Magento observer examples

When it comes to customize the magento code, there are following 4 options:
1. Creating a observer should be the first to consider.
2. Extending and overriding the core modules should be the second to consider when option 1 cannot fails to accomplish the objectives.
3. Copy the core class to the local directory and put it in the same directory path it was in core directory, this is should be avoided.
4. Modify the core class directly, this should definitedly be avoided unless you are just trying to test a code snippet.

So, there are really only two recommended options which is to try to create an observer first, if it doesn’t work out then write a module to extend and override the core module.

Here is an example of how to create an observer in Magento. This module has two observer:
First one: When observed a product is added to the cart, print some messages.
Second one: To observe when a product review is submitted. The default Magento behavior is to save the product review with PENDING status, this observer will auto approve the submitted product review instead.

1. Create a xml file app/etc/modules/MyExtensions_Observerdemo.xml

<?xml version="1.0"?>
<config>
    <modules>
        <MyExtensions_Observerdemo>
            <active>true</active>
            <codePool>local</codePool>
        </MyExtensions_Observerdemo>
    </modules>
</config>

2. Create a xml file app/code/local/MyExtensions/Observerdemo/etc/config.xml

<?xml version="1.0"?>
<config>
    <modules>
        <MyExtensions_Observerdemo>
            <version>0.1.0</version>
        </MyExtensions_Observerdemo>
    </modules>
	<global>
		<models>
			<myextensions_observerdemo>
				<class>MyExtensions_Observerdemo_Model</class>
			</myextensions_observerdemo>
		</models>
		<events>
			<checkout_cart_product_add_after>
				<observers>
					<MyExtensions_Observerdemo_Model_Observer>
						<type>singleton</type>
						<class>MyExtensions_Observerdemo_Model_Observer</class>
						<method>addtocartEvent</method>
					</MyExtensions_Observerdemo_Model_Observer>
				</observers>
			</checkout_cart_product_add_after>
			 
			 <review_save_before>
				<observers>
					<MyExtensions_Observerdemo_Model_Observer>
						<type>singleton</type>
						<class>MyExtensions_Observerdemo_Model_Observer</class>
						<method>autoApproveReview</method>
					</MyExtensions_Observerdemo_Model_Observer>
				</observers>
			</review_save_before>
		</events>
	</global>

</config>

In the above xml file, the tag global means any configuration inside this tag applys to anything happening in frontend or backend (admin panel). This tag can be changed to frontend because the events checkout_cart_product_add_after and review_safe_before both happens on the frontend. Using global is also fine because global applys to both frontend and backend. There is another tag adminhtml, if we change the above global tag to adminhtml, the observer will not fire on the events checkout_cart_product_add_after and review_safe_before because anything inside adminhtml only applys to events happening on the backend. In short:

<global> applys to both frontend and backend
<fronend> applys only applys to frontend
<adminhtml> applys only applys to backend

3. Create a php file app/code/local/MyExtensions/Observerdemo/Model/Observer.php

<?php

class MyExtensions_Observerdemo_Model_Observer {

    public function addtocartEvent(Varien_Event_Observer $observer) {

        $event = $observer->getEvent();  //Gets the event
        $product = $event->getProduct();
        $observermsg = "The event triggered>>> <B>" . $event->getName() . "</B><br/> The added product>>> <B> " . $product->getName()."</B>";
        //Adds the message to the shopping cart
        echo Mage::getSingleton('checkout/session')->addSuccess($observermsg);
    }
	
	public function autoApproveReview(Varien_Event_Observer $observer)
    {
		$event = $observer->getEvent();
		$reviewObjData = $event->getData();
		//print_r($reviewObjData);exit;

		$reviewData=$reviewObjData["data_object"];
		$reviewData->setStatusId(Mage_Review_Model_Review::STATUS_APPROVED);
		
        echo Mage::getSingleton('core/session')->addSuccess("Thank you for your input!!");
	}
}

4. Clear cache, go to the frontend, add a product and submit a review to see what happens.