WP_Widget::form_callback( int|array $widget_args = 1 )

	public function form_callback( $widget_args = 1 ) {
		if ( is_numeric($widget_args) )
			$widget_args = array( 'number' => $widget_args );

		$widget_args = wp_parse_args( $widget_args, array( 'number' => -1 ) );
		$all_instances = $this->get_settings();

		if ( -1 == $widget_args['number'] ) {
			// We echo out a form where 'number' can be set later
			$this->_set('__i__');
			$instance = array();
		} else {
			$this->_set($widget_args['number']);
			$instance = $all_instances[ $widget_args['number'] ];
		}

		/**
		 * Filters the widget instance's settings before displaying the control form.
		 *
		 * Returning false effectively short-circuits display of the control form.
		 *
		 * @since WP-2.8.0
		 *
		 * @param array     $instance The current widget instance's settings.
		 * @param WP_Widget $this     The current widget instance.
		 */
		$instance = apply_filters( 'widget_form_callback', $instance, $this );

		$return = null;
		if ( false !== $instance ) {
			$return = $this->form($instance);

			/**
			 * Fires at the end of the widget control form.
			 *
			 * Use this hook to add extra fields to the widget form. The hook
			 * is only fired if the value passed to the 'widget_form_callback'
			 * hook is not false.
			 *
			 * Note: If the widget has no form, the text echoed from the default
			 * form method can be hidden using CSS.
			 *
			 * @since WP-2.8.0
			 *
			 * @param WP_Widget $this     The widget instance (passed by reference).
			 * @param null      $return   Return null if new fields are added.
			 * @param array     $instance An array of the widget's settings.
			 */
			do_action_ref_array( 'in_widget_form', array( &$this, &$return, $instance ) );
		}
		return $return;
	}