WordPress选择选项值未保存在小部件中

Question

我正在创建一个WordPress小部件，其中的小部件标题和选择下拉选项可用。

以某种方式不保存下拉值。

代码：

if ( ! class_exists( 'ECP_Extended_Widget' ) ) {


    /**
     * Adds ECP_Widget widget.
     */
    class ECP_Extended_Widget extends WP_Widget {

        /**
         * Register widget with WordPress.
         */

        function __construct() {
            add_action( 'widgets_init', array( $this, 'register_ecp_widget' ) );
            parent::__construct(
                'ecp_widget', // Base ID
                esc_html__( 'Event Calendar Pro', 'event-calendar-pro' ),
                array( 'description' => esc_html__( 'Show and Filters Events', 'event-calendar-pro' ) )
            );
        }

        /**
         * Front-end display of widget.
         *
         * @see WP_Widget::widget()
         *
         * @param array $args Widget arguments.
         * @param array $instance Saved values from database.
         */
        public function widget( $widget_args, $instance ) {

            echo $widget_args['before_widget'];
            if ( ! empty( $instance['title'] ) ) {
                $widget_title = apply_filters( 'widget_title', $instance['title'] );
                echo '<button class="event-common-btn">' . $widget_title . '</button>';
            }

            $events_page = ecp_get_settings( 'events_page', 'calendar', 'event_calendar_pro_page_settings' );

            $date = get_query_var( 'date' );

            if ( empty( $date ) ) {
                $date = date( 'Y-m-d', current_time( 'timestamp' ) );
            }

            $args = array(
                'order_by' => 'event_start_date',
                'order'    => 'DESC',
            );

            if ( ! empty( $date ) ) {
                $args['start_date'] = $date;
            }

            $args = apply_filters('ecp_widget_events_query_args', $args);

            $events = ecp_get_event_list( $args );

            ob_start();
            ecp_get_template( 'widget/main.php', [ 'events' => $events, 'events_page' => $events_page ] );
            $html = ob_get_clean();

            echo $html;

            echo $widget_args['after_widget'];
        }

        /**
         * Back-end widget form.
         *
         * @see WP_Widget::form()
         *
         * @param array $instance Previously saved values from database.
         */
        public function form( $instance ) {
            $title = ! empty( $instance['title'] ) ? $instance['title'] : esc_html__( 'Calendar', 'event-calendar-pro' );
            $calendar_id = ! empty( $instance['calendar_id'] ) ? $instance['calendar_id'] : 'sidebar';
            ?>
            <p>
                <label for="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>">
                    <?php esc_attr_e( 'Title:', 'event-calendar-pro' ); ?>
                </label>
                <input class="widefat" id="<?php echo esc_attr( $this->get_field_id( 'title' ) ); ?>" name="<?php echo esc_attr( $this->get_field_name( 'title' ) ); ?>" type="text" value="<?php echo esc_attr( $title ); ?>">
            </p>
            <p>
                <label for="<?php echo esc_attr( $this->get_field_id( 'calendar_id' ) ); ?>">
                    <?php esc_attr_e( 'Calendar ID:', 'event-calendar-pro' ); ?>
                </label>
                <select id="<?php echo esc_attr( $this->get_field_id( 'calendar_id' ) ); ?>" name="<?php echo esc_attr( $this->get_field_name( 'calendar_id' ) ); ?>" type="text">
                    <option value="sidebar" <?php echo $calendar_id == 'sidebar' ? 'selected="selected"' : ''; ?>>Sidebar</option>
                    <option value="home" <?php echo $calendar_id == 'home' ? 'selected="selected"' : ''; ?>>Home</option>
                </select>
            </p>
            <?php
        }

        /**
         * Sanitize widget form values as they are saved.
         *
         * @see WP_Widget::update()
         *
         * @param array $new_instance Values just sent to be saved.
         * @param array $old_instance Previously saved values from database.
         *
         * @return array Updated safe values to be saved.
         */
        public function update( $new_instance, $old_instance ) {
            $instance          = array();
            var_dump( $new_instance );
            var_dump( $instance );
            $instance['title'] = ( ! empty( $new_instance['title'] ) ) ? sanitize_text_field( $new_instance['title'] ) : '';
            $instance['calendar_id'] = ( ! empty( $new_instance['calendar_id'] ) ) ? sanitize_text_field( $new_instance['calendar_id'] ) : '';

            return $instance;
        }

        // register ECP_Widget widget
        public function register_ecp_widget() {
            register_widget( 'ECP_Extended_Widget' );
        }

    }

    $ECP_Extended_Widget = new ECP_Extended_Widget();
}

我想念什么吗？