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.
     */
    protected abstract 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(),
        ];
    }

}