest of the number is not changed.
DrawScreen; // output the changes in ball position to the screen
end;

procedure TForm1.FormPaint(Sender: TObject);
begin // This is called when the form has to be repainted.
// ie. a window was moved infront of it and this window must be painted
DrawScreen; // update the screen
end;

end.

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