class AttributeValueBase

abstract class AttributeValueBase {
    
    /**
     * Renders '$name=""' if $value is an empty string.
     *
     * @see \Drupal\Core\Template\AttributeValueBase::render()
     */
    const RENDER_EMPTY_ATTRIBUTE = TRUE;
    
    /**
     * The value itself.
     *
     * @var mixed
     */
    protected $value;
    
    /**
     * The name of the value.
     *
     * @var mixed
     */
    protected $name;
    
    /**
     * Constructs a \Drupal\Core\Template\AttributeValueBase object.
     */
    public function __construct($name, $value) {
        $this->name = $name;
        $this->value = $value;
    }
    
    /**
     * Returns a string representation of the attribute.
     *
     * While __toString only returns the value in a string form, render()
     * contains the name of the attribute as well.
     *
     * @return string
     *   The string representation of the attribute.
     */
    public function render() {
        $value = (string) $this;
        if (isset($this->value) && static::RENDER_EMPTY_ATTRIBUTE || !empty($value)) {
            return Html::escape($this->name) . '="' . $value . '"';
        }
    }
    
    /**
     * Returns the raw value.
     */
    public function value() {
        return $this->value;
    }
    
    /**
     * Implements the magic __toString() method.
     */
    public abstract function __toString();

}