Logging for Hello World

Now that we have all the tools at our disposable, and, more importantly, understand how logging works in Drupal 8, let's add some logging to our module.

There is one place where we can log an action that may prove helpful. Let's log an info message when an administrator changes the greeting message via the form we wrote. This should naturally happen in the submit handler of SalutationConfigurationForm.

If you remember my rant in the preceding chapter, there is no way we should use a service statically if we can instead inject it, and we can easily inject services into our form. So, let's do this now.

First of all, FormBase already implements ContainerInjectionInterface, so we don't need to implement it in our class, as we are extending from it somewhere down the line. Second of all, ConfigFormBase class we are directly extending already has config.factory injected, so this complicates things for us a bit--well, not really. All we need to do is copy over the constructor and create() method, add our own service, store it in a property, and pass the services it needs to the parent constructor call. It will look like this:

 /**
   * @var \Drupal\Core\Logger\LoggerChannelInterface
   */
  protected $logger;

  /**
   * SalutationConfigurationForm constructor.
   *
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The factory for configuration objects.
   */
  public function __construct(ConfigFactoryInterface $config_factory, LoggerChannelInterface $logger) {
    parent::__construct($config_factory);
    $this->logger = $logger;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('config.factory'),
      $container->get('hello_world.logger.channel.hello_world')
    );
  }

And the relevant use statements at the top:

use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Logger\LoggerChannelInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

As you can see, we get all the services any of the parents need and the one we need (our logger channel) via the create() method. Also, in our constructor, we store the channel as a property and then pass the parent arguments to the parent constructor. Now, we have our hello_world logger channel available in our configuration form class. So, let's use it.

At the end of the submitForm() method, let's add the following line:

$this->logger->info('The Hello World salutation has been changed to @message', ['@message' => $form_state->getValue('salutation')]);

We are logging a regular information message. However, since we also want to log the message that has been set, we use the second argument, which represents an array of context values. Under the hood, the database logger will extract the context variables that start with @, !, or % with the values from the entire context array. This is done using the LogMessageParser service (standard syntax in Drupal), and we'll see more of this in a later chapter when we will discuss internationalization. If you implement your own logger plugin, you will have to handle this yourself as well--but we'll see that in action soon.

Now, we are done with logging a message when the salutation configuration form is saved.