plugin-observer-recipes

Skill: Plugin, Observer, and Preference Recipes

When to use

When a user asks to "intercept a method", "hook into X", "listen to an event", "extend class Y", or "modify the behavior of Z".

Decision guide

Goal	Mechanism	Reason
Modify method inputs or return value	Plugin (before/after/around)	Composable; multiple plugins can coexist
React to a business event	Observer	Decoupled; fire-and-forget
Replace the entire class	Preference	Last resort; use only when plugins cannot reach the target
Add new methods to an interface	Extension attributes + extension_attributes.xml	API-safe; does not break interfaces

Plugin boilerplate

etc/di.xml:

<type name="Magento\Catalog\Model\Product">
    <plugin name="{vendor}_{module}_product" type="{Vendor}\{Module}\Plugin\ProductPlugin" sortOrder="10"/>
</type>

Plugin/ProductPlugin.php:

<?php
declare(strict_types=1);
namespace {Vendor}\{Module}\Plugin;

class ProductPlugin
{
    public function afterGetName(\Magento\Catalog\Model\Product $subject, string $result): string
    {
        return $result . ' (modified)';
    }
}

Observer boilerplate

etc/frontend/events.xml:

<config>
    <event name="catalog_product_load_after">
        <observer name="{vendor}_{module}_product_load"
                  instance="{Vendor}\{Module}\Observer\AfterProductLoad"/>
    </event>
</config>

Observer/AfterProductLoad.php:

<?php
declare(strict_types=1);
namespace {Vendor}\{Module}\Observer;

use Magento\Framework\Event\ObserverInterface;
use Magento\Framework\Event\Observer;

class AfterProductLoad implements ObserverInterface
{
    public function execute(Observer $observer): void
    {
        $product = $observer->getEvent()->getProduct();
        // your logic here
    }
}

Notes

• around plugins wrap the entire method call, including its own plugins below them. Use only when you must conditionally skip the original method.
• Do not declare preferences for classes you do not own if a plugin can achieve the same result.
• Observers cannot return values or modify arguments; they react to data via the event object.