exercise_04-custom-services.md

Drupal 8 Backend Training

Exercise 4:

Creating a custom service.

You may be aware that Drupal has undergone a sea change in its transition from 7 to 8. In addition to (almost) fully adopting an object oriented approach, Drupal 8 draws heavily from external frameworks and libraries. The most significant cornerstone of our new version of Drupal is the Symfony framework, a service oriented full stack php framework. Drupal 8 draws heavily from Symfony, using it for request and response handling, routes and controllers, templating, and, you guessed it, for services and the service container.

What is a service container? A service container is basically just a global object that you can use to retrieve services. What are services? Services are essentially just useful chunks of code that might be reused throughout your site. For example some services provided by core include logger.factory which you can use to create log entries (essentially replacing watchdog from d7), and messenger which you can use instead of drupal_set_message. Pulling these services from the service container allows Drupal to handle dependency management and to ensure that only services being used are actually instantiated. This leads to a cleaner, more organized codebase, while providing performance benefits over other object oriented architectures.

In this exercise we will create a service that flips a string.

1. We can use Drupal console to generate the scaffolding for our service. Navigate to your drupal docroot and run the command:

   lando drupal generate:service

   This will once again start up an interactive prompt -- enter the following:

    Enter the module name [admin_toolbar]:
    > example
   
    Enter the service name [example.default]:
    > example.string_flip
   
    Enter the Class name [DefaultService]:
    > ExampleStringFlip
   
    Create an interface (yes/no) [yes]:
    > no
   
    Do you want to load services from the container? (yes/no) [no]:
    >
   
    Enter the path for the services [/modules/custom/example/src/]:
    >
   
    Do you want proceed with the operation? (yes/no) [yes]:
    >

2. You now have all the scaffolding code needed for a custom service. Open /modules/custom/example/example.services.yml. This is the file that defines your module's services. Observe that our example.string_flip service specifies that it uses the class Drupal\example\ExampleStringFlip. This class can be found in /modules/custom/example/src/ExampleStringFlip.php. Please open that file now.

3. Let's create a method that actually does something! Add the following code to the ExampleStringFlip class:

   public function flipString($string) {
     return strrev($string);
   }

4. Now let's use it in the Controller we created earlier. Edit the getSiteName method in the controller file and make the following changes:

     public function getSiteName() {
       $site_name = $this->config('system.site')->get('name');
       $eman_etis = \Drupal::service('example.string_flip')->flipString($site_name);
   
       return [
         '#type' => 'markup',
         '#markup' => $this->t('The site name is: :site_name, backwards that\'s: :eman_etis', 
           [':site_name' => $site_name, ':eman_etis' => $eman_etis]),
       ];
     }

5. Our custom service is now in the service container and can be accessed using \Drupal::service, however this is not best practice. Can you convert this example to use dependency injection? Hint: how is the controller accessing the config.factory service?

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