What is being passed to an observer

Question

I have been reading a couple of tutorials and am now trying to create my own observer.

The way I understand observers, is that when you observe an event a parameter will be automatically passed to it, but how do I know what is being passed?

I thought the below example would pass Mage_Catalog_Model_Product as a parameter. But when I try to implement this in my code I get the following error ..._Model_Observer::_generateDiscount() must be an instance of Mage_Catalog_Model_Product (I have added my code below)

I'm trying to use the dispatchEvent() in getFinalPrice()

Mage_Catalog_Model_Product_Type_Price.php

 /**
 * Retrieve product final price
 *
 * @param float|null $qty
 * @param Mage_Catalog_Model_Product $product
 * @return float
 */
public function getFinalPrice($qty = null, $product)
{
    if (is_null($qty) && !is_null($product->getCalculatedFinalPrice())) {
        return $product->getCalculatedFinalPrice();
    }

    $finalPrice = $this->getBasePrice($product, $qty);
    $product->setFinalPrice($finalPrice);

    Mage::dispatchEvent('catalog_product_get_final_price', array('product' => $product, 'qty' => $qty));

    $finalPrice = $product->getData('final_price');
    $finalPrice = $this->_applyOptionsPrice($product, $qty, $finalPrice);
    $finalPrice = max(0, $finalPrice);
    $product->setFinalPrice($finalPrice);

    return $finalPrice;
}

My observer

class Mdeprojects_Discount_Model_Observer
{
    public function applyDiscount()
    {
        $this->_generateDiscount();
    }

    protected function _generateDiscount(Mage_Catalog_Model_Product $product)
    {
        $discount = $product->getFinalPrice() / 100 * 50;
        return $discount;
    }
}

I have also tried

protected function _generateDiscount(Varien_Event_Observer $product)
{
    $discount = $product->_data['final_price'] / 100 * 50;
    return $discount;
}

I am misunderstanding how observers work could someone please point out where my error(s) is/are?

Answer 1

Each observer receives as parameter an instance of Varien_Event_Observer that has a member called event that is an instance of Varien_Event.
You can access this member by calling $observer->getEvent().
This event contains all the variables passed to the observer and you can get them by using getData('obj_key') or getObjKey(), wheren obj_key is the key passed when the event is dispatched.
It sounds more complicated than it is. Let's take your example.
The event dispatched is

Mage::dispatchEvent('catalog_product_get_final_price', array('product' => $product, 'qty' => $qty));

This means that your observer should look like this:

class Mdeprojects_Discount_Model_Observer
{
    //main method called by dispatch event that receives a parameter ($observer)
    public function applyDiscount($observer)
    {
        $event = $observer->getEvent();//get the event object
        $product = $event->getProduct();//the key in the dispatchEvent is `product`.
        //or $product = $event->getData('product');
        //pass the product to your protected method. 
        $this->_generateDiscount($product);
    }

    protected function _generateDiscount(Mage_Catalog_Model_Product $product)
    {
        $discount = $product->getFinalPrice() / 100 * 50;
        return $discount;
    }
}

Answer 2

What passed is an Varien_Event_Observer $observer with the following data:

Mage::dispatchEvent('catalog_product_get_final_price', array('product' => $product, 'qty' => $qty));

$observer->getData() => array('product' => $product, 'qty' => $qty) + array('event' => $event)

where $event->getData() = array('product' => $product, 'qty' => $qty)

You can check here: \Mage_Core_Model_App::dispatchEvent