class ListFloatItem

class ListFloatItem extends ListItemBase {
    
    /**
     * {@inheritdoc}
     */
    public static function propertyDefinitions(FieldStorageDefinitionInterface $field_definition) {
        $properties['value'] = DataDefinition::create('float')->setLabel(new TranslatableMarkup('Float value'))
            ->setRequired(TRUE);
        return $properties;
    }
    
    /**
     * {@inheritdoc}
     */
    public static function schema(FieldStorageDefinitionInterface $field_definition) {
        return [
            'columns' => [
                'value' => [
                    'type' => 'float',
                ],
            ],
            'indexes' => [
                'value' => [
                    'value',
                ],
            ],
        ];
    }
    
    /**
     * {@inheritdoc}
     */
    protected function allowedValuesDescription() {
        $description = '<p>' . $this->t('The name will be used in displayed options and edit forms. The value is the stored value, and must be numeric.') . '</p>';
        $description .= '<p>' . $this->t('Allowed HTML tags in labels: @tags', [
            '@tags' => FieldFilteredMarkup::displayAllowedTags(),
        ]) . '</p>';
        return $description;
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function extractAllowedValues($string, $has_data) {
        $values = parent::extractAllowedValues($string, $has_data);
        if ($values) {
            $keys = array_keys($values);
            $labels = array_values($values);
            $keys = array_map(function ($key) {
                // Float keys are represented as strings and need to be disambiguated
                // ('.5' is '0.5').
                return is_numeric($key) ? (string) (double) $key : $key;
            }, $keys);
            return array_combine($keys, $labels);
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function validateAllowedValue($option) {
        if (!is_numeric($option)) {
            return new TranslatableMarkup('Allowed values list: each key must be a valid integer or decimal.');
        }
    }
    
    /**
     * {@inheritdoc}
     */
    public static function simplifyAllowedValues(array $structured_values) {
        $values = [];
        foreach ($structured_values as $item) {
            // Nested elements are embedded in the label.
            if (is_array($item['label'])) {
                $item['label'] = static::simplifyAllowedValues($item['label']);
            }
            // Cast the value to a float first so that .5 and 0.5 are the same value
            // and then cast to a string so that values like 0.5 can be used as array
            // keys.
            // @see http://php.net/manual/language.types.array.php
            $values[(string) (double) $item['value']] = $item['label'];
        }
        return $values;
    }
    
    /**
     * {@inheritdoc}
     */
    protected static function castAllowedValue($value) {
        return (double) $value;
    }
    
    /**
     * {@inheritdoc}
     */
    public function storageSettingsForm(array &$form, FormStateInterface $form_state, $has_data) {
        $element = parent::storageSettingsForm($form, $form_state, $has_data);
        foreach (Element::children($element['allowed_values']['table']) as $delta => $row) {
            // @see https://developer.mozilla.org/en-US/docs/Web/HTML/Element/input/number
            // @see \Drupal\Core\Field\Plugin\Field\FieldWidget\NumberWidget::formElement()
            $element['allowed_values']['table'][$delta]['item']['key']['#step'] = 'any';
            $element['allowed_values']['table'][$delta]['item']['key']['#type'] = 'number';
        }
        return $element;
    }

}