class ColorBackgroudFormatter

class ColorBackgroudFormatter extends FormatterBase {
    
    /**
     * {@inheritdoc}
     */
    public function viewElements(FieldItemListInterface $items, $langcode) {
        $elements = [];
        foreach ($items as $delta => $item) {
            // Set the value of the CSS color property depending on user provided
            // configuration. Individual configuration items can be accessed with
            // $this->getSetting('key') where 'key' is the same as the key in the
            // form array from settingsForm() and what's defined in the configuration
            // schema.
            $text_color = 'inherit';
            if ($this->getSetting('adjust_text_color')) {
                $text_color = $this->lightness($item->value) < 50 ? 'white' : 'black';
            }
            $elements[$delta] = [
                '#type' => 'html_tag',
                '#tag' => 'p',
                '#value' => $this->t('The content area color has been changed to @code', [
                    '@code' => $item->value,
                ]),
                '#attributes' => [
                    'style' => 'background-color: ' . $item->value . '; color: ' . $text_color,
                ],
            ];
        }
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     *
     * Set the default values for the formatter's configuration.
     */
    public static function defaultSettings() {
        // The keys of this array should match the form element names in
        // settingsForm(), and the schema defined in
        // config/schema/field_example.schema.yml.
        return [
            'adjust_text_color' => TRUE,
        ] + parent::defaultSettings();
    }
    
    /**
     * {@inheritdoc}
     *
     * Define the Form API widgets a user should see when configuring the
     * formatter. These are displayed when a user clicks the gear icon in the row
     * for a formatter on the manage display page.
     *
     * The field_ui module takes care of handling submitted form values.
     */
    public function settingsForm(array $form, FormStateInterface $form_state) {
        // Create a new array with one or more form elements. $form is available for
        // context but you should not add your elements to it directly.
        $elements = [];
        // The keys of the array, 'adjust_text_color' in this case, should match
        // what is defined in ::defaultSettings(), and the field_example.schema.yml
        // schema. The values collected by the form will be automatically stored
        // as part of the field instance configuration so you do not need to
        // implement form submission processing.
        $elements['adjust_text_color'] = [
            '#type' => 'checkbox',
            // The current configuration for this setting for the field instance can
            // be accessed via $this->getSetting().
'#default_value' => $this->getSetting('adjust_text_color'),
            '#title' => $this->t('Adjust foreground text color'),
            '#description' => $this->t('Switch the foreground color between black and white depending on lightness of the background color.'),
        ];
        return $elements;
    }
    
    /**
     * {@inheritdoc}
     */
    public function settingsSummary() {
        // This optional summary text is displayed on the manage displayed in place
        // of the formatter configuration form when the form is closed. You'll
        // usually see it in the list of fields on the manage display page where
        // this formatter is used.
        $state = $this->getSetting('adjust_text_color') ? $this->t('yes') : $this->t('no');
        $summary[] = $this->t('Adjust text color: @state', [
            '@state' => $state,
        ]);
        return $summary;
    }
    
    /**
     * Determine lightness of a color.
     *
     * This might not be the best way to determine if the contrast between the
     * foreground and background colors is legible. But it'll work well enough for
     * this demonstration.
     *
     * Logic from https://stackoverflow.com/a/12228730/8616016.
     *
     * @param string $color
     *   A color in hex format, leading '#' is optional.
     *
     * @return float
     *   Percentage of lightness of the provided color.
     */
    protected function lightness(string $color) {
        $hex = ltrim($color, '#');
        // Convert the hex string to RGB.
        $r = hexdec($hex[0] . $hex[1]);
        $g = hexdec($hex[2] . $hex[3]);
        $b = hexdec($hex[4] . $hex[5]);
        // Calculate the HSL lightness value and return that as a percent.
        return (max($r, $g, $b) + min($r, $g, $b)) / 510.0 * 100;
    }

}