Magento :: Layout in Module

Is it possible to create a layout file inside of a module ? How ?

For what:
I want to add a some kind of statistics hit counter for products, and I don't want to override the products class, as that is already done by some module I'm using. Thus I thought it would be best to have a custom module with a block that would be called by a layout statement. Of course I could easily edit my private local.xml or make changes to another layout-xml in the layout folder of my theme, but I want this feature to be available in all themes (independent of any selected theme).

Some constraints:

All code in one single module
... so that it is theme independent
... so that the module can be shared with others without them having to change anything (like theme files), so that the install/load of my module would be enough

I would also accept different approaches for my statistics hit counter loading (using the same constraints)

Solution

Yes it is possible. Just create your layout xml file in the following path: /design/frontend/default/default/layout/yourlayout.xml(or whatever your theme name is), and add a proper statement in your modules etc/config.xml:

<config> 
 <frontend>   
  <layout>
    <updates>
      <yourmoduleshortname>
         <file>yourlayout.xml</file>
      <yourmoduleshortname>
    </updates>
  </layout>  
 </frontend>
</config>

This sample is for frontend user, but adminhtml layouts can be updated in a similar manner. If something doesn't work, be sure to check if your layout is in the proper theme/package directory.

Edit:

Second approach:

You can use a controller of your own, which will extend the core functionality (one of the catalog controllers) - just rewrite it (or just product view action). Inside its action method add something like this:

$thiss->getLayout()->createBlock('namespacename/block','layout-block-name',
array('template' => 'relativepathtotemplate.phtml'));
$this->getLayout()->getBlock('content')->append($block);
run-original-parent-code();

Third approach:

Similar to the previous one, but you can use some event observer, and try Mage::getSingleton('core/layout'), and inject your block there. Not in all events the layout will be already available (try the post_dispatch family).

I don't really recommend the second and third approach, because if someone else wants to find where this 'magic' block comes from, it will most surely look int app/design/(...) directory. Finding it in your controller or model, may be very tricky...

If you don't want to display your statistic counter, you can also use events (like post_dispatch) to count the controller dispatches. Just create an observer attached to it, and store your data in the DB.