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‎book/part09.rst

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Create your own framework... on top of the Symfony2 Components (part 9)
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=======================================================================
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Our framework is still missing a major characteristic of any good framework:
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*extensibility*. Being extensible means that the developer should be able to
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easily hook into the framework life cycle to modify the way the request is
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handled.
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What kind of hooks are we talking about? Authentication or caching for
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instance. To be flexible, hooks must be plug-and-play; the ones you "register"
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for an application are different from the next one depending on your specific
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needs. Many software have a similar concept like Drupal or Wordpress. In some
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languages, there is even a standard like `WSGI`_ in Python or `Rack`_ in Ruby.
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As there is no standard for PHP, we are going to use a well-known design
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pattern, the *Observer*, to allow any kind of behaviors to be attached to our
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framework; the Symfony2 EventDispatcher Component implements a lightweight
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version of this pattern:
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.. code-block:: json
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{
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"require": {
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"symfony/class-loader": "2.1.*",
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"symfony/http-foundation": "2.1.*",
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"symfony/routing": "2.1.*",
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"symfony/http-kernel": "2.1.*",
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"symfony/event-dispatcher": "2.1.*"
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},
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"autoload": {
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"psr-0": { "Simplex": "src/", "Calendar": "src/" }
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}
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}
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How does it work? The *dispatcher*, the central object of the event dispatcher
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system, notifies *listeners* of an *event* dispatched to it. Put another way:
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your code dispatches an event to the dispatcher, the dispatcher notifies all
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registered listeners for the event, and each listener do whatever it wants
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with the event.
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As an example, let's create a listener that transparently adds the Google
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Analytics code to all responses.
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To make it work, the framework must dispatch an event just before returning
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the Response instance::
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<?php
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// example.com/src/Simplex/Framework.php
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namespace Simplex;
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use Symfony\Component\HttpFoundation\Request;
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use Symfony\Component\HttpFoundation\Response;
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use Symfony\Component\Routing\Matcher\UrlMatcherInterface;
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use Symfony\Component\Routing\Exception\ResourceNotFoundException;
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use Symfony\Component\HttpKernel\Controller\ControllerResolverInterface;
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use Symfony\Component\EventDispatcher\EventDispatcher;
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class Framework
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{
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protected $matcher;
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protected $resolver;
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protected $dispatcher;
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public function __construct(EventDispatcher $dispatcher, UrlMatcherInterface $matcher, ControllerResolverInterface $resolver)
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{
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$this->matcher = $matcher;
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$this->resolver = $resolver;
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$this->dispatcher = $dispatcher;
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}
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public function handle(Request $request)
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{
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try {
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$request->attributes->add($this->matcher->match($request->getPathInfo()));
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$controller = $this->resolver->getController($request);
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$arguments = $this->resolver->getArguments($request, $controller);
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$response = call_user_func_array($controller, $arguments);
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} catch (ResourceNotFoundException $e) {
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$response = new Response('Not Found', 404);
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} catch (\Exception $e) {
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$response = new Response('An error occurred', 500);
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}
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// dispatch a response event
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$this->dispatcher->dispatch('response', new ResponseEvent($response, $request));
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return $response;
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}
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}
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Each time the framework handles a Request, a ``ResponseEvent`` event is
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now dispatched::
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<?php
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// example.com/src/Simplex/ResponseEvent.php
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namespace Simplex;
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use Symfony\Component\HttpFoundation\Request;
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use Symfony\Component\HttpFoundation\Response;
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use Symfony\Component\EventDispatcher\Event;
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class ResponseEvent extends Event
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{
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private $request;
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private $response;
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public function __construct(Response $response, Request $request)
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{
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$this->response = $response;
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$this->request = $request;
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}
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public function getResponse()
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{
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return $this->response;
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}
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public function getRequest()
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{
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return $this->request;
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}
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}
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The last step is the creation of the dispatcher in the front controller and
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the registration of a listener for the ``response`` event::
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<?php
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// example.com/web/front.php
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require_once __DIR__.'/../vendor/.composer/autoload.php';
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// ...
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use Symfony\Component\EventDispatcher\EventDispatcher;
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$dispatcher = new EventDispatcher();
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$dispatcher->addListener('response', function (Simplex\ResponseEvent $event) {
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$response = $event->getResponse();
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if ($response->isRedirection()
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|| ($response->headers->has('Content-Type') && false === strpos($response->headers->get('Content-Type'), 'html'))
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|| 'html' !== $event->getRequest()->getRequestFormat()
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) {
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return;
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}
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$response->setContent($response->getContent().'GA CODE');
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});
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$framework = new Simplex\Framework($dispatcher, $matcher, $resolver);
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$response = $framework->handle($request);
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$response->send();
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.. note::
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The listener is just a proof of concept and you should add the Google
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Analytics code just before the body tag.
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As you can see, ``addListener()`` associates a valid PHP callback to a named
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event (``response``); the event name must be the same as the one used in the
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``dispatch()`` call.
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In the listener, we add the Google Analytics code only if the response is not
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a redirection, if the requested format is HTML, and if the response content
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type is HTML (these conditions demonstrate the ease of manipulating the
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Request and Response data from your code).
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So far so good, but let's add another listener on the same event. Let's say
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that I want to set the ``Content-Length`` of the Response if it is not already
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set::
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$dispatcher->addListener('response', function (Simplex\ResponseEvent $event) {
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$response = $event->getResponse();
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$headers = $response->headers;
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if (!$headers->has('Content-Length') && !$headers->has('Transfer-Encoding')) {
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$headers->set('Content-Length', strlen($response->getContent()));
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}
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});
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Depending on whether you have added this piece of code before the previous
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listener registration or after it, you will have the wrong or the right value
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for the ``Content-Length`` header. Sometimes, the order of the listeners
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matter but by default, all listeners are registered with the same priority,
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``0``. To tell the dispatcher to run a listener early, change the priority to
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a positive number; negative numbers can be used for low priority listeners.
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Here, we want the ``Content-Length`` listener to be executed last, so change
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the priority to ``-255``::
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$dispatcher->addListener('response', function (Simplex\ResponseEvent $event) {
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$response = $event->getResponse();
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$headers = $response->headers;
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if (!$headers->has('Content-Length') && !$headers->has('Transfer-Encoding')) {
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$headers->set('Content-Length', strlen($response->getContent()));
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}
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}, -255);
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.. tip::
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When creating your framework, think about priorities (reserve some numbers
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for internal listeners for instance) and document them thoroughly.
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Let's refactor the code a bit by moving the Google listener to its own class::
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<?php
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// example.com/src/Simplex/GoogleListener.php
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namespace Simplex;
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class GoogleListener
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{
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public function onResponse(ResponseEvent $event)
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{
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$response = $event->getResponse();
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if ($response->isRedirection()
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|| ($response->headers->has('Content-Type') && false === strpos($response->headers->get('Content-Type'), 'html'))
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|| 'html' !== $event->getRequest()->getRequestFormat()
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) {
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return;
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}
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$response->setContent($response->getContent().'GA CODE');
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}
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}
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And do the same with the other listener::
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<?php
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// example.com/src/Simplex/ContentLengthListener.php
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namespace Simplex;
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class ContentLengthListener
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{
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public function onResponse(ResponseEvent $event)
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{
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$response = $event->getResponse();
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$headers = $response->headers;
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if (!$headers->has('Content-Length') && !$headers->has('Transfer-Encoding')) {
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$headers->set('Content-Length', strlen($response->getContent()));
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}
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}
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}
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Our front controller should now look like the following::
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$dispatcher = new EventDispatcher();
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$dispatcher->addListener('response', array(new Simplex\ContentLengthListener(), 'onResponse'), -255);
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$dispatcher->addListener('response', array(new Simplex\GoogleListener(), 'onResponse'));
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Even if the code is now nicely wrapped in classes, there is still a slight
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issue: the knowledge of the priorities is "hardcoded" in the front controller,
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instead of being in the listeners themselves. For each application, you have
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to remember to set the appropriate priorities. Moreover, the listener method
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names are also exposed here, which means that refactoring our listeners would
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mean changing all the applications that rely on those listeners. Of course,
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there is a solution: use subscribers instead of listeners::
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$dispatcher = new EventDispatcher();
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$dispatcher->addSubscriber(new Simplex\ContentLengthListener());
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$dispatcher->addSubscriber(new Simplex\GoogleListener());
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A subscriber knowns about all the events it is interested in and pass this
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information to the dispatcher via the ``getSubscribedEvents()`` method. Have a
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look at the new version of the ``GoogleListener``::
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<?php
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// example.com/src/Simplex/GoogleListener.php
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namespace Simplex;
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use Symfony\Component\EventDispatcher\EventSubscriberInterface;
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class GoogleListener implements EventSubscriberInterface
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{
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// ...
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public static function getSubscribedEvents()
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{
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return array('response' => 'onResponse');
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}
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}
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And here is the new version of ``ContentLengthListener``::
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<?php
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// example.com/src/Simplex/ContentLengthListener.php
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namespace Simplex;
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use Symfony\Component\EventDispatcher\EventSubscriberInterface;
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class ContentLengthListener implements EventSubscriberInterface
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{
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// ...
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public static function getSubscribedEvents()
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{
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return array('response' => array('onResponse', -255));
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}
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}
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.. tip::
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A single subscriber can host as many listeners as you want on as many
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events as needed.
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To make your framework truly flexible, don't hesitate to add more events; and
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to make it more awesome out of the box, add more listeners. Again, this series
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is not about creating a generic framework, but one that is tailored to your
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needs. Stop whenever you see fit, and further evolve the code from there.
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.. _`WSGI`: http://www.python.org/dev/peps/pep-0333/#middleware-components-that-play-both-sides
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.. _`Rack`: http://rack.rubyforge.org/

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