[Drupal] Create an event subscriber

Instructions

I will be creating a dedicated module using Drupal Console for this event subscriber. But feel free to use your own if you already have one! (don’t forget to backup)

1. Access the drupal root folder in command line.
2. Generate a new module:
   
   dakwamine@debian-drupal:/var/www/html/drupal$ drupal generate:module Enter the new module name: > Test Module Enter the module machine name [test_module]: > Enter the module Path [/modules/custom]: > Enter module description [My Awesome Module]: > Enter package name [Custom]: > Enter Drupal Core version [8.x]: > Do you want to generate a .module file (yes/no) [yes]: > no Define module as feature (yes/no) [no]: > Do you want to add a composer.json file to your module (yes/no) [yes]: > no Would you like to add module dependencies (yes/no) [no]: > Do you confirm generation? (yes/no) [yes]: > Generated or updated files Site path: /var/www/html/drupal 1 - modules/custom/test_module/test_module.info.yml

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   dakwamine@debian-drupal:/var/www/html/drupal$ drupal generate:module   Enter the new module name: > Test Module             Enter the module machine name [test_module]: >   Enter the module Path [/modules/custom]: >   Enter module description [My Awesome Module]: >   Enter package name [Custom]: >   Enter Drupal Core version [8.x]: >   Do you want to generate a .module file (yes/no) [yes]: > no   Define module as feature (yes/no) [no]: >   Do you want to add a composer.json file to your module (yes/no) [yes]: > no   Would you like to add module dependencies (yes/no) [no]: >     Do you confirm generation? (yes/no) [yes]: >   Generated or updated files Site path: /var/www/html/drupal 1 - modules/custom/test_module/test_module.info.yml

3. Generate the event subscriber. We will subscribe to the kernel.request event:
   
   dakwamine@debian-drupal:/var/www/html/drupal$ drupal generate:event:subscriber Enter the module name [admin_toolbar]: > test_module Enter the service name [test_module.default]: > test_module.event_subscriber_example Class name [DefaultSubscriber]: > TestModuleExampleSubscriber Type the event name or use keyup or keydown. This is optional, press enter to continue Enter event name [ ]: > kernel.request Callback function name to handle event [kernel_request]: > Enter event name [ ]: > Do you want to load services from the container (yes/no) [no]: > Do you confirm generation? (yes/no) [yes]: > Generated or updated files Site path: /var/www/html/drupal 1 - modules/custom/test_module/src/EventSubscriber/TestModuleExampleSubscriber.php 2 - modules/custom/test_module/test_module.services.yml cache:rebuild Rebuilding cache(s), wait a moment please. [OK] Done clearing cache(s).

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   dakwamine@debian-drupal:/var/www/html/drupal$ drupal generate:event:subscriber Enter the module name [admin_toolbar]: > test_module   Enter the service name [test_module.default]: > test_module.event_subscriber_example   Class name [DefaultSubscriber]: > TestModuleExampleSubscriber   Type the event name or use keyup or keydown. This is optional, press enter to continue   Enter event name [ ]: > kernel.request   Callback function name to handle event [kernel_request]: >   Enter event name [ ]: > Do you want to load services from the container (yes/no) [no]: >     Do you confirm generation? (yes/no) [yes]: >   Generated or updated files Site path: /var/www/html/drupal 1 - modules/custom/test_module/src/EventSubscriber/TestModuleExampleSubscriber.php 2 - modules/custom/test_module/test_module.services.yml cache:rebuild   Rebuilding cache(s), wait a moment please.     [OK] Done clearing cache(s).

4. Current state:
   • Tree in test_module

     . ├── src │   └── EventSubscriber │   └── TestModuleExampleSubscriber.php ├── test_module.info.yml └── test_module.services.yml

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     . ├── src │   └── EventSubscriber │       └── TestModuleExampleSubscriber.php ├── test_module.info.yml └── test_module.services.yml

   • ./test_module.info.yml

     YAML

     name: Test Module type: module description: My Awesome Module core: 8.x package: Custom

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     name: Test Module type: module description: My Awesome Module core: 8.x package: Custom

   • ./test_module.services.yml

     YAML

     services: test_module.event_subscriber_example: class: Drupal\test_module\EventSubscriber\TestModuleExampleSubscriber arguments: [] tags: - { name: event_subscriber }

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     services:   test_module.event_subscriber_example:     class: Drupal\test_module\EventSubscriber\TestModuleExampleSubscriber     arguments: []     tags:       - { name: event_subscriber }

   • ./src/EventSubscriber/TestModuleExampleSubscriber.php

     PHP

     <!--?php namespace Drupal\test_module\EventSubscriber; use Symfony\Component\EventDispatcher\EventSubscriberInterface; use Symfony\Component\EventDispatcher\Event; /** * Class TestModuleExampleSubscriber. * * @package Drupal\test_module */ class TestModuleExampleSubscriber implements EventSubscriberInterface { /** * Constructor. */ public function __construct() { } /** * {@inheritdoc} */ static function getSubscribedEvents() { // Array structure recommended in Drupal official docs $events['kernel.request'][] = ['kernel_request']; // Generated by Drupal Console, tested, works the same for now //$events['kernel.request'] = ['kernel_request']; // kernel.request has a constant which can be used as the following //$events[KernelEvents::REQUEST][] = ['kernel_request']; return $events; } /** * This method is called whenever the kernel.request event is * dispatched. * * @param GetResponseEvent $event */ public function kernel_request(Event $event) { drupal_set_message('Event kernel.request thrown by Subscriber in module test_module.', 'status', TRUE); } } </pre-->

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     <!--?php   namespace Drupal\test_module\EventSubscriber;   use Symfony\Component\EventDispatcher\EventSubscriberInterface; use Symfony\Component\EventDispatcher\Event;   /** * Class TestModuleExampleSubscriber. * * @package Drupal\test_module */ class TestModuleExampleSubscriber implements EventSubscriberInterface {     /**    * Constructor.    */   public function __construct() {     }     /**    * {@inheritdoc}    */   static function getSubscribedEvents() {     // Array structure recommended in Drupal official docs     $events['kernel.request'][] = ['kernel_request'];          // Generated by Drupal Console, tested, works the same for now     //$events['kernel.request'] = ['kernel_request'];          // kernel.request has a constant which can be used as the following     //$events[KernelEvents::REQUEST][] = ['kernel_request'];       return $events;   }     /**    * This method is called whenever the kernel.request event is    * dispatched.    *    * @param GetResponseEvent $event    */   public function kernel_request(Event $event) {     drupal_set_message('Event kernel.request thrown by Subscriber in module test_module.', 'status', TRUE);   }   }   </pre-->

5. Install the module using either drupal module:install or drush pm-enable:
   
   dakwamine@debian-drupal:/var/www/html/drupal$ drupal module:install test_module Installing module(s) test_module [OK] The following module(s) were installed successfully: test_module cache:rebuild Rebuilding cache(s), wait a moment please. [OK] Done clearing cache(s).

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   dakwamine@debian-drupal:/var/www/html/drupal$ drupal module:install test_module Installing module(s) test_module     [OK] The following module(s) were installed successfully: test_module     cache:rebuild   Rebuilding cache(s), wait a moment please.     [OK] Done clearing cache(s).

6. Now, refresh your web page and on each request received server side, a message will tell you the event occurred!

Additional Information

• Instructions made on Drupal 8.2.x.
• Using Drupal Console, you can easily inject services such as the current_user one by answering yes when it asks if you want to load services from the container. Please check this article about service dependency injection: [Drupal] Service dependency injection in a service type class.
• After editing your files, remember to run a cache rebuild either with drupal cache:rebuild or drush cache-rebuild.
• Events documentation entry point: Events on Drupal 8. You will find a few core events here.
• The callback function’s name can be changed to whatever you want.