Because your drawOn: method doesn't draw anything and needs help from super.
It seems to me that you need to read more documentation :-)

On 05.02.2015 15:53, nacho wrote:

That work perfectly. What a silly mistake!!!

Of course it should have been position := aPoint.

Now, is it possible write the drawOn: method without calling super drawOn: a Canvas?

I don't completely get it why I should call super drawOn: aCanvas

thanks

nacho

Lic. Ignacio Sniechowski, MBA

Prosavic SRL

Tel: (011) 4542-6714

On Thu, Feb 5, 2015 at 11:28 AM, Trygve Reenskaug [via Smalltalk] <[hidden email]> wrote:

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.
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The essence of object orientation is that objects collaborate  to achieve a goal.
Trygve Reenskaug      [hidden email]
Morgedalsvn. 5A       http://folk.uio.no/trygver/
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Norway                     Tel: (+47) 22 49 57 27

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