/**
  * Return an instance of this class.
  *
  * @since     1.0.0
  *
  * @return    object    A single instance of this class.
  */
 public static function get_instance()
 {
     // If the single instance hasn't been set, set it now.
     if (null == self::$instance) {
         self::$instance = new self();
     }
     return self::$instance;
 }
 /**
  * Initialize the plugin by loading admin scripts & styles and adding a
  * settings page and menu.
  *
  * @since     1.0.0
  */
 private function __construct()
 {
     // Call $plugin_slug from public plugin class.
     $plugin = Gravity_Forms_Math_Captcha::get_instance();
     $this->plugin_slug = $plugin->get_plugin_slug();
     // Add an admin notice if Gravity Forms isn't installed.
     add_action('admin_notices', array($this, 'admin_warning'));
     // Add the math captcha field to the Gravity Forms editor.
     add_filter('gform_add_field_buttons', array($this, 'add_math_captcha_field'));
     add_filter('gform_field_type_title', array($this, 'add_math_captcha_title'), 10, 2);
     add_action('gform_editor_js', array($this, 'math_captcha_gform_editor_js'));
     add_action('gform_field_standard_settings', array($this, 'math_captcha_settings'), 10, 2);
     add_filter('gform_tooltips', array($this, 'math_captcha_type_tooltip'));
 }