Drupal 7 Module Development

Drupal’s Theme Layer
This book is mostly focused on building modules that encapsulate discrete chunks
of functionality. However, since we're ultimately building a web application,
everything outputted by your functionality will need to be marked up with HTML.
Drupal calls the process of wrapping your data in HTML and CSS as theming.
For the next two chapters, we will discuss how your module should integrate
with the theme layer. Chapter 3 will talk about the architecture of the system,
theme functions, templates, render elements, and the theme registry. Chapter 4
will use these newly acquired concepts to integrate an example module with
the theming layer.
Business logic versus presentation logic
So what would be the best way to get our data and functionality marked up? Do we
simply wrap each piece of data in HTML and return the whole as a giant string? Like
the following example:
return '' . $data . '';
Fortunately, we don't. Like all other well-designed applications, Drupal separates its
business logic from its presentation logic. Traditionally, the primary motivations for
this separation of concerns are as follows:
1. To make the code easier to maintain.
2. To make it possible to easily swap out one layer's implementation without
having to re-write the other layers.
As we shall see, Drupal takes the "swap-ability" aspect to the extreme.
As we mentioned in the introduction of this chapter, the default theme selected on
the Appearance page is the most obvious part of the theme layer. Also, you might
think that the theme is responsible for applying the HTML and CSS for the website.
However, there are thousands of contributed modules on drupal.org. Should the
theme be responsible for marking up all of those modules' data? Obviously not.
Since a module is most intimately familiar with its own data and functionality,
it's the module's responsibility to provide the default theme implementation.
As long as the module uses the theme system properly, a theme will be able to
override any HTML and CSS by hot-swapping its own implementation for the
module's implementation.
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