johan/GM_queryContentVar.js

## GM_queryContentVar.js
// Query page javascript for the identifier "name", and call callback(value),
// when found, or undefined, if not found or some error occurred. This works
// only for values that can be JSON serialized -- numbers, strings, booleans,
// null, or nested structures like Arrays and Objects that only contain above
// mentioned types of data.
function queryContentVar(name, callback) {
  // makes a random 20-char lowercase id
  function random() {
    var rand = '';
    while (rand.length < 20)
      rand += String.fromCharCode(97 + Math.random() * 26 | 0);
    return rand;
  }

  // Creates a mostly anonymous <meta> tag in the <head> section, giving it a
  // secret id we'll use for messaging back and forth with content space, and
  // fires pageInit() in the page scope (to listen and respond to queries from
  // us about variables) and registers reply() for answers from it at this end.
  function privInit() {
    var node = document.createElement('meta'), secret,
        head = document.getElementsByTagName('head')[0];
    node.id = secret = random();
    node.addEventListener(id, reply, false);
    head.appendChild(node);
    location.href = 'javascript:('+ pageInit +')('+ JSON.stringify(secret) +')';
    return node;
  }

  // Grabs the meta tag, anonymizes it and listens for incoming queries from us.
  function pageInit(secret) {
    // Replaces given <meta content="variable.name"> with <meta content="value">
    // (after JSON encoding it) and then alerts the caller to grab the response.
    function reply() {
      var name = node.getAttribute('content'),
          mesg = document.createEvent('Events'),
          val;
      try {
        val = eval(node.getAttribute('content'));
        val = JSON.stringify(val);
      } catch(e) { val = ''; }

      node.setAttribute('content', val);
      mesg.initEvent(secret, true, false);
      node.dispatchEvent(mesg);
    }

    var node = document.getElementById(secret);
    node.addEventListener(secret + '?', reply, false);
    node.removeAttribute('id');
  }

  var self = queryContentVar,
      node = self.node = self.node || privInit(),
        id = self.secret = self.secret || node.id,
      mesg = document.createEvent('Events');

  mesg.initEvent(secret + '?', true, false);
  node.setAttribute('content', name);
  self.callback = callback;
  node.dispatchEvent(mesg);

  // Picks up the response from content space, decodes it, passes it on to given
  // response callback and resets everything (just in case) to handle new calls.
  function reply() {
    var cb = self.callback,
        is = node.getAttribute('content');
    if (cb) cb(is === '' ? undefined : JSON.parse(is));
    node.removeAttribute('name');
    delete self.callback;
  }
}
	// Query page javascript for the identifier "name", and call callback(value),
	// when found, or undefined, if not found or some error occurred. This works
	// only for values that can be JSON serialized -- numbers, strings, booleans,
	// null, or nested structures like Arrays and Objects that only contain above
	// mentioned types of data.
	function queryContentVar(name, callback) {
	// makes a random 20-char lowercase id
	function random() {
	var rand = '';
	while (rand.length < 20)
	rand += String.fromCharCode(97 + Math.random() * 26 \| 0);
	return rand;
	}

	// Creates a mostly anonymous <meta> tag in the <head> section, giving it a
	// secret id we'll use for messaging back and forth with content space, and
	// fires pageInit() in the page scope (to listen and respond to queries from
	// us about variables) and registers reply() for answers from it at this end.
	function privInit() {
	var node = document.createElement('meta'), secret,
	head = document.getElementsByTagName('head')[0];
	node.id = secret = random();
	node.addEventListener(id, reply, false);
	head.appendChild(node);
	location.href = 'javascript:('+ pageInit +')('+ JSON.stringify(secret) +')';
	return node;
	}

	// Grabs the meta tag, anonymizes it and listens for incoming queries from us.
	function pageInit(secret) {
	// Replaces given <meta content="variable.name"> with <meta content="value">
	// (after JSON encoding it) and then alerts the caller to grab the response.
	function reply() {
	var name = node.getAttribute('content'),
	mesg = document.createEvent('Events'),
	val;
	try {
	val = eval(node.getAttribute('content'));
	val = JSON.stringify(val);
	} catch(e) { val = ''; }

	node.setAttribute('content', val);
	mesg.initEvent(secret, true, false);
	node.dispatchEvent(mesg);
	}

	var node = document.getElementById(secret);
	node.addEventListener(secret + '?', reply, false);
	node.removeAttribute('id');
	}

	var self = queryContentVar,
	node = self.node = self.node \|\| privInit(),
	id = self.secret = self.secret \|\| node.id,
	mesg = document.createEvent('Events');

	mesg.initEvent(secret + '?', true, false);
	node.setAttribute('content', name);
	self.callback = callback;
	node.dispatchEvent(mesg);

	// Picks up the response from content space, decodes it, passes it on to given
	// response callback and resets everything (just in case) to handle new calls.
	function reply() {
	var cb = self.callback,
	is = node.getAttribute('content');
	if (cb) cb(is === '' ? undefined : JSON.parse(is));
	node.removeAttribute('name');
	delete self.callback;
	}
	}