Dispatcher Filters

Deprecated since version 3.3.0: As of 3.3.0 Dispatcher Filters are deprecated. You should use Middleware instead now.

There are several reasons to want a piece of code to be run before any controller code is executed or right before the response is sent to the client, such as response caching, header tuning, special authentication or just to provide access to a mission-critical API response in lesser time than a complete request dispatching cycle would take.

CakePHP provides a clean interface for attaching filters to the dispatch cycle. It is similar to a middleware layer, but re-uses the existing event subsystem used in other parts of CakePHP. Since they do not work exactly like traditional middleware, we refer to them as Dispatcher Filters.

Built-in Filters

CakePHP comes with several dispatcher filters built-in. They handle common features that all applications are likely to need. The built-in filters are:

  • AssetFilter checks whether the request is referring to a theme or plugin asset file, such as a CSS, JavaScript or image file stored in either a plugin’s webroot folder or the corresponding one for a Theme. It will serve the file accordingly if found, stopping the rest of the dispatching cycle:

    // Use options to set cacheTime for your static assets
    // If not set, this defaults to +1 hour
    DispatcherFactory::add('Asset', ['cacheTime' => '+24 hours']);
    
  • RoutingFilter applies application routing rules to the request URL. Populates $request->getParam() with the results of routing.

  • ControllerFactory uses $request->getParam() to locate the controller that will handle the current request.

  • LocaleSelector enables automatic language switching from the Accept-Language header sent by the browser.

Using Filters

Filters are usually enabled in your application’s bootstrap.php file, but you could load them any time before the request is dispatched. Adding and removing filters is done through Cake\Routing\DispatcherFactory. By default, the CakePHP application template comes with a couple filter classes already enabled for all requests; let’s take a look at how they are added:

DispatcherFactory::add('Routing');
DispatcherFactory::add('ControllerFactory');

// Plugin syntax is also possible
DispatcherFactory::add('PluginName.DispatcherName');

// Use options to set priority
DispatcherFactory::add('Asset', ['priority' => 1]);

Dispatcher filters with higher priority (lower numbers) - will be executed first. Priority defaults to 10.

While using the string name is convenient, you can also pass instances into add():

use Cake\Routing\Filter\RoutingFilter;

DispatcherFactory::add(new RoutingFilter());

Configuring Filter Order

When adding filters, you can control the order they are invoked in using event handler priorities. While filters can define a default priority using the $_priority property, you can set a specific priority when attaching the filter:

DispatcherFactory::add('Asset', ['priority' => 1]);
DispatcherFactory::add(new AssetFilter(['priority' => 1]));

The higher the priority the later this filter will be invoked.

Conditionally Applying Filters

If you don’t want to run a filter on every request, you can use conditions to only apply it some of the time. You can apply conditions using the for and when options. The for option lets you match on URL substrings, while the when option allows you to run a callable:

// Only runs on requests starting with `/blog`
DispatcherFactory::add('BlogHeader', ['for' => '/blog']);

// Only run on GET requests.
DispatcherFactory::add('Cache', [
    'when' => function ($request, $response) {
        return $request->is('get');
    }
]);

The callable provided to when should return true when the filter should run. The callable can expect to get the current request and response as arguments.

Building a Filter

To create a filter, define a class in src/Routing/Filter. In this example, we’ll be making a filter that adds a tracking cookie for the first landing page. First, create the file. Its contents should look like:

namespace App\Routing\Filter;

use Cake\Event\Event;
use Cake\Routing\DispatcherFilter;

class TrackingCookieFilter extends DispatcherFilter
{

    public function beforeDispatch(Event $event)
    {
        $request = $event->getData('request');
        $response = $event->getData('response');
        if (!$request->getCookie('landing_page')) {
            $response->cookie([
                'name' => 'landing_page',
                'value' => $request->here(),
                'expire' => '+ 1 year',
            ]);
        }
    }
}

Save this file into src/Routing/Filter/TrackingCookieFilter.php. As you can see, like other classes in CakePHP, dispatcher filters have a few conventions:

  • Class names end in Filter.
  • Classes are in the Routing\Filter namespace. For example, App\Routing\Filter.
  • Generally filters extend Cake\Routing\DispatcherFilter.

DispatcherFilter exposes two methods that can be overridden in subclasses, they are beforeDispatch() and afterDispatch(). These methods are executed before or after any controller is executed respectively. Both methods receive a Cake\Event\Event object containing the ServerRequest and Response objects (Cake\Http\ServerRequest and Cake\Http\Response instances) inside the $data property.

While our filter was pretty simple, there are a few other interesting things we can do in filter methods. By returning an Response object, you can short-circuit the dispatch process and prevent the controller from being called. When returning a response, you should also remember to call $event->stopPropagation() so other filters are not called.

Note

When a beforeDispatch method returns a response, the controller, and afterDispatch event will not be invoked.

Let’s now create another filter for altering response headers in any public page, in our case it would be anything served from the PagesController:

namespace App\Routing\Filter;

use Cake\Event\Event;
use Cake\Routing\DispatcherFilter;

class HttpCacheFilter extends DispatcherFilter
{

    public function afterDispatch(Event $event)
    {
        $request = $event->getData('request');
        $response = $event->getData('response');

        if ($response->statusCode() === 200) {
            $response->sharable(true);
            $response->expires(strtotime('+1 day'));
        }
    }
}


// In our bootstrap.php
DispatcherFactory::add('HttpCache', ['for' => '/pages'])

This filter will send a expiration header to 1 day in the future for all responses produced by the pages controller. You could of course do the same in the controller, this is just an example of what could be done with filters. For instance, instead of altering the response, you could cache it using Cake\Cache\Cache and serve the response from the beforeDispatch() callback.

While powerful, dispatcher filters have the potential to make your application more difficult to maintain. Filters are an extremely powerful tool when used wisely and adding response handlers for each URL in your app is not a good use for them. Keep in mind that not everything needs to be a filter; Controllers and Components are usually a more accurate choice for adding any request handling code to your app.