在greasemonkey脚本中使用谷歌日历

Question

我正在尝试在greasemonkey脚本中使用google calender api。这是我的代码灵感来自google calender api教程：

// ==UserScript==
// @name        Google Calender
// @namespace   xyz
// @include     *
// @version     1
// ==/UserScript==

var $ = unsafeWindow.jQuery;

API_js_callback = "https://apis.google.com/js/api.js";

var script = document.createElement('script');
script.src = API_js_callback;
var head = document.getElementsByTagName("head")[0];
(head || document.body).appendChild(script);

//this is not working
var gapi = unsafeWindow.gapi;


$(document).ready(function()
                  {
    // Client ID and API key from the Developer Console
    var CLIENT_ID = 'XXX';

    // Array of API discovery doc URLs for APIs used by the quickstart
    var DISCOVERY_DOCS = ["https://www.googleapis.com/discovery/v1/apis/calendar/v3/rest"];

    // Authorization scopes required by the API; multiple scopes can be
    // included, separated by spaces.
    var SCOPES = "https://www.googleapis.com/auth/calendar.readonly";

    handleClientLoad();

    /**
       *  On load, called to load the auth2 library and API client library.
       */
    function handleClientLoad() {
        gapi.load('client:auth2', initClient);
    }

    /**
       *  Initializes the API client library and sets up sign-in state
       *  listeners.
       */
    function initClient() {
     .........
    }
});

在handleClientLoad方法中，我得到以下异常：

Cannot read property 'load' of undefined

有什么想法吗？