[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
   

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).
   
   
   

4. Current state:

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

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

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

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).
   
   

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.