class BooleanFormatter

class BooleanFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public static function defaultSettings() {
        $settings = [];
        // Fall back to field settings by default.
        $settings['format'] = 'default';
        $settings['format_custom_false'] = '';
        $settings['format_custom_true'] = '';
        return $settings;
    }
    
    /**
     * Gets the available format options.
     *
     * @return array|string
     *   A list of output formats. Each entry is keyed by the machine name of the
     *   format. The value is an array, of which the first item is the result for
     *   boolean TRUE, the second is for boolean FALSE. The value can be also an
     *   array, but this is just the case for the custom format.
     */
    protected function getOutputFormats() {
        $formats = [
            'default' => [
                $this->getFieldSetting('on_label'),
                $this->getFieldSetting('off_label'),
            ],
            'yes-no' => [
                $this->t('Yes'),
                $this->t('No'),
            ],
            'true-false' => [
                $this->t('True'),
                $this->t('False'),
            ],
            'on-off' => [
                $this->t('On'),
                $this->t('Off'),
            ],
            'enabled-disabled' => [
                $this->t('Enabled'),
                $this->t('Disabled'),
            ],
            'boolean' => [
                1,
                0,
            ],
            'unicode-yes-no' => [
                '✔',
                '✖',
            ],
            'custom' => $this->t('Custom'),
        ];
        return $formats;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        $form = parent::settingsForm($form, $form_state);
        $formats = [];
        foreach ($this->getOutputFormats() as $format_name => $format) {
            if (is_array($format)) {
                if ($format_name == 'default') {
                    $formats[$format_name] = $this->t('Field settings (@on_label / @off_label)', [
                        '@on_label' => $format[0],
                        '@off_label' => $format[1],
                    ]);
                }
                else {
                    $formats[$format_name] = $this->t('@on_label / @off_label', [
                        '@on_label' => $format[0],
                        '@off_label' => $format[1],
                    ]);
                }
            }
            else {
                $formats[$format_name] = $format;
            }
        }
        $field_name = $this->fieldDefinition
            ->getName();
        $form['format'] = [
            '#type' => 'select',
            '#title' => $this->t('Output format'),
            '#default_value' => $this->getSetting('format'),
            '#options' => $formats,
        ];
        $form['format_custom_true'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Custom output for TRUE'),
            '#default_value' => $this->getSetting('format_custom_true'),
            '#states' => [
                'visible' => [
                    'select[name="fields[' . $field_name . '][settings_edit_form][settings][format]"]' => [
                        'value' => 'custom',
                    ],
                ],
            ],
        ];
        $form['format_custom_false'] = [
            '#type' => 'textfield',
            '#title' => $this->t('Custom output for FALSE'),
            '#default_value' => $this->getSetting('format_custom_false'),
            '#states' => [
                'visible' => [
                    'select[name="fields[' . $field_name . '][settings_edit_form][settings][format]"]' => [
                        'value' => 'custom',
                    ],
                ],
            ],
        ];
        return $form;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        $summary = [];
        $setting = $this->getSetting('format');
        if ($setting == 'custom') {
            $summary[] = $this->t('Custom text: @true_label / @false_label', [
                '@true_label' => $this->getSetting('format_custom_true'),
                '@false_label' => $this->getSetting('format_custom_false'),
            ]);
        }
        else {
            $formats = $this->getOutputFormats();
            $summary[] = $this->t('Display: @true_label / @false_label', [
                '@true_label' => $formats[$setting][0],
                '@false_label' => $formats[$setting][1],
            ]);
        }
        return $summary;
    }
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        $formats = $this->getOutputFormats();
        foreach ($items as $delta => $item) {
            $format = $this->getSetting('format');
            if ($format == 'custom') {
                $elements[$delta] = [
                    '#markup' => $item->value ? $this->getSetting('format_custom_true') : $this->getSetting('format_custom_false'),
                ];
            }
            else {
                $elements[$delta] = [
                    '#markup' => $item->value ? $formats[$format][0] : $formats[$format][1],
                ];
            }
        }
        return $elements;
    }

}