Hi Mathieu,

I've been thinking about how to respond to your question. My first thought was to jump right in and help, but since this is a home work assignment it is probably best to help in a round about way. What I didn't want to do was to discourage others that are working on homework from asking questions.

So in order to help you I propose the following. I will help to explain the patterns you mentioned, and I will critique your solution, all without helping you solve the problem you are trying to solve, so that you can benefit from doing and learning from your homework assignment.

First the critique: I really don't like your implementation of using a dictionary to handle your collection. It seems to me that you are trying to accomplish something but I don't see what the benefit of that implementation is. Can you tell me what functionality the dictionary adds? What is the benefit of the dictionary over a simple collection?

The visitor pattern is a very useful and translates into passing an object (the visitor) around. The idea here is that the functionality that is needed can be written into the visitor and that way functionality can be added without modifying the objects that it visits. Some people do modify the visited objects to double dispatch methods back to the visitor in order to control which methods are called on the visitor.

So examples are usually good so let's try a visitor example. Say we have a sales tax rate for different states. We would like to build a simple sales tax object that is able to calculate the taxes that need to be paid for each product that is sold. The tax rules for each state are encoded in visitor objects but the visitor is passed to the product to calculate the tax. Each product may have a different requirement for taxing, say food has one tax rate and regular goods have another and luxury items have another tax.

This could be accomplished without modifying the object if each object could respond to product type in which case our tax object could be called by passing it around.

ShoppingBag>>calcTax "return to the sender the calculated tax for the contents of the shopping bag for the shopping bags state" aState := self myState. "find the appropriate visitor " aVisitor := StateTax forState: aState. ^aVisotor calcTax: self. StateTax>>calcTax: aShoppingBag "return to the sender the calculated tax of all items in aShoppingBag" ^aShoppingBag contents inject: Float zero into: [:aSum :anItem | aSum := aSum + (self taxFor: anItem) ]. StateTax>>taxFor: anItem "return the tax for anItem type" anItem taxType isNoTaxProduct "food" ifTure: [^Float zero]. taxRate := (anItem taxType isLuxuryTaxProduct) ifTrue: [self luxuryTaxRate] ifFalse: [self regularTaxRate]. ^anItem baseCost * taxRate

Ok this is not really the best way to do thing since it would be better to have a double dispatch way to respond to taxRate. Instead by modifying the visited objects with an accept type method we can send the method back to the visitor. Product>>taxRate: aVisitor "this is an accept method" "return the tax rate for this product type" ^aVisitor regularTaxRate Product subclass: LuxuryProduct>>taxRate: aVisitor "return the tax rate for this product type" ^aVisitor luxuryTaxRate Product subclass: TaxFreeProduct>>taxRate: aVisitor "return the tax rate for this product type" ^Float zero

This changes the visitor method taxFor: to the following StateTax>>taxFor: anItem "return the tax for anItem type" ^anItem baseCost * (anItem taxRate: self)

You can see that each subclass of StateTax now simply has to respond to the three tax rates.

To summarize you can use the visitor method to add functionality to an object without modifying it or by adding a double dispatch modification the functionality stays with the new visitor object but is controlled by the visited objects.

This is particularly useful for printing!

The example assumes that you can create a method to find the proper StateTax subclass to use as a visitor, and a factory method to create products of the proper subclass of Product.

I hope that helps with visitor, if not please feel free to ask questions.

Happy coding!!

Ron Teitelbaum President / Principal Software Engineer US Medical Record Specialists Ron@USMedRec.com

From: math Sent: Thursday, May 25, 2006 10:22 AM

...

Subject: Design with Visitor From: "Mathieu SUEN" mathk.sue@gmail.com Date: Wed, 24 May 2006 23:14:58 +0200 To: beginners-bounces@lists.squeakfoundation.org

Hi squeakers,

I have to build a lite code for home work. The objective is to gave to user a possibility to store his sport performance.

So I have design a Class Performance(with PerformanceCollection) and I wanted to let things like TimeLaps, Date, Distance, and so on. for a later implementation (if we wanted to add others).

So I provide a Measure abstract class.

Hence I add a dictionary in my Performance with the class for key and the instance for value:

for example:

Performance>>addMeasure: aMeasure dictionaryOfMesure at: aMeasure class put: aMeasure

Now I have to bring them in a Seaside UI so I guess that Visitor Pattern ( ;-) ) can help me but my complex structure mess my brain. So I ask you if you think that can be possible(and how) or other pattern can be more suitable for this problem.

Thank you for your attention (: _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners

Thinking forward I see that 2 Performance instance couldn't be compare so befor asking how to print them I will think about a other structure. I think I will use Builder or Factory Method pattern.

Thanks _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners

Pay attention mathieu. Design patterns can be evil too. You do not really need a Visitor for your application. Your performance classes are your domain and your seaside components your UI pointing to the domain objects.

Why do you need a PerfomanceCollection class?

Why do you want to keep in your dictionary the class of its objects?

See you monday :)

Stef On 25 mai 06, at 16:22, math wrote:

...

Subject: Design with Visitor From: "Mathieu SUEN" mathk.sue@gmail.com Date: Wed, 24 May 2006 23:14:58 +0200 To: beginners-bounces@lists.squeakfoundation.org

Hi squeakers,

I have to build a lite code for home work. The objective is to gave to user a possibility to store his sport performance.

So I have design a Class Performance(with PerformanceCollection) and I wanted to let things like TimeLaps, Date, Distance, and so on. for a later implementation (if we wanted to add others).

So I provide a Measure abstract class.

Hence I add a dictionary in my Performance with the class for key and the instance for value:

for example:

Performance>>addMeasure: aMeasure dictionaryOfMesure at: aMeasure class put: aMeasure

Now I have to bring them in a Seaside UI so I guess that Visitor Pattern ( ;-) ) can help me but my complex structure mess my brain. So I ask you if you think that can be possible(and how) or other pattern can be more suitable for this problem.

Thank you for your attention (: _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners

Thinking forward I see that 2 Performance instance couldn't be compare so befor asking how to print them I will think about a other structure. I think I will use Builder or Factory Method pattern.

Thanks _______________________________________________ Beginners mailing list Beginners@lists.squeakfoundation.org http://lists.squeakfoundation.org/mailman/listinfo/beginners