Saturday, May 22, 2010

Templates and inheritance

In every web site project I have been working on there inevitably came that moment that someone wanted to have a copy of the site with some small changes, or that someone wanted to use a piece the website in something else. In both cases it was all about (initially) some small changes, what they call as skin or a branded site. With programming part we know what to do then - we design base classes for the core undifferentiated functionality and inherit from them in the targeted classes. Most templating engines do have some notion of search path - so this should be doable with templates - but it always proves to be much more messy than it initially seems to be. I'll try to catalogue here all the use cases and how they compare to the standard inheritance in object oriented programming languages and describe them in terms of operations on the search path.

Initial setup

In 'normal' programming languages the atomic piece that can be inherited and overloaded and that is searched on the inheritance path is a method - in case of templates the atomic part is a whole template. The granularity is defined differently, but we can do analogues things with both. Here I will concentrate on the dispatching model familiar for users of most of the MVC frameworks with controllers and their methods as the endpoints (called actions). In this setting all the methods could have a matching template file and inheriting and overriding a template could work on the same level as inheriting and overriding an action.

Basic case

Lets say I have a controller 'MyApp::Controller::Book' with a few actions like 'view', 'list', 'edit' (a CRUD controller) and matching templates in 'templates/Book'. I would like that when calling the render from that controller I did not need to specify the full path of the templates - but just call them by name 'view'. Also when they have a common subcomponents in the same directory they should be able to [% INCLUDE date_formatting %] them by name. This is just like calling methods in a class and it can be accomplished by adding the 'templates/Book' directory at the start of the search path.

Library component

Sometimes we would like to use ready made components from some published library, maybe a CRUD controller for our Books. We inherit from the library class in our 'MyApp::Controller::Book' and we would like its templates to be inherited in a similar way. That means adding '/usr/lib/something/something/CRUD/templates' to our search path somewhere after 'templates/Book'.

Subclassing our own controllers

If our 'MyApp::Controller::Book' is a subclass of our own ''MyApp::Controller::Item' class - this is a very similar case to the above. Here as well we want to insert the search path of the parent class somewhere after 'templates/Book. But in this case what we insert is 'template/Item' - a path relative to our application root.

Application wide components

This is solved simply by adding an application wide element in the search path. It would be analogues to adding stuff to a common base class in standard programming - much more rare there.

Subclassing a whole site

This is the case of 'branded' sites where we would like to subclass everything at once. It is maybe the most common case - but also the most difficult because there are no good analogies/solutions in standard object oriented programming. If 'MyNewApp' inherits from 'MyApp' that does not automatically mean that 'MyNewApp::Controller::Book' springs to existence and can serve book information in the new app (but I've heard there are existing Object Oriented languages that work like that and some people are thinking about implementing this in Perl). So I think we need to first think over how this is handled in the main framework and then we can deduce something about how to treat templates from that. For the limited case of application wide components this can be solved by adding the new template root to the beginning of the search path - but it gets more complicated for all the other cases described above.


I mostly wrote that to have it all in one place - but I think it does show the need to make the template search path dependent on the controller used.

1 comment:

Anonymous said...

A different way to tackle these problems is to look at the resulting html and change that the way you like it. Take a look at Genshi, e.g., which is a templating system to do just that. (I really hate this thing, but it is a possibility).