我试图从我在小部件中编写的自定义函数访问$instance变量，但我一直收到一个“变量未定义”错误。

如何重新创建$实例变量？

public function __construct() {

    parent::__construct(
        \'cst_twitter_widget\',
        __( \'CST Twitter Tweets\', \'chicagosuntimes\' ),
        array(
            \'description\' => __( \'Displays Tweets from a given list url.\', \'mythemename\' ),
        )
    );

    add_action( \'wp_head\', array( $this, \'cst_twitter_feed_ajaxurl\' ) );
    add_action( \'wp_ajax_refresh_tweets\', array( $this, \'refresh_tweets\' ) );

}

/**
 * Set the AJAX URL for the Twitter Feed Widget
 */
public function cst_twitter_feed_ajaxurl() {
?>
    <script type="text/javascript">
    var ajaxurl = \'<?php echo admin_url(\'admin-ajax.php\'); ?>\';
    </script>
<?php
}

/**
 * Display the updated Twitter Feed
 */
public function refresh_tweets() {

    //my custom function, where i\'m trying to re-create the $instance and access the values
    $instance = self::$instance;
    echo $instance[\'cst_twitter_screen_name\']; //the variable i\'m trying to access
    die();
}

public function widget( $args, $instance ) {
    //the widget instance
}

public function form( $instance ) {

    //the form
}

public function update( $new_instance, $old_instance ) {

    $instance = $old_instance;
    $instance[\'cst_twitter_screen_name\'] = $new_instance[\'cst_twitter_screen_name\'];

    return $instance;

}