public function setTest()
{
    $test = true;

    $event = new Event($this, array(
        'test' => $test
    ));

    $this->onAfterSetTest($event);

    return $test;
}

public function onAfterSetTest(Event $event)
{
    $this->raiseEvent('onAfterSetTest', $event);
}

craft()->on('myPlugin_myService.afterSetTest', function(Event $event)
{
    $event->params['test'] = false;
});

public function setTest()
{
    // ...

    $test = $event->params['test']

    return $test;
}