AttributeBoolean example

    if ($name == 'class' && !is_array($value)) {
      // Cast the value to string in case it implements MarkupInterface.       $value = [(string) $value];
    }
    if (is_array($value)) {
      // Cast the value to an array if the value was passed in as a string.       // @todo Decide to fix all the broken instances of class as a string       // in core or cast them.       $value = new AttributeArray($name, $value);
    }
    elseif (is_bool($value)) {
      $value = new AttributeBoolean($name, $value);
    }
    // As a development aid, we allow the value to be a safe string object.     elseif ($value instanceof MarkupInterface) {
      // Attributes are not supposed to display HTML markup, so we just convert       // the value to plain text.       $value = PlainTextOutput::renderFromHtml($value);
      $value = new AttributeString($name, $value);
    }
    elseif (!is_object($value)) {
      $value = new AttributeString($name, $value);
    }