Event-driven architecture and hooks in PHP

Question

I am planning on working on a game that has a PHP back-end to communicate with the data repository. I was thinking about it and concluded that the best design paradigm to follow for our game would be event driven. I am looking to have an achievement system (similar to the badges system of this website) and basically I would like to be able to hook these "achievement checks" into a number of different events that occur in the game. ie:

When a user does action X hook Y is fired and all attached functions are called to check against an achievement requirement.

In structuring the architecture like this I will allow for new achievements to be added easily as all I will have to do is add the checking function to the proper hook and everything else will fall into place.

I'm not sure if this is a great explanation of what I intend to do, but in any case I am looking for the following:

1. Good reference material on how to code an event-driven application
2. Code snippet(s) showing how to put a "hook" in a function in PHP
3. Code snippet(s) showing how to attach a function to the "hook" mentioned in point 2

I have a few ideas as to how to accomplish 2) and 3) but I was hoping that somebody well-versed in the matter could shed some light on best practices.

Thank you in advance!

Answer 1

Good reference material on how to code an event-driven application

You can either do this with "dumb" callbacks (Demo):

class Hooks
{
    private $hooks;
    public function __construct()
    {
        $this->hooks = array();
    }
    public function add($name, $callback) {
        // callback parameters must be at least syntactically
        // correct when added.
        if (!is_callable($callback, true))
        {
            throw new InvalidArgumentException(sprintf('Invalid callback: %s.', print_r($callback, true)));
        }
        $this->hooks[$name][] = $callback;
    }
    public function getCallbacks($name)
    {
        return isset($this->hooks[$name]) ? $this->hooks[$name] : array();
    }
    public function fire($name)
    {
        foreach($this->getCallbacks($name) as $callback)
        {
            // prevent fatal errors, do your own warning or
            // exception here as you need it.
            if (!is_callable($callback))
                continue;

            call_user_func($callback);
        }
    }
}

$hooks = new Hooks;
$hooks->add('event', function() {echo 'morally disputed.';});
$hooks->add('event', function() {echo 'explicitly called.';});
$hooks->fire('event');

Or implementing a pattern often used in event-driven applications: Observer Pattern.

Code snippet(s) showing how to put a "hook" in a function in PHP

The manual link above (callbacks can be stored into a variable) and some PHP code examples for the Observer Pattern.

Answer 2

For PHP I've regulary integrated the Symfony Event Component: http://components.symfony-project.org/event-dispatcher/.

Here's a short example below, which you can find expanded in Symfony's Recipe section.

<?php

class Foo
{
  protected $dispatcher = null;

    // Inject the dispatcher via the constructor
  public function __construct(sfEventDispatcher $dispatcher)
  {
    $this->dispatcher = $dispatcher;
  }

  public function sendEvent($foo, $bar)
  {
    // Send an event
    $event = new sfEvent($this, 'foo.eventName', array('foo' => $foo, 'bar' => $bar));
    $this->dispatcher->notify($event);
  }
}


class Bar
{
  public function addBarMethodToFoo(sfEvent $event)
  {
    // respond to event here.
  }
}


// Somewhere, wire up the Foo event to the Bar listener
$dispatcher->connect('foo.eventName', array($bar, 'addBarMethodToFoo'));

?>

This is the system we integrated into a shopping cart to create a game-like shopping experience, hooking user actions into game-events. When the user performed specific actions, php fired events causing rewards to be triggered.

Example 1: if the user clicked a specific button 10 times, they received a star.

Example 2: when the user refers a friend and that friend signs up an event is fired rewarding the original referrer with points.

Answer 3

Check out CodeIgniter as it has hooks built right in.

Simply enable hooks:

$config['enable_hooks'] = TRUE;

And then define your hook:

 $hook['post_controller_constructor'] = array(
                                'class'    => 'Hooks',
                                'function' => 'session_check',
                                'filename' => 'hooks.php',
                                'filepath' => 'hooks',
                                'params'   => array()
                                ); 

Then use it in your class:

<?php

    class Hooks {
        var $CI;

        function Hooks() {
            $this->CI =& get_instance();
        }

        function session_check() {
            if(!$this->CI->session->userdata("logged_in") && $this->CI->uri->uri_string != "/user/login")
                redirect('user/login', 'location');
        }
    }

?>