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From Sylvain Wallez <sylv...@apache.org>
Subject Re: [RT] Attribute Rendering and CForms Convertors
Date Thu, 04 Nov 2004 14:27:46 GMT
Daniel Fagerstrom wrote:

> Bart Molenkamp wrote:
>
>> I've also been thinking about a simple method for displaying object
>>
>> instances of different classes. E.g. I get an object from the flow
>> layer, I need to decide how to format it. Instances of class "A" are
>> formatted differently than instances from class "B". Now, this could be
>> done using <jx:choose>, but that doesn't make the code more readable:
>>
>> <!-- is object instanceof MyClass -->
>> <jx:when test="object.getClass().getName() == 'com.MyClass'">
>>
>> Even worse when you want to check if it is instanceof an interface:
>>
>> <!-- is object instanceof MyInterface -->
>> <jx:when
>> test="${java.lang.Class.forName('com.MyInterface').isAssignableFrom(
>> object.getClass())}">
>>  
>>
> I don't know your use cases, so maybe not the following is applicable 
> at all, anyway:
>
> In the (often cited on this list) article: Enforcing Strict Model View 
> Separation in Template Engines, by Terence Parr 
> http://www.cs.usfca.edu/~parrt/papers/mvc.templates.pdf, a number of 
> rules for strict separation between model and view are given. One of 
> them is that the view shouldn't make any assumptions about data types 
> of the model data. The view should only get displayable strings.


I don't agree with this rule as, in order to avoid the view to know 
about the model, it requires the model to know about the view. IMO, the 
view should be given the necessary pointers to access the model data it 
needs. The granularity of these pointers is the key point that has to be 
chosen wisely.

Your convertor proposal below can be a good way for the controller not 
to know too much about the view needs and for the view not to depend too 
much on the implementation details of the model.

> If one start to depend on data types as in your code above, the 
> template files must be supported by programmers rather than web 
> designers. And the separation of concern between view and model start 
> to get entangeled. Ok, knowing what Terence Parr _don't_ want us to do 
> is not very helpfull ;)
>
> As a solution to the problem he sugests MVCR 
> (Model-View-Controller-Renderer), where the renderer is responsible 
> for converting (Java) data types to displayable strings. The renderer 
> can be responsible for localization of numbers etc as well. The 
> renderer is an extra step between model and view.
>
> The simplest possible renderer is to just implement toString() in the 
> classes one is going to access in the view. But a better SoC is to 
> have a separate rendering component. In this case the object from the 
> model is first accessed by an expression in a suitable expression 
> language and then the object is rendered to a displayable string by 
> the rendering component and at last the displayable string is emited 
> by the template engine.
>
> So, how would the rendering component work? I think that we basically 
> could reuse and extend the Convertor idea from CForms. The renderer 
> check the type of the input object (and possibly the locale) and 
> applies the apropriate convertor. The coupling between object type and 
> convertor is decribed in a configuration file, much as in the form 
> definition in CForms. It should also be easy to use custom converters, 
> for own data types.
>
> Typically one can reuse the same rendering configuration file for all 
> templates.
>
>                                                           ---o0o---
>
> I think the renderer component idea could give a better SoC in CForms 
> as well. I have had a feeling that data type convertion is not a 
> natural part of the model (form definition) for a while. A problem is 
> that conversion configuration must be repeated each time the data type 
> is used.


The repeated configuration is a problem that will be solved once we have 
the ability to define widget types and repositories [1].

> Ok, the idea is as follows: we have a converter component, that is 
> like the renderer component above, but bidirectional. I.e. both 
> rendering: data type -> displayable string and input conversion: input 
> string -> data type. The converter is configured in the same way as 
> described for the renderer above.
>
> The convertor is used as a step between the form instance and the view 
> for rendering the data types. And between the request object and the 
> form instance during the population of the form instance.
>
> Typically only one convertor configuration is needed for all ones 
> forms. And we could distribute a localized default convertor 
> configuration with Cocoon.
>
>                                                            ---o0o---
>
> So, the basic idea is to factor out the type conversion functionallity 
> from CForms and make it available for all rendering and input handling 
> needs in Cocoon.


This is an interesting idea, which could be even more useful if the 
convertors were able not only to produce strings, but also XML snippets 
for output-only data. For example, we may want an email address to be 
rendered as a <a href="mailto.."> or negative numbers being displayed in 
red, or enclosed in parenthesis.

What that means is that a convertor should be able to do the following 
conversions:
- string+locale --> object : parsing request parameters
- object+locale --> string : producing the value of an input
- object+locale+channel --> xml fragment : producing the output view for 
a particular channel (html, wap, fo, etc).

WDYT?

Sylvain

[1] http://wiki.apache.org/cocoon/WoodyScratchpad

-- 
Sylvain Wallez                                  Anyware Technologies
http://www.apache.org/~sylvain           http://www.anyware-tech.com
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