The HttpKernel Component: the Controller Resolver

You might think that our framework is already pretty solid and you are probably right. But let's see how we can improve it nonetheless.

Right now, all our examples use procedural code, but remember that controllers can be any valid PHP callbacks. Let's convert our controller to a proper class:

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class LeapYearController
{
    public function index($request): Response
    {
        if (is_leap_year($request->attributes->get('year'))) {
            return new Response('Yep, this is a leap year!');
        }

        return new Response('Nope, this is not a leap year.');
    }
}

Update the route definition accordingly:

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$routes->add('leap_year', new Routing\Route('/is_leap_year/{year}', [
    'year' => null,
    '_controller' => [new LeapYearController(), 'index'],
]));

The move is pretty straightforward and makes a lot of sense as soon as you create more pages but you might have noticed a non-desirable side effect... The LeapYearController class is always instantiated, even if the requested URL does not match the leap_year route. This is bad for one main reason: performance-wise, all controllers for all routes must now be instantiated for every request. It would be better if controllers were lazy-loaded so that only the controller associated with the matched route is instantiated.

To solve this issue, and a bunch more, let's install and use the HttpKernel component:

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$ composer require symfony/http-kernel

The HttpKernel component has many interesting features, but the ones we need right now are the controller resolver and argument resolver. A controller resolver knows how to determine the controller to execute and the argument resolver determines the arguments to pass to it, based on a Request object. All controller resolvers implement the following interface:

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namespace Symfony\Component\HttpKernel\Controller;

// ...
interface ControllerResolverInterface
{
    public function getController(Request $request);
}

The getController() method relies on the same convention as the one we have defined earlier: the _controller request attribute must contain the controller associated with the Request. Besides the built-in PHP callbacks, getController() also supports strings composed of a class name followed by two colons and a method name as a valid callback, like 'class::method':

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$routes->add('leap_year', new Routing\Route('/is_leap_year/{year}', [
    'year' => null,
    '_controller' => 'LeapYearController::index',
]));

To make this code work, modify the framework code to use the controller resolver from HttpKernel:

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use Symfony\Component\HttpKernel;

$controllerResolver = new HttpKernel\Controller\ControllerResolver();
$argumentResolver = new HttpKernel\Controller\ArgumentResolver();

$controller = $controllerResolver->getController($request);
$arguments = $argumentResolver->getArguments($request, $controller);

$response = call_user_func_array($controller, $arguments);

Now, let's see how the controller arguments are guessed. getArguments() introspects the controller signature to determine which arguments to pass to it by using the native PHP reflection. This method is defined in the following interface:

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namespace Symfony\Component\HttpKernel\Controller;

// ...
interface ArgumentResolverInterface
{
    public function getArguments(Request $request, $controller);
}

The index() method needs the Request object as an argument. getArguments() knows when to inject it properly if it is type-hinted correctly:

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public function index(Request $request)

// won't work
public function index($request)

More interesting, getArguments() is also able to inject any Request attribute; if the argument has the same name as the corresponding attribute:

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public function index(int $year)

You can also inject the Request and some attributes at the same time (as the matching is done on the argument name or a type hint, the arguments order does not matter):

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public function index(Request $request, int $year)

public function index(int $year, Request $request)

Finally, you can also define default values for any argument that matches an optional attribute of the Request:

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public function index(int $year = 2012)

Let's inject the $year request attribute for our controller:

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class LeapYearController
{
    public function index(int $year): Response
    {
        if (is_leap_year($year)) {
            return new Response('Yep, this is a leap year!');
        }

        return new Response('Nope, this is not a leap year.');
    }
}

The resolvers also take care of validating the controller callable and its arguments. In case of a problem, it throws an exception with a nice message explaining the problem (the controller class does not exist, the method is not defined, an argument has no matching attribute, ...).

Let's conclude with the new version of our framework:

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// example.com/web/front.php
require_once __DIR__.'/../vendor/autoload.php';

use Symfony\Component\HttpFoundation\Request;
use Symfony\Component\HttpFoundation\Response;
use Symfony\Component\HttpKernel;
use Symfony\Component\Routing;

function render_template(Request $request): Response
{
    extract($request->attributes->all(), EXTR_SKIP);
    ob_start();
    include sprintf(__DIR__.'/../src/pages/%s.php', $_route);

    return new Response(ob_get_clean());
}

$request = Request::createFromGlobals();
$routes = include __DIR__.'/../src/app.php';

$context = new Routing\RequestContext();
$context->fromRequest($request);
$matcher = new Routing\Matcher\UrlMatcher($routes, $context);

$controllerResolver = new HttpKernel\Controller\ControllerResolver();
$argumentResolver = new HttpKernel\Controller\ArgumentResolver();

try {
    $request->attributes->add($matcher->match($request->getPathInfo()));

    $controller = $controllerResolver->getController($request);
    $arguments = $argumentResolver->getArguments($request, $controller);

    $response = call_user_func_array($controller, $arguments);
} catch (Routing\Exception\ResourceNotFoundException $exception) {
    $response = new Response('Not Found', 404);
} catch (Exception $exception) {
    $response = new Response('An error occurred', 500);
}

$response->send();

Think about it once more: our framework is more robust and more flexible than ever and it still has less than 50 lines of code.