class Mapping

abstract class Mapping extends StylePluginBase {
  
  /**
   * Do not use grouping.
   *
   * @var bool
   */
  protected $usesGrouping = FALSE;
  
  /**
   * Use fields without a row plugin.
   *
   * @var bool
   */
  protected $usesFields = TRUE;
  
  /**
   * Builds the list of field mappings.
   *
   * @return array
   *   An associative array, keyed by the field name, containing the following
   *   key-value pairs:
   *   - #title: The human-readable label for this field.
   *   - #default_value: The default value for this field. If not provided, an
   *     empty string will be used.
   *   - #description: A description of this field.
   *   - #required: Whether this field is required.
   *   - #filter: (optional) A method on the plugin to filter field options.
   *   - #toggle: (optional) If this select should be toggled by a checkbox.
   */
  abstract protected function defineMapping();
  
  /**
   * {@inheritdoc}
   */
  protected function defineOptions() {
    $options = parent::defineOptions();
    // Parse the mapping and add a default for each.
    foreach ($this->defineMapping() as $key => $value) {
      $default = !empty($value['#multiple']) ? [] : '';
      $options['mapping']['contains'][$key] = [
        'default' => $value['#default_value'] ?? $default,
      ];
      if (!empty($value['#toggle'])) {
        $options['mapping']['contains']["toggle_{$key}"] = [
          'default' => FALSE,
        ];
      }
    }
    return $options;
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildOptionsForm(&$form, FormStateInterface $form_state) {
    parent::buildOptionsForm($form, $form_state);
    // Get the mapping.
    $mapping = $this->defineMapping();
    // Restrict the list of defaults to the mapping, in case they have changed.
    $options = array_intersect_key($this->options['mapping'], $mapping);
    // Get the labels of the fields added to this display.
    $field_labels = $this->displayHandler
      ->getFieldLabels();
    // Provide some default values.
    $defaults = [
      '#type' => 'select',
      '#required' => FALSE,
      '#multiple' => FALSE,
    ];
    // For each mapping, add a select element to the form.
    foreach ($options as $key => $value) {
      // If the field is optional, add a 'None' value to the top of the options.
      $field_options = [];
      $required = !empty($mapping[$key]['#required']);
      if (!$required && empty($mapping[$key]['#multiple'])) {
        $field_options = [
          '' => $this->t('- None -'),
        ];
      }
      $field_options += $field_labels;
      // Optionally filter the available fields.
      if (isset($mapping[$key]['#filter'])) {
        $this->view
          ->initHandlers();
        $filter = $mapping[$key]['#filter'];
        $this::$filter($field_options);
        unset($mapping[$key]['#filter']);
      }
      // These values must always be set.
      $overrides = [
        '#options' => $field_options,
        '#default_value' => $options[$key],
      ];
      // Optionally allow the select to be toggleable.
      if (!empty($mapping[$key]['#toggle'])) {
        $form['mapping']["toggle_{$key}"] = [
          '#type' => 'checkbox',
          '#title' => $this->t('Use a custom %field_name', [
            '%field_name' => strtolower($mapping[$key]['#title']),
          ]),
          '#default_value' => $this->options['mapping']["toggle_{$key}"],
        ];
        $overrides['#states']['visible'][':input[name="style_options[mapping][' . "toggle_{$key}" . ']"]'] = [
          'checked' => TRUE,
        ];
      }
      $form['mapping'][$key] = $overrides + $mapping[$key] + $defaults;
    }
  }
  
  /**
   * Overrides Drupal\views\Plugin\views\style\StylePluginBase::render().
   *
   * Provides the mapping definition as an available variable.
   */
  public function render() {
    return [
      '#theme' => $this->themeFunctions(),
      '#view' => $this->view,
      '#options' => $this->options,
      '#rows' => $this->view->result,
      '#mapping' => $this->defineMapping(),
    ];
  }

}