Some Delphi errors - delphi

Question

I try to make class Ball which should be in Unit and then I need to draw Ball on form with using Canvas. Actually I never trying OOP in Delphi before (all I rember is simple exercises in school in Pascal) so I got many problems. Oh.
So, here the code
unit with Ball class
unit Unit2;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ExtCtrls;
type
MyPoint = record
x, y: integer;
end;
Ball = class
Pos:MyPoint;
Vel:MyPoint;
Rad:integer;
Can:TCanvas;
procedure BallCreate(crd, spd:MyPoint; Sender: TObject);
procedure BallDraw(Sender: TObject);
procedure BallMove();
private
{ Private declarations }
public
{ Public declarations }
end;
var
posX, posY, speedX, speedY, radius:Integer;
implementation
procedure Ball.BallMove;
begin
if((posX + radius > 700) or (posX - radius < 0)) then speedX:= (-speedX);
if((posY + radius > 500) or (posY - radius < 0)) then speedY:= (-speedY);
posX:=posX+speedX;
posY:=posY+speedY;
end;
procedure Ball.BallCreate(crd, spd:MyPoint; Sender: TObject);
begin
Vel.x:=3;
Vel.y:=3;
pos.X:=crd.x;
pos.Y:=crd.y;
radius:=30;
end;
procedure Ball.BallDraw(Sender: TObject);
begin
with Can do
begin
brush.Style:=bsSolid;
brush.Color:=clRed;
ellipse((pos.X-radius),(pos.Y-radius),(pos.X+radius),(pos.Y+radius));
end;
end;
end.
unit with Form
unit Unit1;
interface
uses
Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
Dialogs, StdCtrls, ExtCtrls, Unit2;
type
TForm1 = class(TForm)
Timer1: TTimer;
Button1: TButton;
procedure FormCreate(Sender: TObject);
procedure Button1Click(Sender: TObject);
procedure Timer1Timer(Sender: TObject);
private
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
x1,y1,x2,y2,x,y:integer;
posX, posY, speedX, speedY, radius:Integer;
f:boolean;
obj:Ball;
p:MyPoint;
s:MyPoint;
implementation
{$R *.dfm}
{procedure TForm1.BallMove;
begin
if((posX + radius > ClientWidth) or (posX - radius < 0)) then speedX:= (-speedX);
if((posY + radius > ClientHeight) or (posY - radius < 0)) then speedY:= (-speedY);
posX:=posX+speedX;
posY:=posY+speedY;
end; }
procedure TForm1.FormCreate(Sender: TObject);
begin
Timer1.Enabled:=false;
Timer1.Interval:=5;
p.x:= Round(ClientWidth/2);
p.y:= Round(ClientHeight/2);
s.y:=3;
s.x:=s.y;
obj.BallCreate(p,s,Sender);
end;
procedure TForm1.Button1Click(Sender: TObject);
begin
if not f then
begin
Timer1.Enabled:=true;
Button1.Caption:='Ñòîï';
f:=not f;
end
else
begin
Timer1.Enabled:=false;
Button1.Caption:='Ïóñê';
f:=not f;
end;
end;
procedure TForm1.Timer1Timer(Sender: TObject);
begin
obj.BallDraw(Sender);
obj.BallMove;
end;
end.
When I try to Run it it says that
raised exception class EAccessViolation with message 'Access violation at address 0044DE7B in module Project1.exe. Write of address 000000C'
and in the code those strokes are highlighted red
Vel.x:=3;
and
with Can do
I don't understand whats wrong and how i sholud declare and use Canvas here properly. Maybe you've got some examples with OOP stuff in units with Canvas in Delphi?

Answer 1

You declared a Can:TCanvas; variable but it's not created anywhere.
You can use the Main form canvas, for that you should pass it to Ball for exmaple in the Ball constructor like:
TBall = class
...
public
constructor Create(crd, spd:MyPoint; ACanvas:TCanvas);
....
implementation
...
constructor TBall.Create(crd, spd:MyPoint; ACanvas:TCanvas);
begin
Can := ACavas;
...
Then, you are not properly creating and instance of Ball:
obj.BallCreate(p,s,Sender);
to create an instance you have to call the class constructor like
obj := TBall.Create(crd, spd, Self.Canvas);
By the way the "T" before Ball is just a convention to name a class in Delphi