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From Sylvain Wallez <sylv...@apache.org>
Subject Re: [RFC] "Cocoon Templates": Name and Tag Interface
Date Thu, 02 Dec 2004 11:13:39 GMT
Daniel Fagerstrom wrote:

> As you probably have noticed, we are working on a replacement on JXTG.
>
> The goal is that it, as far as it is desirable, should be back 
> compatible with JXTG. But instead of being implemented as a huge (and 
> somewhat scary) monolith, it will be implemented as a taglib 
> framework. By implementing it that way it will be much more open for 
> further development and the goal is to implement taglibs similar to 
> ESQL etc so that it can work as an XSP replacement as well. It should 
> be at least as performant as JXTG.
>
> Jonas has submitted an initial implementation based on the design 
> discussions on the list, that Leszek is going to commit.
>
>                              ---o0o---
>
> We need your input on chosing a name for the framework. And also on 
> what data that should be available for the taglibs and in what way the 
> tags should be given access to this data. The design of the tag 
> "interface" will be an important question for you when you are going 
> to implement tags in the future. And it would also be good if we could 
> agree about the main direction of the tag design quite soon, so that 
> we can start the development of taglibs in parallel with the framework 
> development. Please spend some brain cycles on this.
>
>
> Naming
> ======
>
> The goal is that the new template framework should replace JXTG and 
> XSP as the "recomended" template system for Cocoon users. It is a 
> community effort rather than a one man show. It is based on our 
> previous experince of templating solutions.
>
> This should IMO be reflected in the naming. The name should reflect 
> the intended purpose rather than being a "brand" name. "JX" in 
> JXTemplateGenerator comes from JXPath and as we are going to provide 
> support for other expression languages as well, JXTG 2.0 would not be 
> a logical name for the framework IMHO.
> I would propose that we (analogous with CForms) refer to it as 
> "template" in block name, package name, name spaces and so on. When it 
> actually delivers what is promised, we can refer to it as Cocoon 
> Template or CTemplate.
>
> To show that we care about our users previous investments in Cocoon 
> technology, we can say that JXTG has been re-implemented as a Cocoon 
> Template tag lib.
>
> Of course other and better naming sugestions are most welcome.
>
>
> Tag "Interface"
> ===============
>
> What data should be available for the taglib writer and how should it 
> be made accessible? What about thread safety?
>
> Expressions
> -----------
>
> This is somewhat OT in the current context but IMO expressions, (the 
> stuff inside ${}), should only have access to FOM and preferably just 
> read access. IMHO expressions should not have side effects.


FOM sounds ok, or at least some "cocoon" object giving access to flow 
view data (if any, as it can be used outside of flowscript), request, 
session, etc.

Expressions should have no side effects although this is difficult to 
enforce strictly as the EL must be able to call a method and we cannot 
control what happens within the method.

> What Data in Tags?
> ------------------
>
> A tag need access to:
>
> * FOM
> * The current XMLConsumer for output
> * The content of its attributes
> * The content of its body (the possibility of executing the body and 
> geting the result)


+1 The two last items are especially important to implement the CForms 
template language (see jx-macros.xml).

> The following data is also useful for some tag libs:
>
> * A variable stack for handling local variables in tags or macros
> * The tag repository
> * The source resolver


That's not needed, as it can be looked up in the ServiceManager.

> * The component manager (e.g. for geting a DB connection)
> * Its parent tag
> * The content of its body (the possiblity to get the body unevaluated)


+1

> IMHO tags should behave as if they are side effect free. Side efects 
> should be performed in flowscripts. From this we can infer that we 
> only should have read access to the above data. But in practice it 
> might be hard to enforce and furthermore even if the tag behave as it 
> is side effect free, it might be better (or the only possiblity) to 
> implement it in terms of side effects.
>
> Implementation details are discussed in the section "Adding Executable 
> Tags" in 
> http://marc.theaimsgroup.com/?l=xml-cocoon-dev&m=110132582421156&w=2.
>
> Have I forgotten something or should we remove something?
>
> How to Make Data Accessible in Tags
> -----------------------------------
>
> There are two main type of events in the life cycle of a tag: setup 
> and execution. For a thread safe tag, the setup phase can be divided 
> in: script compile time setup and execution time setup.
>
> The relation between the tag and the framework has many parallels with 
> the relation between a component and a component manager, so similar 
> solutions are possible.
>
> We can have reflection based solution with setter injection of 
> attributes and possibly the rest of data also. We can have interface 
> driven injection, (Avalon framework style), where the tag implements 
> various interfaces for geting the various input data types.
>
> We can also have more "static" solutions where the tag has to 
> implement a certain interface and get all the data with methods in the 
> interface or in one large context object.
>
> Reflection based methods are more flexible, but should probably only 
> be performed during script compile time setup, due to keep good 
> performance.
>
> See the above reference for examples and more details.
>
> Thread Safe or Not?
> -------------------
>
> Thread safe tags can have better performance as more work can be done 
> during script compile time, but they might be harder to write. What 
> should we chose or should we support both types?


I don't see how we can possibly cache precompiled templates if some tag 
instances are not threadsafe. So IMO *all* tags should be threadsafe.

If we make the parallel with the TreeProcessor, tags should receive 
something similar to InvokeContext. This is an object that holds all 
execution-time data such as the generator's <map:parameter>, the object 
model (or maybe not as it's available through the Context) and the 
variable stack.

For some tags to be able to store execution-time data, the invocation 
object should also be able to hold some arbitrary attributes, possibly 
associated to the tag instance itself. That provides support for both 
tag-related data (e.g. some data created on start that is needed on end 
such as SQL connections) or inter-tag communication such as <ft:class> 
definitions later used by <ft:new> tags.

Something like:
    void setAttribute(String name, Object value);
    Object getAttribute(String name);
    void setTagAtttribute(Tag tag, String name, Object value);
    Object getTagAttribute(Tag tag, String name);

Sylvain

-- 
Sylvain Wallez                                  Anyware Technologies
http://www.apache.org/~sylvain           http://www.anyware-tech.com
{ XML, Java, Cocoon, OpenSource }*{ Training, Consulting, Projects }


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