FooterBlock.php

<?php

namespace Drupal\project_core\Plugin\Block;

use Drupal\Core\Block\BlockBase;
use Drupal\Core\Cache\Cache;
use Drupal\Core\Config\ConfigFactoryInterface;
use Drupal\Core\Form\FormStateInterface;
use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
use Symfony\Component\DependencyInjection\ContainerInterface;

/**
 * Provides a 'Project Footer' Block.
 *
 * @Block(
 *   id = "project_footer",
 *   admin_label = @Translation("Platform Footer"),
 * )
 */
class ProjectFooterBlock extends BlockBase implements ContainerFactoryPluginInterface {

  /**
   * Stores the configuration factory.
   *
   * @var \Drupal\Core\Config\ConfigFactoryInterface
   */
  protected $configFactory;

  /**
   * Creates a SystemBrandingBlock instance.
   *
   * @param array $configuration
   *   A configuration array containing information about the plugin instance.
   * @param string $plugin_id
   *   The plugin_id for the plugin instance.
   * @param mixed $plugin_definition
   *   The plugin implementation definition.
   * @param \Drupal\Core\Config\ConfigFactoryInterface $config_factory
   *   The factory for configuration objects.
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, ConfigFactoryInterface $config_factory) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->configFactory = $config_factory;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('config.factory')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function blockForm($form, FormStateInterface $form_state) {
    $copyright = $this->configFactory->get('project_core.settings')->get('copyright');

    $form['footer'] = [
      '#type' => 'fieldset',
      '#title' => $this->t('Footer elements'),
      '#description' => $this->t('Update footer elements here. Changes will be reflected on the <a href="@config">project configuration form</a>.', ['@config' => '/admin/config/system/project']),
    ];
    $form['footer']['copyright'] = [
      '#type' => 'textarea',
      '#rows' => 3,
      '#title' => $this->t('Copyright'),
      '#default_value' => $copyright,
      '#description' => $this->t('Provide copy to display as a copyright statement in the footer.'),
      '#attributes' => [
        'placeholder' => '© ' . date("Y") . ' ' . $this->t('Contra legem facit qui id facit quod lex prohibet.'),
      ],
    ];

    return $form;
  }

  /**
   * {@inheritdoc}
   */
  public function blockSubmit($form, FormStateInterface $form_state) {
    // Retrieve the configuration, set values and save config.
    $footer = $form_state->getValue('footer');
    $this->configFactory->getEditable('project_core.settings')
      ->set('copyright', $footer['copyright'])
      ->save();
  }

  /**
   * {@inheritdoc}
   */
  public function build() {
    $build = [];
    $copyright = $this->configFactory->get('project_core.settings')->get('copyright');

    $build['copyright'] = [
      '#markup' => '<p>' . $copyright . '</p>',
    ];

    return $build;
  }

  /**
   * {@inheritdoc}
   */
  public function getCacheTags() {
    return Cache::mergeTags(
      parent::getCacheTags(),
      $this->configFactory->get('project_core.settings')->getCacheTags()
    );
  }

}