[Newbies] dependency mechanism

[Shawn MacIntyre]

I was reading Smalltalk by Example yesterday and I had the exact same  
problem as Chris.

I did some exploring of Object's dependency protocol and rewrote the  
methods like this:

MyStock>>price: aPrice
	price := aPrice.
	self changed: #price with: self

MyStockWatcher>>update: aSymbol with: aStock
	Transcript cr; print: thisContext; tab; print: aSymbol; tab.
	aSymbol == #price ifTrue:
		[ Transcript
			nextPutAll: 'Price changed to: '; print: aStock price].
	Transcript flush.

This works like a charm. But I am passing the parent rather then just  
the old value because I don't want to store the parent as an instance  
variable. Is that a bad thing to do? I'm not sure ... still learning  
right from wrong. :) I don't like it as much as what is in the book  
because I don't feel I should have to store either the parent or the  
old value in an instance variable... it seems less elegant.

Seeing Hilaire's explication of the the events protocol, I read  
through the events protocol and rewrote the methods as such:

MyStock>>price: aPrice
	price := aPrice.
	self triggerEvent: #price with: self

MyStockWatcher>>initialize: aStock
	aStock when: #price send: #updatePrice: to: self

MyStockWatcher>>updatePrice: aStock
	Transcript
		cr; print: thisContext; tab;
		nextPutAll: 'Price changed to: ';
		print: aStock price;
		flush.

This works; but, I am still not passing the old value as well. For  
anyone reading this: any comments on style for doing it this way?

Thanks!

On May 31, 2006, at 14:18, Hilaire Fernandes wrote:

> What about using the methods: #triggerEvent:with: and #when:send:to:
>
> obj when: #price send: #updatePrice: to: anObject2
>
> obj triggerEvent: #price with: oldPrice
>
> where
>
> Object2>>updatePrice: aPrice
>
> Hilaire
>
> Chris Kassopulo a écrit :
>> Greetings,
>>
>> The basic dependency example from "Smalltalk By Example" uses
>> Visualworks changed:with and update:with:from: methods like this:
>>
>> price: aPrice
>> | oldPrice |
>> oldPrice := self price.
>> price := aPrice.
>> self changed: #price with: oldPrice
>>
>> update: aSymbol with: oldPrice from: aStock
>>     (aSymbol == #price)  ifTrue:
>>         [Transcript
>>             cr; show: thisContext; tab; show: aSymbol; tab;
>>             nextPutAll: 'Price changed from: '; show: oldPrice; tab;
>>             nextPutAll: 'To: '; show: aStock price; flush]
>>
>> The problem is that Squeak only implements update:with: so the
>> aStock price messaging doesn't work.  My solution was to follow
>> VW's implementation by making the following changes:
>>
>> Modified the changed:with: method to include from:.
>>
>> changed: anAspect with: anObject
>> self dependents do: [:aDependent | aDependent update: anAspect with:
>> anObject from: self]
>>
>> Added an update:with:from: method.
>>
>> update: anAspect with: anObject from: aSender
>> ^ self update: anAspect with: anObject
>>
>> It took a while to see the problem and without looking at VW,
>> I never would have been able to figure a way.  Is my fix
>> reasonable?
>>
>> VW also implements expressInterestIn:for:sendBack: and
>> onChangeSend:to: methods.  Anyone familiar with them and care to
>> provide pointers on the possibility of implementing something
>> similar in Squeak?
>>
>> Thanks,
>> Chris
>>
>>
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>>
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--
Shawn MacIntyre
Edmonton, Alberta, Canada