! This is a private undocumented service !

@name $browser
@requires $log
@description
This object has two goals:

- hide all the global state in the browser caused by the window object
- abstract away all the browser specific features and inconsistencies

For tests we provide {@link ngMock.$browser mock implementation} of the `$browser`
service, which can be used for convenient testing of the application without the interaction with
the real browser apis.
 
@param {object} window The global window object.
@param {object} document jQuery wrapped document.
@param {object} $log window.console or an object with the same interface.
@param {object} $sniffer $sniffer service
function Browser(window, document, $log, $sniffer, $$taskTrackerFactory) {
  var self = this,
      location = window.location,
      history = window.history,
      setTimeout = window.setTimeout,
      clearTimeout = window.clearTimeout,
      pendingDeferIds = {},
      taskTracker = $$taskTrackerFactory($log);

  self.isMock = false;

  //////////////////////////////////////////////////////////////
  // Task-tracking API
  //////////////////////////////////////////////////////////////

  // TODO(vojta): remove this temporary api
  self.$$completeOutstandingRequest = taskTracker.completeTask;
  self.$$incOutstandingRequestCount = taskTracker.incTaskCount;

  // TODO(vojta): prefix this method with $$ ?
  self.notifyWhenNoOutstandingRequests = taskTracker.notifyWhenNoPendingTasks;

  //////////////////////////////////////////////////////////////
  // URL API
  //////////////////////////////////////////////////////////////

  var cachedState, lastHistoryState,
      lastBrowserUrl = location.href,
      baseElement = document.find('base'),
      pendingLocation = null,
      getCurrentState = !$sniffer.history ? noop : function getCurrentState() {
        try {
          return history.state;
        } catch (e) {
          // MSIE can reportedly throw when there is no state (UNCONFIRMED).
        }
      };

  cacheState();

  /**
   * @name $browser#url
   *
   * @description
   * GETTER:
   * Without any argument, this method just returns current value of `location.href` (with a
   * trailing `#` stripped of if the hash is empty).
   *
   * SETTER:
   * With at least one argument, this method sets url to new value.
   * If html5 history api supported, `pushState`/`replaceState` is used, otherwise
   * `location.href`/`location.replace` is used.
   * Returns its own instance to allow chaining.
   *
   * NOTE: this api is intended for use only by the `$location` service. Please use the
   * {@link ng.$location $location service} to change url.
   *
   * @param {string} url New url (when used as setter)
   * @param {boolean=} replace Should new url replace current history record?
   * @param {object=} state State object to use with `pushState`/`replaceState`
   */
  self.url = function(url, replace, state) {
    // In modern browsers `history.state` is `null` by default; treating it separately
    // from `undefined` would cause `$browser.url('/foo')` to change `history.state`
    // to undefined via `pushState`. Instead, let's change `undefined` to `null` here.
    if (isUndefined(state)) {
      state = null;
    }

    // Android Browser BFCache causes location, history reference to become stale.
    if (location !== window.location) location = window.location;
    if (history !== window.history) history = window.history;

    // setter
    if (url) {
      var sameState = lastHistoryState === state;

      // Don't change anything if previous and current URLs and states match. This also prevents
      // IE<10 from getting into redirect loop when in LocationHashbangInHtml5Url mode.
      // See https://github.com/angular/angular.js/commit/ffb2701
      if (lastBrowserUrl === url && (!$sniffer.history || sameState)) {
        return self;
      }
      var sameBase = lastBrowserUrl && stripHash(lastBrowserUrl) === stripHash(url);
      lastBrowserUrl = url;
      lastHistoryState = state;
      // Don't use history API if only the hash changed
      // due to a bug in IE10/IE11 which leads
      // to not firing a `hashchange` nor `popstate` event
      // in some cases (see #9143).
      if ($sniffer.history && (!sameBase || !sameState)) {
        history[replace ? 'replaceState' : 'pushState'](state, '', url);
        cacheState();
      } else {
        if (!sameBase) {
          pendingLocation = url;
        }
        if (replace) {
          location.replace(url);
        } else if (!sameBase) {
          location.href = url;
        } else {
          location.hash = getHash(url);
        }
        if (location.href !== url) {
          pendingLocation = url;
        }
      }
      if (pendingLocation) {
        pendingLocation = url;
      }
      return self;
    // getter
    } else {
      // - pendingLocation is needed as browsers don't allow to read out
      //   the new location.href if a reload happened or if there is a bug like in iOS 9 (see
      //   https://openradar.appspot.com/22186109).
      return trimEmptyHash(pendingLocation || location.href);
    }
  };

  /**
   * @name $browser#state
   *
   * @description
   * This method is a getter.
   *
   * Return history.state or null if history.state is undefined.
   *
   * @returns {object} state
   */
  self.state = function() {
    return cachedState;
  };

  var urlChangeListeners = [],
      urlChangeInit = false;

  function cacheStateAndFireUrlChange() {
    pendingLocation = null;
    fireStateOrUrlChange();
  }

  // This variable should be used *only* inside the cacheState function.
  var lastCachedState = null;
  function cacheState() {
    // This should be the only place in $browser where `history.state` is read.
    cachedState = getCurrentState();
    cachedState = isUndefined(cachedState) ? null : cachedState;

    // Prevent callbacks fo fire twice if both hashchange & popstate were fired.
    if (equals(cachedState, lastCachedState)) {
      cachedState = lastCachedState;
    }

    lastCachedState = cachedState;
    lastHistoryState = cachedState;
  }

  function fireStateOrUrlChange() {
    var prevLastHistoryState = lastHistoryState;
    cacheState();

    if (lastBrowserUrl === self.url() && prevLastHistoryState === cachedState) {
      return;
    }

    lastBrowserUrl = self.url();
    lastHistoryState = cachedState;
    forEach(urlChangeListeners, function(listener) {
      listener(self.url(), cachedState);
    });
  }

  /**
   * @name $browser#onUrlChange
   *
   * @description
   * Register callback function that will be called, when url changes.
   *
   * It's only called when the url is changed from outside of AngularJS:
   * - user types different url into address bar
   * - user clicks on history (forward/back) button
   * - user clicks on a link
   *
   * It's not called when url is changed by $browser.url() method
   *
   * The listener gets called with new url as parameter.
   *
   * NOTE: this api is intended for use only by the $location service. Please use the
   * {@link ng.$location $location service} to monitor url changes in AngularJS apps.
   *
   * @param {function(string)} listener Listener function to be called when url changes.
   * @return {function(string)} Returns the registered listener fn - handy if the fn is anonymous.
   */
  self.onUrlChange = function(callback) {
    // TODO(vojta): refactor to use node's syntax for events
    if (!urlChangeInit) {
      // We listen on both (hashchange/popstate) when available, as some browsers don't
      // fire popstate when user changes the address bar and don't fire hashchange when url
      // changed by push/replaceState

      // html5 history api - popstate event
      if ($sniffer.history) jqLite(window).on('popstate', cacheStateAndFireUrlChange);
      // hashchange event
      jqLite(window).on('hashchange', cacheStateAndFireUrlChange);

      urlChangeInit = true;
    }

    urlChangeListeners.push(callback);
    return callback;
  };

  /**
   * @private
   * Remove popstate and hashchange handler from window.
   *
   * NOTE: this api is intended for use only by $rootScope.
   */
  self.$$applicationDestroyed = function() {
    jqLite(window).off('hashchange popstate', cacheStateAndFireUrlChange);
  };

  /**
   * Checks whether the url has changed outside of AngularJS.
   * Needs to be exported to be able to check for changes that have been done in sync,
   * as hashchange/popstate events fire in async.
   */
  self.$$checkUrlChange = fireStateOrUrlChange;

  //////////////////////////////////////////////////////////////
  // Misc API
  //////////////////////////////////////////////////////////////

  /**
   * @name $browser#baseHref
   *
   * @description
   * Returns current <base href>
   * (always relative - without domain)
   *
   * @returns {string} The current base href
   */
  self.baseHref = function() {
    var href = baseElement.attr('href');
    return href ? href.replace(/^(https?:)?\/\/[^/]*/, '') : '';
  };

  /**
   * @name $browser#defer
   * @param {function()} fn A function, who's execution should be deferred.
   * @param {number=} [delay=0] Number of milliseconds to defer the function execution.
   * @param {string=} [taskType=DEFAULT_TASK_TYPE] The type of task that is deferred.
   * @returns {*} DeferId that can be used to cancel the task via `$browser.defer.cancel()`.
   *
   * @description
   * Executes a fn asynchronously via `setTimeout(fn, delay)`.
   *
   * Unlike when calling `setTimeout` directly, in test this function is mocked and instead of using
   * `setTimeout` in tests, the fns are queued in an array, which can be programmatically flushed
   * via `$browser.defer.flush()`.
   *
   */
  self.defer = function(fn, delay, taskType) {
    var timeoutId;

    delay = delay || 0;
    taskType = taskType || taskTracker.DEFAULT_TASK_TYPE;

    taskTracker.incTaskCount(taskType);
    timeoutId = setTimeout(function() {
      delete pendingDeferIds[timeoutId];
      taskTracker.completeTask(fn, taskType);
    }, delay);
    pendingDeferIds[timeoutId] = taskType;

    return timeoutId;
  };


  /**
   * @name $browser#defer.cancel
   *
   * @description
   * Cancels a deferred task identified with `deferId`.
   *
   * @param {*} deferId Token returned by the `$browser.defer` function.
   * @returns {boolean} Returns `true` if the task hasn't executed yet and was successfully
   *                    canceled.
   */
  self.defer.cancel = function(deferId) {
    if (pendingDeferIds.hasOwnProperty(deferId)) {
      var taskType = pendingDeferIds[deferId];
      delete pendingDeferIds[deferId];
      clearTimeout(deferId);
      taskTracker.completeTask(noop, taskType);
      return true;
    }
    return false;
  };

}