Bug 1463878 - Add SessionStateAggregator.js (mostly lifted from desktop code with modifications to reflect GV's needs) and code to send incremental session storage updates to Java. r=snorp,JanH
authorDylan Roeh <droeh@mozilla.com>
Fri, 22 Mar 2019 00:51:11 +0000
changeset 465570 51b9ea3597634038f0b5df7ac79f3476d3287785
parent 465569 a1c5e97960d018765655172190d7661c89039694
child 465571 f857b15c8d3c48cda12f50db07445b4a04ce1743
push id35744
push userapavel@mozilla.com
push dateFri, 22 Mar 2019 16:44:08 +0000
treeherdermozilla-central@e66a2b59914d [default view] [failures only]
perfherder[talos] [build metrics] [platform microbench] (compared to previous push)
reviewerssnorp, JanH
bugs1463878
milestone68.0a1
first release with
nightly linux32
nightly linux64
nightly mac
nightly win32
nightly win64
last release without
nightly linux32
nightly linux64
nightly mac
nightly win32
nightly win64
Bug 1463878 - Add SessionStateAggregator.js (mostly lifted from desktop code with modifications to reflect GV's needs) and code to send incremental session storage updates to Java. r=snorp,JanH Differential Revision: https://phabricator.services.mozilla.com/D23696
mobile/android/chrome/geckoview/GeckoViewProgressChild.js
mobile/android/chrome/geckoview/SessionStateAggregator.js
mobile/android/chrome/geckoview/geckoview.js
mobile/android/chrome/geckoview/jar.mn
mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
--- a/mobile/android/chrome/geckoview/GeckoViewProgressChild.js
+++ b/mobile/android/chrome/geckoview/GeckoViewProgressChild.js
@@ -253,11 +253,10 @@ const ProgressTracker = {
     data.prev = progress;
 
     if (progress >= 100) {
       PAGE_LOAD_PROGRESS_PROBE.finish();
     }
   },
 };
 
-
 const {debug, warn} = GeckoViewProgressChild.initLogging("GeckoViewProgress"); // eslint-disable-line no-unused-vars
 const module = GeckoViewProgressChild.create(this);
new file mode 100644
--- /dev/null
+++ b/mobile/android/chrome/geckoview/SessionStateAggregator.js
@@ -0,0 +1,595 @@
+/* -*- indent-tabs-mode: nil; js-indent-level: 2 -*- */
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+const {GeckoViewChildModule} = ChromeUtils.import("resource://gre/modules/GeckoViewChildModule.jsm");
+ChromeUtils.import("resource://gre/modules/XPCOMUtils.jsm", this);
+ChromeUtils.import("resource://gre/modules/Timer.jsm", this);
+const {Services} = ChromeUtils.import("resource://gre/modules/Services.jsm", this);
+
+ChromeUtils.defineModuleGetter(this, "SessionHistory",
+  "resource://gre/modules/sessionstore/SessionHistory.jsm");
+
+const NO_INDEX = Number.MAX_SAFE_INTEGER;
+const LAST_INDEX = Number.MAX_SAFE_INTEGER - 1;
+const DEFAULT_INTERVAL_MS = 1500;
+
+// This pref controls whether or not we send updates to the parent on a timeout
+// or not, and should only be used for tests or debugging.
+const TIMEOUT_DISABLED_PREF = "browser.sessionstore.debug.no_auto_updates";
+
+const PREF_INTERVAL = "browser.sessionstore.interval";
+
+class Handler {
+  constructor(store) {
+    this.store = store;
+  }
+
+  get mm() {
+    return this.store.mm;
+  }
+
+  get eventDispatcher() {
+    return this.store.eventDispatcher;
+  }
+
+  get messageQueue() {
+    return this.store.messageQueue;
+  }
+
+  get stateChangeNotifier() {
+    return this.store.stateChangeNotifier;
+  }
+}
+
+/**
+ * Listens for state change notifcations from webProgress and notifies each
+ * registered observer for either the start of a page load, or its completion.
+ */
+class StateChangeNotifier extends Handler {
+  constructor(store) {
+    super(store);
+
+    this._observers = new Set();
+    const ifreq = this.mm.docShell.QueryInterface(Ci.nsIInterfaceRequestor);
+    const webProgress = ifreq.getInterface(Ci.nsIWebProgress);
+    webProgress.addProgressListener(this, Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
+  }
+
+  /**
+   * Adds a given observer |obs| to the set of observers that will be notified
+   * when when a new document starts or finishes loading.
+   *
+   * @param obs (object)
+   */
+  addObserver(obs) {
+    this._observers.add(obs);
+  }
+
+  /**
+   * Notifies all observers that implement the given |method|.
+   *
+   * @param method (string)
+   */
+  notifyObservers(method) {
+    for (let obs of this._observers) {
+      if (typeof obs[method] == "function") {
+        obs[method]();
+      }
+    }
+  }
+
+  /**
+   * @see nsIWebProgressListener.onStateChange
+   */
+  onStateChange(webProgress, request, stateFlags, status) {
+    // Ignore state changes for subframes because we're only interested in the
+    // top-document starting or stopping its load.
+    if (!webProgress.isTopLevel || webProgress.DOMWindow != this.mm.content) {
+      return;
+    }
+
+    // onStateChange will be fired when loading the initial about:blank URI for
+    // a browser, which we don't actually care about. This is particularly for
+    // the case of unrestored background tabs, where the content has not yet
+    // been restored: we don't want to accidentally send any updates to the
+    // parent when the about:blank placeholder page has loaded.
+    if (!this.mm.docShell.hasLoadedNonBlankURI) {
+      return;
+    }
+
+    if (stateFlags & Ci.nsIWebProgressListener.STATE_START) {
+      this.notifyObservers("onPageLoadStarted");
+    } else if (stateFlags & Ci.nsIWebProgressListener.STATE_STOP) {
+      this.notifyObservers("onPageLoadCompleted");
+    }
+  }
+}
+StateChangeNotifier.prototype.QueryInterface =
+  ChromeUtils.generateQI([Ci.nsIWebProgressListener,
+                          Ci.nsISupportsWeakReference]);
+
+/**
+ * Listens for changes to the session history. Whenever the user navigates
+ * we will collect URLs and everything belonging to session history.
+ *
+ * Causes a SessionStore:update message to be sent that contains the current
+ * session history.
+ *
+ * Example:
+ *   {entries: [{url: "about:mozilla", ...}, ...], index: 1}
+ */
+class SessionHistoryListener extends Handler {
+  constructor(store) {
+    super(store);
+
+    this._fromIdx = NO_INDEX;
+
+
+    // The state change observer is needed to handle initial subframe loads.
+    // It will redundantly invalidate with the SHistoryListener in some cases
+    // but these invalidations are very cheap.
+    this.stateChangeNotifier.addObserver(this);
+
+    // By adding the SHistoryListener immediately, we will unfortunately be
+    // notified of every history entry as the tab is restored. We don't bother
+    // waiting to add the listener later because these notifications are cheap.
+    // We will likely only collect once since we are batching collection on
+    // a delay.
+    this.mm.docShell.QueryInterface(Ci.nsIWebNavigation)
+      .sessionHistory.legacySHistory.addSHistoryListener(this);
+
+    // Listen for page title changes.
+    this.mm.addEventListener("DOMTitleChanged", this);
+  }
+
+  uninit() {
+    const sessionHistory = this.mm.docShell.QueryInterface(Ci.nsIWebNavigation).sessionHistory;
+    if (sessionHistory) {
+      sessionHistory.legacySHistory.removeSHistoryListener(this);
+    }
+  }
+
+  collect() {
+    // We want to send down a historychange even for full collects in case our
+    // session history is a partial session history, in which case we don't have
+    // enough information for a full update. collectFrom(-1) tells the collect
+    // function to collect all data avaliable in this process.
+    if (this.mm.docShell) {
+      this.collectFrom(-1);
+    }
+  }
+
+  // History can grow relatively big with the nested elements, so if we don't have to, we
+  // don't want to send the entire history all the time. For a simple optimization
+  // we keep track of the smallest index from after any change has occured and we just send
+  // the elements from that index. If something more complicated happens we just clear it
+  // and send the entire history. We always send the additional info like the current selected
+  // index (so for going back and forth between history entries we set the index to LAST_INDEX
+  // if nothing else changed send an empty array and the additonal info like the selected index)
+  collectFrom(idx) {
+    if (this._fromIdx <= idx) {
+      // If we already know that we need to update history fromn index N we can ignore any changes
+      // tha happened with an element with index larger than N.
+      // Note: initially we use NO_INDEX which is MAX_SAFE_INTEGER which means we don't ignore anything
+      // here, and in case of navigation in the history back and forth we use LAST_INDEX which ignores
+      // only the subsequent navigations, but not any new elements added.
+      return;
+    }
+
+    this._fromIdx = idx;
+    this.messageQueue.push("historychange", () => {
+      if (this._fromIdx === NO_INDEX) {
+        return null;
+      }
+
+      let history = SessionHistory.collect(this.mm.docShell, this._fromIdx);
+      this._fromIdx = NO_INDEX;
+      return history;
+    });
+  }
+
+  handleEvent(event) {
+    this.collect();
+  }
+
+  onPageLoadCompleted() {
+    this.collect();
+  }
+
+  onPageLoadStarted() {
+    this.collect();
+  }
+
+  OnHistoryNewEntry(newURI, oldIndex) {
+    // We ought to collect the previously current entry as well, see bug 1350567.
+    // TODO: Reenable partial history collection for performance
+    // this.collectFrom(oldIndex);
+    this.collect();
+  }
+
+  OnHistoryGotoIndex(index, gotoURI) {
+    // We ought to collect the previously current entry as well, see bug 1350567.
+    // TODO: Reenable partial history collection for performance
+    // this.collectFrom(LAST_INDEX);
+    this.collect();
+  }
+
+  OnHistoryPurge(numEntries) {
+    this.collect();
+  }
+
+  OnHistoryReload(reloadURI, reloadFlags) {
+    this.collect();
+    return true;
+  }
+
+  OnHistoryReplaceEntry(index) {
+    this.collect();
+  }
+}
+SessionHistoryListener.prototype.QueryInterface =
+  ChromeUtils.generateQI([Ci.nsISHistoryListener,
+                          Ci.nsISupportsWeakReference]);
+
+/**
+ * Listens for scroll position changes. Whenever the user scrolls the top-most
+ * frame we update the scroll position and will restore it when requested.
+ *
+ * Causes a SessionStore:update message to be sent that contains the current
+ * scroll positions as a tree of strings. If no frame of the whole frame tree
+ * is scrolled this will return null so that we don't tack a property onto
+ * the tabData object in the parent process.
+ *
+ * Example:
+ *   {scroll: "100,100", zoom: {resolution: "1.5", displaySize:
+ *   {height: "1600", width: "1000"}}, children:
+ *   [null, null, {scroll: "200,200"}]}
+ */
+class ScrollPositionListener extends Handler {
+  constructor(store) {
+    super(store);
+
+    SessionStoreUtils.addDynamicFrameFilteredListener(
+      this.mm, "mozvisualscroll", this,
+      /* capture */ false, /* system group */ true);
+
+    SessionStoreUtils.addDynamicFrameFilteredListener(
+      this.mm, "mozvisualresize", this,
+      /* capture */ false, /* system group */ true);
+
+    this.stateChangeNotifier.addObserver(this);
+  }
+
+  handleEvent() {
+    this.messageQueue.push("scroll", () => this.collect());
+  }
+
+  onPageLoadCompleted() {
+    this.messageQueue.push("scroll", () => this.collect());
+  }
+
+  onPageLoadStarted() {
+    this.messageQueue.push("scroll", () => null);
+  }
+
+  collect() {
+    // TODO: Keep an eye on bug 1525259; we may not have to manually store zoom
+    // Save the current document resolution.
+    let zoom = 1;
+    const scrolldata = SessionStoreUtils.collectScrollPosition(this.mm.content) || {};
+    const domWindowUtils = this.mm.content.windowUtils;
+    zoom = domWindowUtils.getResolution();
+    scrolldata.zoom = {};
+    scrolldata.zoom.resolution = zoom;
+
+    // Save some data that'll help in adjusting the zoom level
+    // when restoring in a different screen orientation.
+    const displaySize = {};
+    const width = {}, height = {};
+    domWindowUtils.getContentViewerSize(width, height);
+
+    displaySize.width = width.value;
+    displaySize.height = height.value;
+
+    scrolldata.zoom.displaySize = displaySize;
+
+    return scrolldata;
+  }
+}
+
+/**
+ * Listens for changes to input elements. Whenever the value of an input
+ * element changes we will re-collect data for the current frame tree and send
+ * a message to the parent process.
+ *
+ * Causes a SessionStore:update message to be sent that contains the form data
+ * for all reachable frames.
+ *
+ * Example:
+ *   {
+ *     formdata: {url: "http://mozilla.org/", id: {input_id: "input value"}},
+ *     children: [
+ *       null,
+ *       {url: "http://sub.mozilla.org/", id: {input_id: "input value 2"}}
+ *     ]
+ *   }
+ */
+class FormDataListener extends Handler {
+  constructor(store) {
+    super(store);
+
+    SessionStoreUtils.addDynamicFrameFilteredListener(this.mm, "input", this, true);
+    this.stateChangeNotifier.addObserver(this);
+  }
+
+  handleEvent() {
+    this.messageQueue.push("formdata", () => this.collect());
+  }
+
+  onPageLoadStarted() {
+    this.messageQueue.push("formdata", () => null);
+  }
+
+  collect() {
+    return SessionStoreUtils.collectFormData(this.mm.content);
+  }
+}
+
+/**
+ * A message queue that takes collected data and will take care of sending it
+ * to the chrome process. It allows flushing using synchronous messages and
+ * takes care of any race conditions that might occur because of that. Changes
+ * will be batched if they're pushed in quick succession to avoid a message
+ * flood.
+ */
+class MessageQueue extends Handler {
+  constructor(store) {
+    super(store);
+
+    /**
+     * A map (string -> lazy fn) holding lazy closures of all queued data
+     * collection routines. These functions will return data collected from the
+     * docShell.
+     */
+    this._data = new Map();
+
+    /**
+     * The delay (in ms) used to delay sending changes after data has been
+     * invalidated.
+     */
+    this.BATCH_DELAY_MS = 1000;
+
+    /**
+     * The minimum idle period (in ms) we need for sending data to chrome process.
+     */
+    this.NEEDED_IDLE_PERIOD_MS = 5;
+
+    /**
+     * Timeout for waiting an idle period to send data. We will set this from
+     * the pref "browser.sessionstore.interval".
+     */
+    this._timeoutWaitIdlePeriodMs = null;
+
+    /**
+     * The current timeout ID, null if there is no queue data. We use timeouts
+     * to damp a flood of data changes and send lots of changes as one batch.
+     */
+    this._timeout = null;
+
+    /**
+     * Whether or not sending batched messages on a timer is disabled. This should
+     * only be used for debugging or testing. If you need to access this value,
+     * you should probably use the timeoutDisabled getter.
+     */
+    this._timeoutDisabled = false;
+
+    /**
+     * True if there is already a send pending idle dispatch, set to prevent
+     * scheduling more than one. If false there may or may not be one scheduled.
+     */
+    this._idleScheduled = false;
+
+    this.timeoutDisabled =
+      Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF, false);
+    this._timeoutWaitIdlePeriodMs =
+      Services.prefs.getIntPref(PREF_INTERVAL, DEFAULT_INTERVAL_MS);
+
+    Services.prefs.addObserver(TIMEOUT_DISABLED_PREF, this);
+    Services.prefs.addObserver(PREF_INTERVAL, this);
+  }
+
+  /**
+   * True if batched messages are not being fired on a timer. This should only
+   * ever be true when debugging or during tests.
+   */
+  get timeoutDisabled() {
+    return this._timeoutDisabled;
+  }
+
+  /**
+   * Disables sending batched messages on a timer. Also cancels any pending
+   * timers.
+   */
+  set timeoutDisabled(val) {
+    this._timeoutDisabled = val;
+
+    if (val && this._timeout) {
+      clearTimeout(this._timeout);
+      this._timeout = null;
+    }
+
+    return val;
+  }
+
+  uninit() {
+    this.cleanupTimers();
+  }
+
+  /**
+   * Cleanup pending idle callback and timer.
+   */
+  cleanupTimers() {
+    this._idleScheduled = false;
+    if (this._timeout) {
+      clearTimeout(this._timeout);
+      this._timeout = null;
+    }
+  }
+
+  observe(subject, topic, data) {
+    if (topic == "nsPref:changed") {
+      switch (data) {
+        case TIMEOUT_DISABLED_PREF:
+          this.timeoutDisabled =
+            Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF, false);
+          break;
+        case PREF_INTERVAL:
+          this._timeoutWaitIdlePeriodMs =
+            Services.prefs.getIntPref(PREF_INTERVAL, DEFAULT_INTERVAL_MS);
+          break;
+        default:
+          debug `Received unknown message: ${data}`;
+          break;
+      }
+    }
+  }
+
+  /**
+   * Pushes a given |value| onto the queue. The given |key| represents the type
+   * of data that is stored and can override data that has been queued before
+   * but has not been sent to the parent process, yet.
+   *
+   * @param key (string)
+   *        A unique identifier specific to the type of data this is passed.
+   * @param fn (function)
+   *        A function that returns the value that will be sent to the parent
+   *        process.
+   */
+  push(key, fn) {
+    this._data.set(key, fn);
+
+    if (!this._timeout && !this._timeoutDisabled) {
+      // Wait a little before sending the message to batch multiple changes.
+      this._timeout = setTimeoutWithTarget(
+        () => this.sendWhenIdle(), this.BATCH_DELAY_MS, this.mm.tabEventTarget);
+    }
+  }
+
+  /**
+   * Sends queued data when the remaining idle time is enough or waiting too
+   * long; otherwise, request an idle time again. If the |deadline| is not
+   * given, this function is going to schedule the first request.
+   *
+   * @param deadline (object)
+   *        An IdleDeadline object passed by idleDispatch().
+   */
+  sendWhenIdle(deadline) {
+    if (!this.mm.content) {
+      // The frameloader is being torn down. Nothing more to do.
+      return;
+    }
+
+    if (deadline) {
+      if (deadline.didTimeout || deadline.timeRemaining() > this.NEEDED_IDLE_PERIOD_MS) {
+        this.send();
+        return;
+      }
+    } else if (this._idleScheduled) {
+      // Bail out if there's a pending run.
+      return;
+    }
+    ChromeUtils.idleDispatch((deadline_) => this.sendWhenIdle(deadline_),
+                             {timeout: this._timeoutWaitIdlePeriodMs});
+    this._idleScheduled = true;
+  }
+
+  /**
+   * Sends queued data to the chrome process.
+   *
+   * @param options (object)
+   *        {flushID: 123} to specify that this is a flush
+   *        {isFinal: true} to signal this is the final message sent on unload
+   */
+  send(options = {}) {
+    // Looks like we have been called off a timeout after the tab has been
+    // closed. The docShell is gone now and we can just return here as there
+    // is nothing to do.
+    if (!this.mm.docShell) {
+      return;
+    }
+
+    this.cleanupTimers();
+
+    const flushID = (options && options.flushID) || 0;
+
+    let data = {};
+    for (let [key, func] of this._data) {
+      let value = func();
+
+      if (value || (key != "storagechange" && key != "historychange")) {
+        data[key] = value;
+      }
+    }
+
+    this._data.clear();
+
+    try {
+      // Send all data to the parent process.
+      this.eventDispatcher.sendRequest({
+        type: "GeckoView:StateUpdated",
+        data, flushID,
+        isFinal: options.isFinal || false,
+        epoch: this.store.epoch,
+      });
+    } catch (ex) {
+      if (ex && ex.result == Cr.NS_ERROR_OUT_OF_MEMORY) {
+        warn `Failed to save session state`;
+      }
+    }
+  }
+}
+
+class SessionStateAggregator extends GeckoViewChildModule {
+  constructor(aModuleName, aMessageManager) {
+    super(aModuleName, aMessageManager);
+
+    this.mm = aMessageManager;
+    this.messageQueue = new MessageQueue(this);
+    this.stateChangeNotifier = new StateChangeNotifier(this);
+
+    this.handlers = [
+      new FormDataListener(this),
+      new SessionHistoryListener(this),
+      new ScrollPositionListener(this),
+      this.stateChangeNotifier,
+      this.messageQueue,
+    ];
+  }
+
+  flush({id}) {
+    // Flush the message queue, send the latest updates.
+    this.messageQueue.send({flushID: id});
+  }
+
+  onUnload() {
+    // Upon frameLoader destruction, send a final update message to
+    // the parent and flush all data currently held in the child.
+    this.messageQueue.send({isFinal: true});
+
+    for (let handler of this.handlers) {
+      if (handler.uninit) {
+        handler.uninit();
+      }
+    }
+
+    // We don't need to take care of any StateChangeNotifier observers as they
+    // will die with the content script.
+  }
+}
+
+const {debug, warn} = SessionStateAggregator.initLogging("SessionStateAggregator"); // eslint-disable-line no-unused-vars
+const module = SessionStateAggregator.create(this);
--- a/mobile/android/chrome/geckoview/geckoview.js
+++ b/mobile/android/chrome/geckoview/geckoview.js
@@ -452,16 +452,21 @@ function startup() {
     onInit: {
       resource: "resource://gre/modules/GeckoViewTab.jsm",
     },
   }, {
     name: "GeckoViewContentBlocking",
     onEnable: {
       resource: "resource://gre/modules/GeckoViewContentBlocking.jsm",
     },
+  }, {
+    name: "SessionStateAggregator",
+    onInit: {
+      frameScript: "chrome://geckoview/content/SessionStateAggregator.js",
+    },
   }]);
 
   Services.tm.dispatchToMainThread(() => {
     // This should always be the first thing we do here - any additional delayed
     // initialisation tasks should be added between "browser-delayed-startup-finished"
     // and "browser-idle-startup-tasks-finished".
 
     // Bug 1496684: Various bits of platform stuff depend on this notification
--- a/mobile/android/chrome/geckoview/jar.mn
+++ b/mobile/android/chrome/geckoview/jar.mn
@@ -15,10 +15,11 @@ geckoview.jar:
   content/GeckoViewContentChild.js
   content/GeckoViewMediaChild.js
   content/GeckoViewNavigationChild.js
   content/GeckoViewProgressChild.js
   content/GeckoViewPromptChild.js
   content/GeckoViewScrollChild.js
   content/GeckoViewSettingsChild.js
   content/GeckoViewSelectionActionChild.js
+  content/SessionStateAggregator.js
 
 % content branding %content/branding/
--- a/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
+++ b/mobile/android/geckoview/src/main/java/org/mozilla/geckoview/GeckoSession.java
@@ -572,17 +572,18 @@ public class GeckoSession implements Par
 
     private final GeckoSessionHandler<ProgressDelegate> mProgressHandler =
         new GeckoSessionHandler<ProgressDelegate>(
             "GeckoViewProgress", this,
             new String[]{
                 "GeckoView:PageStart",
                 "GeckoView:PageStop",
                 "GeckoView:ProgressChanged",
-                "GeckoView:SecurityChanged"
+                "GeckoView:SecurityChanged",
+                "GeckoView:StateUpdated"
             }
         ) {
             @Override
             public void handleMessage(final ProgressDelegate delegate,
                                       final String event,
                                       final GeckoBundle message,
                                       final EventCallback callback) {
                 if ("GeckoView:PageStart".equals(event)) {
@@ -592,16 +593,18 @@ public class GeckoSession implements Par
                     delegate.onPageStop(GeckoSession.this,
                                         message.getBoolean("success"));
                 } else if ("GeckoView:ProgressChanged".equals(event)) {
                     delegate.onProgressChange(GeckoSession.this,
                                               message.getInt("progress"));
                 } else if ("GeckoView:SecurityChanged".equals(event)) {
                     final GeckoBundle identity = message.getBundle("identity");
                     delegate.onSecurityChange(GeckoSession.this, new ProgressDelegate.SecurityInformation(identity));
+                } else if ("GeckoView:StateUpdated".equals(event)) {
+                    final GeckoBundle update = message.getBundle("data");
                 }
             }
         };
 
     private final GeckoSessionHandler<ScrollDelegate> mScrollHandler =
         new GeckoSessionHandler<ScrollDelegate>(
             "GeckoViewScroll", this,
             new String[]{ "GeckoView:ScrollChanged" }