How to call observer in controller

Question

I want to call observer in controller using if condition this is my code

app\code\local\EPayment\EPayment\controllers\IndexController.php

class EPayment_EPayment_IndexController extends Mage_Core_Controller_Front_Action{
    public function getepaymentAction()
    {
        $params = $this->getRequest()->getParams();
        $left_amount = $params['left_amount'];
        if($left_amount == true){
          // call observer here
           $this->getResponse()->setHeader('Content-type', 'application/json');
          $this->getResponse()->setBody($left_amount);
        }
    }
}

app\code\local\EPayment\EPayment\Model\Observer.php

class EPayment_EPayment_Model_Observer
{
    public function setDiscount($observer)
    {
        // some codes here
    }
}

The above code $left_amount is true call Observer.php.

Note: I write code for set discount in Observer.php.

Answer 1

To dispatch event you have to write:

Mage::dispatchEvent('YOUR_EVENT_NAME', [YOUR_DATA]);

For example:

Mage::dispatchEvent('my_event', ['order' => $order]);

You should put this code inside of if in controller. Then you must configure event listener in etc/config.xml:

<events>
  <my_event>
    <observers>
      <some_fancy_observer_name>
        <type>singleton</type>
        <class>EPayment_EPayment_Model_Observer</class>
        <method> setDiscount </method>
      </some_fancy_observer_name>
     </observers>
  </some_fancy_observer_name>
</events>

And then put required logic inside of your observer.