我不断得到回应0 而不是string, 这是我的功用。为什么？

代码来自functions.php

// enque test.js
function add_script() {
    wp_enqueue_script(
        \'test\',
        get_stylesheet_directory_uri() . \'/js/test.js\',
        array( \'jquery\' )
    );
    wp_localize_script(\'test\', \'ajax_object\', array( \'ajax_url\' => admin_url( \'admin-ajax.php\' )));
}
add_action( \'wp_enqueue_scripts\', \'add_script\' );

// ajax function
function my_AJAX_processing_function(){
   echo \'string\';
}
add_action(\'wp_ajax_nopriv_ACTION_NAME\', \'my_AJAX_processing_function\');

$(function () {

    var data = {
        action: \'ACTION_NAME\'
    };

    jQuery.post(ajax_object.ajax_url, data, function(response) {
        alert(\'Got this from the server: \' + response);
    });

});