class Element

class Element {
    
    /**
     * Checks if the key is a property.
     *
     * @param string $key
     *   The key to check.
     *
     * @return bool
     *   TRUE of the key is a property, FALSE otherwise.
     */
    public static function property($key) {
        return $key[0] == '#';
    }
    
    /**
     * Gets properties of a structured array element (keys beginning with '#').
     *
     * @param array $element
     *   An element array to return properties for.
     *
     * @return array
     *   An array of property keys for the element.
     */
    public static function properties(array $element) {
        return array_filter(array_keys($element), 'static::property');
    }
    
    /**
     * Checks if the key is a child.
     *
     * @param string $key
     *   The key to check.
     *
     * @return bool
     *   TRUE if the element is a child, FALSE otherwise.
     */
    public static function child($key) {
        return !isset($key[0]) || $key[0] != '#';
    }
    
    /**
     * Identifies the children of an element array, optionally sorted by weight.
     *
     * The children of a element array are those key/value pairs whose key does
     * not start with a '#'. See drupal_render() for details.
     *
     * @param array $elements
     *   The element array whose children are to be identified. Passed by
     *   reference.
     * @param bool $sort
     *   Boolean to indicate whether the children should be sorted by weight.
     *
     * @return array
     *   The array keys of the element's children.
     */
    public static function children(array &$elements, $sort = FALSE) {
        // Do not attempt to sort elements which have already been sorted.
        $sort = isset($elements['#sorted']) ? !$elements['#sorted'] : $sort;
        // Filter out properties from the element, leaving only children.
        $count = count($elements);
        $child_weights = [];
        $i = 0;
        $sortable = FALSE;
        foreach ($elements as $key => $value) {
            if (is_int($key) || $key === '' || $key[0] !== '#') {
                if (is_array($value)) {
                    if (isset($value['#weight'])) {
                        $weight = $value['#weight'];
                        $sortable = TRUE;
                    }
                    else {
                        $weight = 0;
                    }
                    // Supports weight with up to three digit precision and conserve
                    // the insertion order.
                    $child_weights[$key] = floor($weight * 1000) + $i / $count;
                }
                elseif (isset($value)) {
                    trigger_error(new FormattableMarkup('"@key" is an invalid render array key', [
                        '@key' => $key,
                    ]), E_USER_ERROR);
                }
            }
            $i++;
        }
        // Sort the children if necessary.
        if ($sort && $sortable) {
            asort($child_weights);
            // Put the sorted children back into $elements in the correct order, to
            // preserve sorting if the same element is passed through
            // \Drupal\Core\Render\Element::children() twice.
            foreach ($child_weights as $key => $weight) {
                $value = $elements[$key];
                unset($elements[$key]);
                $elements[$key] = $value;
            }
            $elements['#sorted'] = TRUE;
        }
        return array_keys($child_weights);
    }
    
    /**
     * Returns the visible children of an element.
     *
     * @param array $elements
     *   The parent element.
     *
     * @return array
     *   The array keys of the element's visible children.
     */
    public static function getVisibleChildren(array $elements) {
        $visible_children = [];
        foreach (static::children($elements) as $key) {
            $child = $elements[$key];
            // Skip value and hidden elements, since they are not rendered.
            if (!static::isVisibleElement($child)) {
                continue;
            }
            $visible_children[$key] = $child;
        }
        return array_keys($visible_children);
    }
    
    /**
     * Determines if an element is visible.
     *
     * @param array $element
     *   The element to check for visibility.
     *
     * @return bool
     *   TRUE if the element is visible, otherwise FALSE.
     */
    public static function isVisibleElement($element) {
        return (!isset($element['#type']) || !in_array($element['#type'], [
            'value',
            'hidden',
            'token',
        ])) && (!isset($element['#access']) || ($element['#access'] instanceof AccessResultInterface && $element['#access']->isAllowed() || $element['#access'] === TRUE));
    }
    
    /**
     * Sets HTML attributes based on element properties.
     *
     * @param array $element
     *   The renderable element to process. Passed by reference.
     * @param array $map
     *   An associative array whose keys are element property names and whose
     *   values are the HTML attribute names to set on the corresponding
     *   property; e.g., array('#propertyname' => 'attributename'). If both names
     *   are identical except for the leading '#', then an attribute name value is
     *   sufficient and no property name needs to be specified.
     */
    public static function setAttributes(array &$element, array $map) {
        foreach ($map as $property => $attribute) {
            // If the key is numeric, the attribute name needs to be taken over.
            if (is_int($property)) {
                $property = '#' . $attribute;
            }
            // Do not overwrite already existing attributes.
            if (isset($element[$property]) && !isset($element['#attributes'][$attribute])) {
                $element['#attributes'][$attribute] = $element[$property];
            }
        }
    }
    
    /**
     * Indicates whether the given element is empty.
     *
     * An element that only has #cache set is considered empty, because it will
     * render to the empty string.
     *
     * @param array $elements
     *   The element.
     *
     * @return bool
     *   Whether the given element is empty.
     */
    public static function isEmpty(array $elements) {
        return empty($elements) || count($elements) === 1 && array_keys($elements) === [
            '#cache',
        ];
    }

}