(FFOS Gecko & Gaia) OTA - Do real check

checkForUpdates: function UC_checkForUpdates(listener, force) {
  LOG("Checker: checkForUpdates, force: " + force);
  if (!listener)
    throw Cr.NS_ERROR_NULL_POINTER;

  // 通知‘update-check-start‘事件，监听者是谁呢？还记得上一篇分析的UpdatePrompt实现了nsIObserver吗？
  Services.obs.notifyObservers(null, "update-check-start", null);

　// 这个函数很重要，它通过获取很多settings和preferences，还会通过libutils库，来获取很多系统参数，拼接成一个url，
  // 这个url就是OTA server URL
  var url = this.getUpdateURL(force);
  if (!url || (!this.enabled && !force))
    return;

  // 通过XHR发送GET请求，获取update信息
  this._request = Cc["@mozilla.org/xmlextras/xmlhttprequest;1"].
                  createInstance(Ci.nsISupports);
  // This is here to let unit test code override XHR
  if (this._request.wrappedJSObject) {
    this._request = this._request.wrappedJSObject;
  }
  this._request.open("GET", url, true);
  var allowNonBuiltIn = !getPref("getBoolPref",
                                 PREF_APP_UPDATE_CERT_REQUIREBUILTIN, true);
  this._request.channel.notificationCallbacks = new gCertUtils.BadCertHandler(allowNonBuiltIn);
  // Prevent the request from reading from the cache.
  this._request.channel.loadFlags |= Ci.nsIRequest.LOAD_BYPASS_CACHE;
  // Prevent the request from writing to the cache.
  this._request.channel.loadFlags |= Ci.nsIRequest.INHIBIT_CACHING;

  this._request.overrideMimeType("text/xml");
  // The Cache-Control header is only interpreted by proxies and the
  // final destination. It does not help if a resource is already
  // cached locally.
  this._request.setRequestHeader("Cache-Control", "no-cache");
  // HTTP/1.0 servers might not implement Cache-Control and
  // might only implement Pragma: no-cache
  this._request.setRequestHeader("Pragma", "no-cache");

  var self = this;
  // 设置XHR event listener
  this._request.addEventListener("error", function(event) { self.onError(event); } ,false);
  this._request.addEventListener("load", function(event) { self.onLoad(event); }, false);

  LOG("Checker:checkForUpdates - sending request to: " + url);
  this._request.send(null);
  // 将传进来的listener保存下来，在XHR的event handler中回调
  this._callback = listener;
},

onLoad: function UC_onLoad(event) {
  LOG("Checker:onLoad - request completed downloading document");

  var prefs = Services.prefs;
  var certs = null;
  if (!getPref("getCharPref", PREF_APP_UPDATE_URL_OVERRIDE, null) &&
      getPref("getBoolPref", PREF_APP_UPDATE_CERT_CHECKATTRS, true)) {
    certs = gCertUtils.readCertPrefs(PREF_APP_UPDATE_CERTS_BRANCH);
  }

  try {
    // Analyze the resulting DOM and determine the set of updates.
　　// 之前没有找到对于request response解析的部分，其实秘密就在于这里，Checker中并没有显示的定义_updates属性，而是定义了一个get _updates()函数。
   // 这是js的特性，如果定义了get/set property()函数，就表示可以get/set这个以property命名的属性。

    var updates = this._updates;
    LOG("Checker:onLoad - number of updates available: " + updates.length);
    var allowNonBuiltIn = !getPref("getBoolPref",
                                   PREF_APP_UPDATE_CERT_REQUIREBUILTIN, true);
    gCertUtils.checkCert(this._request.channel, allowNonBuiltIn, certs);

    if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_CERT_ERRORS))
      Services.prefs.clearUserPref(PREF_APP_UPDATE_CERT_ERRORS);

    if (Services.prefs.prefHasUserValue(PREF_APP_UPDATE_BACKGROUNDERRORS))
      Services.prefs.clearUserPref(PREF_APP_UPDATE_BACKGROUNDERRORS);

    // Tell the callback about the updates


    this._callback.onCheckComplete(event.target, updates, updates.length);
  }
  catch (e) {
    LOG("Checker:onLoad - there was a problem checking for updates. " +
        "Exception: " + e);
    var request = event.target;
    var status = this._getChannelStatus(request);
    LOG("Checker:onLoad - request.status: " + status);
    var update = new Update(null);
    update.errorCode = status;
    update.statusText = getStatusTextFromCode(status, 404);

    if (this._isHttpStatusCode(status)) {
      update.errorCode = HTTP_ERROR_OFFSET + status;
    }
    if (e.result && e.result == Cr.NS_ERROR_ILLEGAL_VALUE) {
      update.errorCode = updates[0] ? CERT_ATTR_CHECK_FAILED_HAS_UPDATE
                                    : CERT_ATTR_CHECK_FAILED_NO_UPDATE;
    }

    this._callback.onError(request, update);
  }

  this._callback = null;
  this._request = null;
},

  /**
　　* Returns an array of nsIUpdate objects discovered by the update check.
　　* @throws if the XML document element node name is not updates.
　　*/

get _updates() {
  var updatesElement = this._request.responseXML.documentElement;
  if (!updatesElement) {
    LOG("Checker:_updates get - empty updates document?!");
    return [];
  }

  if (updatesElement.nodeName != "updates") {
    LOG("Checker:_updates get - unexpected node name!");
    throw new Error("Unexpected node name, expected: updates, got: " +
                    updatesElement.nodeName);
  }

  const ELEMENT_NODE = Ci.nsIDOMNode.ELEMENT_NODE;
  var updates = [];
  for (var i = 0; i < updatesElement.childNodes.length; ++i) {
    var updateElement = updatesElement.childNodes.item(i);
    if (updateElement.nodeType != ELEMENT_NODE ||
        updateElement.localName != "update")
      continue;

    updateElement.QueryInterface(Ci.nsIDOMElement);
    let update;
    try {
      update = new Update(updateElement);
    } catch (e) {
      LOG("Checker:_updates get - invalid <update/>, ignoring...");
      continue;
    }
    update.serviceURL = this.getUpdateURL(this._forced);
    update.channel = UpdateChannel.get();
    updates.push(update);
  }
  return updates;
},