class FormHelper

class FormHelper {
    
    /**
     * Rewrites #states selectors in a render element.
     *
     * When a structure of elements is being altered, their HTML selectors may
     * change. In such cases calling this method will check if there are any
     * states in element and its children, and rewrite selectors in those states.
     *
     * @param array $elements
     *   A render array element having a #states property.
     * @param string $search
     *   A partial or entire jQuery selector string to replace in #states.
     * @param string $replace
     *   The string to replace all instances of $search with.
     *
     * @see self::processStates()
     */
    public static function rewriteStatesSelector(array &$elements, $search, $replace) {
        if (!empty($elements['#states'])) {
            foreach ($elements['#states'] as $state => $ids) {
                static::processStatesArray($elements['#states'][$state], $search, $replace);
            }
        }
        foreach (Element::children($elements) as $key) {
            static::rewriteStatesSelector($elements[$key], $search, $replace);
        }
    }
    
    /**
     * Helps recursively rewrite #states selectors.
     *
     * Not to be confused with self::processStates(), which just prepares states
     * for rendering.
     *
     * @param array $conditions
     *   States conditions array.
     * @param string $search
     *   A partial or entire jQuery selector string to replace in #states.
     * @param string $replace
     *   The string to replace all instances of $search with.
     *
     * @see self::rewriteStatesSelector()
     */
    protected static function processStatesArray(array &$conditions, $search, $replace) {
        // Retrieve the keys to make it easy to rename a key without changing the
        // order of an array.
        $keys = array_keys($conditions);
        $update_keys = FALSE;
        foreach ($conditions as $id => $values) {
            if (strpos($id, $search) !== FALSE) {
                $update_keys = TRUE;
                $new_id = str_replace($search, $replace, $id);
                // Replace the key and keep the array in the same order.
                $index = array_search($id, $keys, TRUE);
                $keys[$index] = $new_id;
            }
            elseif (is_array($values)) {
                static::processStatesArray($conditions[$id], $search, $replace);
            }
        }
        // Updates the states conditions keys if necessary.
        if ($update_keys) {
            $conditions = array_combine($keys, array_values($conditions));
        }
    }
    
    /**
     * Adds JavaScript to change the state of an element based on another element.
     *
     * A "state" means a certain property of a DOM element, such as "visible" or
     * "checked", which depends on a state or value of another element on the
     * page. In general, states are HTML attributes and DOM element properties,
     * which are applied initially, when page is loaded, depending on elements'
     * default values, and then may change due to user interaction.
     *
     * Since states are driven by JavaScript only, it is important to understand
     * that all states are applied on presentation only, none of the states force
     * any server-side logic, and that they will not be applied for site visitors
     * without JavaScript support. All modules implementing states have to make
     * sure that the intended logic also works without JavaScript being enabled.
     *
     * #states is an associative array in the form of:
     * @code
     * [
     *   STATE1 => CONDITIONS_ARRAY1,
     *   STATE2 => CONDITIONS_ARRAY2,
     *   ...
     * ]
     * @endcode
     * Each key is the name of a state to apply to the element, such as 'visible'.
     * Each value is a list of conditions that denote when the state should be
     * applied.
     *
     * Multiple different states may be specified to act on complex conditions:
     * @code
     * [
     *   'visible' => CONDITIONS,
     *   'checked' => OTHER_CONDITIONS,
     * ]
     * @endcode
     *
     * Every condition is a key/value pair, whose key is a jQuery selector that
     * denotes another element on the page, and whose value is an array of
     * conditions, which must bet met on that element:
     * @code
     * [
     *   'visible' => [
     *     JQUERY_SELECTOR => REMOTE_CONDITIONS,
     *     JQUERY_SELECTOR => REMOTE_CONDITIONS,
     *     ...
     *   ],
     * ]
     * @endcode
     * All conditions must be met for the state to be applied.
     *
     * Each remote condition is a key/value pair specifying conditions on the
     * other element that need to be met to apply the state to the element:
     * @code
     * [
     *   'visible' => [
     *     ':input[name="remote_checkbox"]' => ['checked' => TRUE],
     *   ],
     * ]
     * @endcode
     *
     * For example, to show a textfield only when a checkbox is checked:
     * @code
     * $form['toggle_me'] = [
     *   '#type' => 'checkbox',
     *   '#title' => t('Tick this box to type'),
     * ];
     * $form['settings'] = [
     *   '#type' => 'textfield',
     *   '#states' => [
     *     // Only show this field when the 'toggle_me' checkbox is enabled.
     *     'visible' => [
     *       ':input[name="toggle_me"]' => ['checked' => TRUE],
     *     ],
     *   ],
     * ];
     * @endcode
     *
     * The following states may be applied to an element:
     * - enabled
     * - disabled
     * - required
     * - optional
     * - visible
     * - invisible
     * - checked
     * - unchecked
     * - expanded
     * - collapsed
     *
     * The following states may be used in remote conditions:
     * - empty
     * - filled
     * - checked
     * - unchecked
     * - expanded
     * - collapsed
     * - value
     *
     * The following states exist for both elements and remote conditions, but are
     * not fully implemented and may not change anything on the element:
     * - relevant
     * - irrelevant
     * - valid
     * - invalid
     * - touched
     * - untouched
     * - readwrite
     * - readonly
     *
     * When referencing select lists and radio buttons in remote conditions, a
     * 'value' condition must be used:
     * @code
     *   '#states' => [
     *     // Show the settings if 'bar' has been selected for 'foo'.
     *     'visible' => [
     *       ':input[name="foo"]' => ['value' => 'bar'],
     *     ],
     *   ],
     * @endcode
     *
     * @param array $elements
     *   A render array element having a #states property as described above.
     *
     * @see \Drupal\form_test\Form\JavascriptStatesForm
     * @see \Drupal\FunctionalJavascriptTests\Core\Form\JavascriptStatesTest
     */
    public static function processStates(array &$elements) {
        $elements['#attached']['library'][] = 'core/drupal.states';
        // Elements of '#type' => 'item' are not actual form input elements, but we
        // still want to be able to show/hide them. Since there's no actual HTML
        // input element available, setting #attributes does not make sense, but a
        // wrapper is available, so setting #wrapper_attributes makes it work.
        $key = $elements['#type'] == 'item' ? '#wrapper_attributes' : '#attributes';
        $elements[$key]['data-drupal-states'] = Json::encode($elements['#states']);
    }

}