I've tested your code in Squeak and found another bug (I should have been more careful when I read your code. Shame on me.)

In class Ball you have the methods
	position: aPoint
		super position: aPoint.
This does not set a value to your position variable in class Ball, but actually sets the upper-left corner of the bounds.
You should have said position := aPoint.
--------------------------------	 
The following now works in mySqueak image:
EllipseMorph subclass: #Ball
	instanceVariableNames: 'position'
	classVariableNames: ''
	poolDictionaries: '' "this part is needed in Squeak"
	category: 'ZZZtesting'

drawOn: aCanvas
	self borderWidth:10; borderColor: Color green.
	super drawOn: aCanvas.

position
	^ position.

position: aPoint
	position := aPoint.
I then execute the following in a workspace
��� | aBall |
��� aBall := Ball new.
��� aBall position: ( 10@10 ).
��� aBall openInWorld.

and it seems to work. But closer scrutiny shows that it ignores your instance variable (position). when I move the ball around.
The reason is that EllipseMorph� is a superclass of your Ball and cannot possibly know about an instance variable (position) that is defined in the subclass (Ball).

I ignored Ball>position since the EllipseMorph doesn't use it:

EllipseMorph subclass: #Ball
��� instanceVariableNames: ''
��� classVariableNames: ''
��� poolDictionaries: ''
��� category: 'ZZZtesting'

��� position
��� ��� ^ super position.

��� position: aPoint
�� ��� super position: aPoint.
This looks better.
I you really want to replace the EllipseMorph>position you must go deeper into how the EllipseMorph handles its position and bounds . This will not be trivial. Much easier to invent a new instance variable and use it in the drawOn: method.

Good luck with your apprenticeship









On 05.02.2015 11:56, nacho wrote:
@Trygve
I've checked and it doesn't work. I can't even bring the halo on to delete
the morph. The debugger keeps popping.
Thanks anyway to all
Nacho




-----
Nacho
Smalltalker apprentice.
Buenos Aires, Argentina.
--
View this message in context: http://forum.world.st/Question-on-Morphic-drawOn-method-tp4803695p4803895.html
Sent from the Pharo Smalltalk Users mailing list archive at Nabble.com.

--

The essence of object orientation is that objects collaborate� to achieve a goal.
Trygve Reenskaug������mailto: trygver@ifi.uio.no
Morgedalsvn. 5A�������http://folk.uio.no/trygver/
N-0378 Oslo�������������http://fullOO.info
Norway���������������������Tel: (+47) 22 49 57 27