Magento: How do you disable or change how a core observer method works

Question

I have been wondering about this for a while. What if Magento has written a core Observer class and performs a functionality that you do not want it to execute, or you want to rewrite it? Is there a way to say, don't use this method in an Observer, just use mine. If I setup a method for my own Observer won't it just do the core functionality first and then whatever I have implement?

For example, Magento wants to save some data to the database in an Observer method, but I don't want it to save that data at all, I want it to save some other data that I have added columns or attributes to the database for.

Answer 1

Standard caveat about class overrides being the last resort for implementing your own functionality

It's probably possible to fiddle with the loading of the Magento global config to strip out a core Magento Observer, but there's no supported way for doing it.

However, consider how the Core observers are configured.

<adminhtml>
    <events>
        <cms_wysiwyg_config_prepare>
            <observers>
                <widget_observer>
                    <class>widget/observer</class>
                    <method>prepareWidgetsPluginConfig</method>
                </widget_observer>
            </observers>
        </cms_wysiwyg_config_prepare>
    </events>
</adminhtml>

Observers are Magento Model classes. If a Model's been configured with the URI/path based syntax

<class>widget/observer</class>

as opposed to a full PHP class name

<class>Mage_Widget_Model_Observer</class>

you can create an override for the Observer Model class (just as you can for any other Model). If a PHP class name's been used, you're out of luck. (You could place a local file in local/Mage/Widget/Model/Observer.php if you're willing to take on the responsibility of maintenance, but I don't recommend it)

So, to override the above observer, you would

1. Create a custom module that include an override for the class Mage_Widget_Model_Observer.

2. In your override class, either redeclare prepareWidgetsPluginConfig to do what you want OR override the specific method. This could include a empty method to completely remove the functionality.

Answer 2

A better method of doing this is to simply re-declare the observer's definition in a config.xml file.

For example, I needed to disable the enterprise_persistent_cart observer which was declared on event controller_action_layout_generate_blocks_after by module Enterprise_Persistent.

The declaration in Enterprise_Persistent's config.xml file is such

<frontend>
    <events>
        <controller_action_layout_generate_blocks_after>
            <observers>
                <enterprise_persistent_cart>
                    <class>enterprise_persistent/observer</class>
                    <method>removeCartLink</method>
                </enterprise_persistent_cart>
            </observers>
       </controller_action_layout_generate_blocks_after>

So I created a module, and in my module's config.xml, I did the following:

<frontend>
    <events>
        <controller_action_layout_generate_blocks_after>
            <observers>
                <enterprise_persistent_cart>
                    <type>disabled</type>
                </enterprise_persistent_cart>
            </observers>
        </controller_action_layout_generate_blocks_after>

I also made my module depend on the Enterprise_Persistent module. This is necessary to make sure my module's config.xml is processed AFTER the Enterprise_Persistent module's config.xml. I did this by doing the following in my module's app/etc/modules My_Module.xml module declaration file

<config>
    <modules>
        <Atlex_AddCartLinkToHeader>
            <active>true</active>
            <codePool>local</codePool>
            <depends>
                <Enterprise_Persistent/>
            </depends>
        </Atlex_AddCartLinkToHeader>
    </modules>
</config>

For each event, there can only be one observer action declared for a given observer name. Therefore, as long as my module's config.xml is processed after the Enterprise_Persistent module's config.xml file, my observer declaration for enterprise_persistent_cart will be the observer action which is excuted.

The <type> disabled </type> node will disable the observer from firing. If you wanted to override the observer to execute your own method, then you would simply replace the <type> node with your observer's <class> and <method> nodes.

This allows you to override/disable observers without overriding core classes. It's more extendable for future developers.