How to override a service class in Drupal?

In Drupal, you can override a service class by creating a custom module and using the service decoration pattern. Here are the steps to do so:

1. Create a custom module: Create a custom Drupal module if you haven't already. You can do this by creating a new folder with the module name in the modules/custom directory of your Drupal installation, and adding a *.info.yml file to define the module.

2. Define the service in your module: In your custom module, create a *.services.yml file to define the service you want to override. For example, if you want to override a service with the ID my_service_id, your *.services.yml file could look like this:
    

   services:
     my_service_id:
       class: Drupal\my_module\MyCustomService
       tags:
         - { name: my_custom_service_tag }
   

   Note that my_service_id should match the service ID of the service you want to override, and Drupal\my_module\MyCustomService should be the class name of your custom service that will override the original service.

3. Create the custom service class: Create a new PHP class in your custom module that extends the original service class you want to override. For example, if the original service class is Drupal\some_module\SomeService, your custom service class could look like this:
    

   namespace Drupal\my_module;
   
   use Drupal\some_module\SomeService;
   
   class MyCustomService extends SomeService {
     // Implement your custom logic here.
   }
   

4. Declare the service decoration: In your custom service class, you can declare the service decoration by adding the @Decorate annotation above the class definition, and specifying the original service class you want to override, as well as the priority of the decoration. For example:

   namespace Drupal\my_module;
   
   use Drupal\some_module\SomeService;
   use Drupal\Core\Annotation\Translation;
   use Drupal\Core\Annotation\Decorate;
   
   /**
    * @Decorate(serviceId="some_module.some_service_id", priority=100)
    */
   class MyCustomService extends SomeService {
     // Implement your custom logic here.
   }
   

5. Clear Drupal's cache: After creating the custom service class, you need to clear Drupal's cache to ensure that the new service decoration is recognized by Drupal. You can do this by running the following Drush command: `drush cr`
6. Use the custom service: You can now use the custom service in your module or other parts of your Drupal site, and Drupal will automatically use the overridden service with your custom logic.

By following these steps, you can successfully override a service class in Drupal using the service decoration pattern in a custom module. There is a simpler way to do the same thing and that is to use a Service Provider class to override any service that can be fetched using the ContainerBuilder of the symfony dependency injection component. See how to do that here.