The Tor Browser / file revision

 # 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/.

 Cu.import("resource://gre/modules/TelemetryStopwatch.jsm", this);

 // Simple gestures support

 //

 // As per bug #412486, web content must not be allowed to receive any

 // simple gesture events.  Multi-touch gesture APIs are in their

 // infancy and we do NOT want to be forced into supporting an API that

 // will probably have to change in the future.  (The current Mac OS X

 // API is undocumented and was reverse-engineered.)  Until support is

 // implemented in the event dispatcher to keep these events as

 // chrome-only, we must listen for the simple gesture events during

 // the capturing phase and call stopPropagation on every event.

 let gGestureSupport = {

   _currentRotation: 0,

   _lastRotateDelta: 0,

   _rotateMomentumThreshold: .75,

   /**

    * Add or remove mouse gesture event listeners

    *

    * @param aAddListener

    *        True to add/init listeners and false to remove/uninit

    */

   init: function GS_init(aAddListener) {

     const gestureEvents = ["SwipeGestureStart",

       "SwipeGestureUpdate", "SwipeGestureEnd", "SwipeGesture",

       "MagnifyGestureStart", "MagnifyGestureUpdate", "MagnifyGesture",

       "RotateGestureStart", "RotateGestureUpdate", "RotateGesture",

       "TapGesture", "PressTapGesture"];

     let addRemove = aAddListener ? window.addEventListener :

       window.removeEventListener;

     gestureEvents.forEach(function (event) addRemove("Moz" + event, this, true),

                           this);

   },

   /**

    * Dispatch events based on the type of mouse gesture event. For now, make

    * sure to stop propagation of every gesture event so that web content cannot

    * receive gesture events.

    *

    * @param aEvent

    *        The gesture event to handle

    */

   handleEvent: function GS_handleEvent(aEvent) {

     if (!Services.prefs.getBoolPref(

            "dom.debug.propagate_gesture_events_through_content")) {

       aEvent.stopPropagation();

     }

     // Create a preference object with some defaults

     let def = function(aThreshold, aLatched)

       ({ threshold: aThreshold, latched: !!aLatched });

     switch (aEvent.type) {

       case "MozSwipeGestureStart":

         if (this._setupSwipeGesture(aEvent)) {

           aEvent.preventDefault();

         }

         break;

       case "MozSwipeGestureUpdate":

         aEvent.preventDefault();

         this._doUpdate(aEvent);

         break;

       case "MozSwipeGestureEnd":

         aEvent.preventDefault();

         this._doEnd(aEvent);

         break;

       case "MozSwipeGesture":

         aEvent.preventDefault();

         this.onSwipe(aEvent);

         break;

       case "MozMagnifyGestureStart":

         aEvent.preventDefault();

 #ifdef XP_WIN

         this._setupGesture(aEvent, "pinch", def(25, 0), "out", "in");

 #else

         this._setupGesture(aEvent, "pinch", def(150, 1), "out", "in");

 #endif

         break;

       case "MozRotateGestureStart":

         aEvent.preventDefault();

         this._setupGesture(aEvent, "twist", def(25, 0), "right", "left");

         break;

       case "MozMagnifyGestureUpdate":

       case "MozRotateGestureUpdate":

         aEvent.preventDefault();

         this._doUpdate(aEvent);

         break;

       case "MozTapGesture":

         aEvent.preventDefault();

         this._doAction(aEvent, ["tap"]);

         break;

       case "MozRotateGesture":

         aEvent.preventDefault();

         this._doAction(aEvent, ["twist", "end"]);

         break;

       /* case "MozPressTapGesture":

         break; */

     }

   },

   /**

    * Called at the start of "pinch" and "twist" gestures to setup all of the

    * information needed to process the gesture

    *

    * @param aEvent

    *        The continual motion start event to handle

    * @param aGesture

    *        Name of the gesture to handle

    * @param aPref

    *        Preference object with the names of preferences and defaults

    * @param aInc

    *        Command to trigger for increasing motion (without gesture name)

    * @param aDec

    *        Command to trigger for decreasing motion (without gesture name)

    */

   _setupGesture: function GS__setupGesture(aEvent, aGesture, aPref, aInc, aDec) {

     // Try to load user-set values from preferences

     for (let [pref, def] in Iterator(aPref))

       aPref[pref] = this._getPref(aGesture + "." + pref, def);

     // Keep track of the total deltas and latching behavior

     let offset = 0;

     let latchDir = aEvent.delta > 0 ? 1 : -1;

     let isLatched = false;

     // Create the update function here to capture closure state

     this._doUpdate = function GS__doUpdate(aEvent) {

       // Update the offset with new event data

       offset += aEvent.delta;

       // Check if the cumulative deltas exceed the threshold

       if (Math.abs(offset) > aPref["threshold"]) {

         // Trigger the action if we don't care about latching; otherwise, make

         // sure either we're not latched and going the same direction of the

         // initial motion; or we're latched and going the opposite way

         let sameDir = (latchDir ^ offset) >= 0;

         if (!aPref["latched"] || (isLatched ^ sameDir)) {

           this._doAction(aEvent, [aGesture, offset > 0 ? aInc : aDec]);

           // We must be getting latched or leaving it, so just toggle

           isLatched = !isLatched;

         }

         // Reset motion counter to prepare for more of the same gesture

         offset = 0;

       }

     };

     // The start event also contains deltas, so handle an update right away

     this._doUpdate(aEvent);

   },

   /**

    * Checks whether a swipe gesture event can navigate the browser history or

    * not.

    *

    * @param aEvent

    *        The swipe gesture event.

    * @return true if the swipe event may navigate the history, false othwerwise.

    */

   _swipeNavigatesHistory: function GS__swipeNavigatesHistory(aEvent) {

     return this._getCommand(aEvent, ["swipe", "left"])

               == "Browser:BackOrBackDuplicate" &&

            this._getCommand(aEvent, ["swipe", "right"])

               == "Browser:ForwardOrForwardDuplicate";

   },

   /**

    * Sets up swipe gestures. This includes setting up swipe animations for the

    * gesture, if enabled.

    *

    * @param aEvent

    *        The swipe gesture start event.

    * @return true if swipe gestures could successfully be set up, false

    *         othwerwise.

    */

   _setupSwipeGesture: function GS__setupSwipeGesture(aEvent) {

     if (!this._swipeNavigatesHistory(aEvent)) {

       return false;

     }

     let isVerticalSwipe = false;

     if (aEvent.direction == aEvent.DIRECTION_UP) {

       if (content.pageYOffset > 0) {

         return false;

       }

       isVerticalSwipe = true;

     } else if (aEvent.direction == aEvent.DIRECTION_DOWN) {

       if (content.pageYOffset < content.scrollMaxY) {

         return false;

       }

       isVerticalSwipe = true;

     }

     if (isVerticalSwipe) {

       // Vertical overscroll has been temporarily disabled until bug 939480 is

       // fixed.

       return false;

     }

     let canGoBack = gHistorySwipeAnimation.canGoBack();

     let canGoForward = gHistorySwipeAnimation.canGoForward();

     let isLTR = gHistorySwipeAnimation.isLTR;

     if (canGoBack) {

       aEvent.allowedDirections |= isLTR ? aEvent.DIRECTION_LEFT :

                                           aEvent.DIRECTION_RIGHT;

     }

     if (canGoForward) {

       aEvent.allowedDirections |= isLTR ? aEvent.DIRECTION_RIGHT :

                                           aEvent.DIRECTION_LEFT;

     }

     gHistorySwipeAnimation.startAnimation(isVerticalSwipe);

     this._doUpdate = function GS__doUpdate(aEvent) {

       gHistorySwipeAnimation.updateAnimation(aEvent.delta);

     };

     this._doEnd = function GS__doEnd(aEvent) {

       gHistorySwipeAnimation.swipeEndEventReceived();

       this._doUpdate = function (aEvent) {};

       this._doEnd = function (aEvent) {};

     }

     return true;

   },

   /**

    * Generator producing the powerset of the input array where the first result

    * is the complete set and the last result (before StopIteration) is empty.

    *

    * @param aArray

    *        Source array containing any number of elements

    * @yield Array that is a subset of the input array from full set to empty

    */

   _power: function GS__power(aArray) {

     // Create a bitmask based on the length of the array

     let num = 1 << aArray.length;

     while (--num >= 0) {

       // Only select array elements where the current bit is set

       yield aArray.reduce(function (aPrev, aCurr, aIndex) {

         if (num & 1 << aIndex)

           aPrev.push(aCurr);

         return aPrev;

       }, []);

     }

   },

   /**

    * Determine what action to do for the gesture based on which keys are

    * pressed and which commands are set, and execute the command.

    *

    * @param aEvent

    *        The original gesture event to convert into a fake click event

    * @param aGesture

    *        Array of gesture name parts (to be joined by periods)

    * @return Name of the executed command. Returns null if no command is

    *         found.

    */

   _doAction: function GS__doAction(aEvent, aGesture) {

     let command = this._getCommand(aEvent, aGesture);

     return command && this._doCommand(aEvent, command);

   },

   /**

    * Determine what action to do for the gesture based on which keys are

    * pressed and which commands are set

    *

    * @param aEvent

    *        The original gesture event to convert into a fake click event

    * @param aGesture

    *        Array of gesture name parts (to be joined by periods)

    */

   _getCommand: function GS__getCommand(aEvent, aGesture) {

     // Create an array of pressed keys in a fixed order so that a command for

     // "meta" is preferred over "ctrl" when both buttons are pressed (and a

     // command for both don't exist)

     let keyCombos = [];

     ["shift", "alt", "ctrl", "meta"].forEach(function (key) {

       if (aEvent[key + "Key"])

         keyCombos.push(key);

     });

     // Try each combination of key presses in decreasing order for commands

     for (let subCombo of this._power(keyCombos)) {

       // Convert a gesture and pressed keys into the corresponding command

       // action where the preference has the gesture before "shift" before

       // "alt" before "ctrl" before "meta" all separated by periods

       let command;

       try {

         command = this._getPref(aGesture.concat(subCombo).join("."));

       } catch (e) {}

       if (command)

         return command;

     }

     return null;

   },

   /**

    * Execute the specified command.

    *

    * @param aEvent

    *        The original gesture event to convert into a fake click event

    * @param aCommand

    *        Name of the command found for the event's keys and gesture.

    */

   _doCommand: function GS__doCommand(aEvent, aCommand) {

     let node = document.getElementById(aCommand);

     if (node) {

       if (node.getAttribute("disabled") != "true") {

         let cmdEvent = document.createEvent("xulcommandevent");

         cmdEvent.initCommandEvent("command", true, true, window, 0,

                                   aEvent.ctrlKey, aEvent.altKey,

                                   aEvent.shiftKey, aEvent.metaKey, aEvent);

         node.dispatchEvent(cmdEvent);

       }

     }

     else {

       goDoCommand(aCommand);

     }

   },

   /**

    * Handle continual motion events.  This function will be set by

    * _setupGesture or _setupSwipe.

    *

    * @param aEvent

    *        The continual motion update event to handle

    */

   _doUpdate: function(aEvent) {},

   /**

    * Handle gesture end events.  This function will be set by _setupSwipe.

    *

    * @param aEvent

    *        The gesture end event to handle

    */

   _doEnd: function(aEvent) {},

   /**

    * Convert the swipe gesture into a browser action based on the direction.

    *

    * @param aEvent

    *        The swipe event to handle

    */

   onSwipe: function GS_onSwipe(aEvent) {

     // Figure out which one (and only one) direction was triggered

     for (let dir of ["UP", "RIGHT", "DOWN", "LEFT"]) {

       if (aEvent.direction == aEvent["DIRECTION_" + dir]) {

         this._coordinateSwipeEventWithAnimation(aEvent, dir);

         break;

       }

     }

   },

   /**

    * Process a swipe event based on the given direction.

    *

    * @param aEvent

    *        The swipe event to handle

    * @param aDir

    *        The direction for the swipe event

    */

   processSwipeEvent: function GS_processSwipeEvent(aEvent, aDir) {

     this._doAction(aEvent, ["swipe", aDir.toLowerCase()]);

   },

   /**

    * Coordinates the swipe event with the swipe animation, if any.

    * If an animation is currently running, the swipe event will be

    * processed once the animation stops. This will guarantee a fluid

    * motion of the animation.

    *

    * @param aEvent

    *        The swipe event to handle

    * @param aDir

    *        The direction for the swipe event

    */

   _coordinateSwipeEventWithAnimation:

   function GS__coordinateSwipeEventWithAnimation(aEvent, aDir) {

     if ((gHistorySwipeAnimation.isAnimationRunning()) &&

         (aDir == "RIGHT" || aDir == "LEFT")) {

       gHistorySwipeAnimation.processSwipeEvent(aEvent, aDir);

     }

     else {

       this.processSwipeEvent(aEvent, aDir);

     }

   },

   /**

    * Get a gesture preference or use a default if it doesn't exist

    *

    * @param aPref

    *        Name of the preference to load under the gesture branch

    * @param aDef

    *        Default value if the preference doesn't exist

    */

   _getPref: function GS__getPref(aPref, aDef) {

     // Preferences branch under which all gestures preferences are stored

     const branch = "browser.gesture.";

     try {

       // Determine what type of data to load based on default value's type

       let type = typeof aDef;

       let getFunc = "get" + (type == "boolean" ? "Bool" :

                              type == "number" ? "Int" : "Char") + "Pref";

       return gPrefService[getFunc](branch + aPref);

     }

     catch (e) {

       return aDef;

     }

   },

   /**

    * Perform rotation for ImageDocuments

    *

    * @param aEvent

    *        The MozRotateGestureUpdate event triggering this call

    */

   rotate: function(aEvent) {

     if (!(content.document instanceof ImageDocument))

       return;

     let contentElement = content.document.body.firstElementChild;

     if (!contentElement)

       return;

     // If we're currently snapping, cancel that snap

     if (contentElement.classList.contains("completeRotation"))

       this._clearCompleteRotation();

     this.rotation = Math.round(this.rotation + aEvent.delta);

     contentElement.style.transform = "rotate(" + this.rotation + "deg)";

     this._lastRotateDelta = aEvent.delta;

   },

   /**

    * Perform a rotation end for ImageDocuments

    */

   rotateEnd: function() {

     if (!(content.document instanceof ImageDocument))

       return;

     let contentElement = content.document.body.firstElementChild;

     if (!contentElement)

       return;

     let transitionRotation = 0;

     // The reason that 360 is allowed here is because when rotating between

     // 315 and 360, setting rotate(0deg) will cause it to rotate the wrong

     // direction around--spinning wildly.

     if (this.rotation <= 45)

       transitionRotation = 0;

     else if (this.rotation > 45 && this.rotation <= 135)

       transitionRotation = 90;

     else if (this.rotation > 135 && this.rotation <= 225)

       transitionRotation = 180;

     else if (this.rotation > 225 && this.rotation <= 315)

       transitionRotation = 270;

     else

       transitionRotation = 360;

     // If we're going fast enough, and we didn't already snap ahead of rotation,

     // then snap ahead of rotation to simulate momentum

     if (this._lastRotateDelta > this._rotateMomentumThreshold &&

         this.rotation > transitionRotation)

       transitionRotation += 90;

     else if (this._lastRotateDelta < -1 * this._rotateMomentumThreshold &&

              this.rotation < transitionRotation)

       transitionRotation -= 90;

     // Only add the completeRotation class if it is is necessary

     if (transitionRotation != this.rotation) {

       contentElement.classList.add("completeRotation");

       contentElement.addEventListener("transitionend", this._clearCompleteRotation);

     }

     contentElement.style.transform = "rotate(" + transitionRotation + "deg)";

     this.rotation = transitionRotation;

   },

   /**

    * Gets the current rotation for the ImageDocument

    */

   get rotation() {

     return this._currentRotation;

   },

   /**

    * Sets the current rotation for the ImageDocument

    *

    * @param aVal

    *        The new value to take.  Can be any value, but it will be bounded to

    *        0 inclusive to 360 exclusive.

    */

   set rotation(aVal) {

     this._currentRotation = aVal % 360;

     if (this._currentRotation < 0)

       this._currentRotation += 360;

     return this._currentRotation;

   },

   /**

    * When the location/tab changes, need to reload the current rotation for the

    * image

    */

   restoreRotationState: function() {

     // Bug 863514 - Make gesture support work in electrolysis

     if (gMultiProcessBrowser)

       return;

     if (!(content.document instanceof ImageDocument))

       return;

     let contentElement = content.document.body.firstElementChild;

     let transformValue = content.window.getComputedStyle(contentElement, null)

                                        .transform;

     if (transformValue == "none") {

       this.rotation = 0;

       return;

     }

     // transformValue is a rotation matrix--split it and do mathemagic to

     // obtain the real rotation value

     transformValue = transformValue.split("(")[1]

                                    .split(")")[0]

                                    .split(",");

     this.rotation = Math.round(Math.atan2(transformValue[1], transformValue[0]) *

                                (180 / Math.PI));

   },

   /**

    * Removes the transition rule by removing the completeRotation class

    */

   _clearCompleteRotation: function() {

     let contentElement = content.document &&

                          content.document instanceof ImageDocument &&

                          content.document.body &&

                          content.document.body.firstElementChild;

     if (!contentElement)

       return;

     contentElement.classList.remove("completeRotation");

     contentElement.removeEventListener("transitionend", this._clearCompleteRotation);

   },

 };

 // History Swipe Animation Support (bug 678392)

 let gHistorySwipeAnimation = {

   active: false,

   isLTR: false,

   /**

    * Initializes the support for history swipe animations, if it is supported

    * by the platform/configuration.

    */

   init: function HSA_init() {

     if (!this._isSupported())

       return;

     this.active = false;

     this.isLTR = document.documentElement.mozMatchesSelector(

                                             ":-moz-locale-dir(ltr)");

     this._trackedSnapshots = [];

     this._startingIndex = -1;

     this._historyIndex = -1;

     this._boxWidth = -1;

     this._boxHeight = -1;

     this._maxSnapshots = this._getMaxSnapshots();

     this._lastSwipeDir = "";

     this._direction = "horizontal";

     // We only want to activate history swipe animations if we store snapshots.

     // If we don't store any, we handle horizontal swipes without animations.

     if (this._maxSnapshots > 0) {

       this.active = true;

       gBrowser.addEventListener("pagehide", this, false);

       gBrowser.addEventListener("pageshow", this, false);

       gBrowser.addEventListener("popstate", this, false);

       gBrowser.addEventListener("DOMModalDialogClosed", this, false);

       gBrowser.tabContainer.addEventListener("TabClose", this, false);

     }

   },

   /**

    * Uninitializes the support for history swipe animations.

    */

   uninit: function HSA_uninit() {

     gBrowser.removeEventListener("pagehide", this, false);

     gBrowser.removeEventListener("pageshow", this, false);

     gBrowser.removeEventListener("popstate", this, false);

     gBrowser.removeEventListener("DOMModalDialogClosed", this, false);

     gBrowser.tabContainer.removeEventListener("TabClose", this, false);

     this.active = false;

     this.isLTR = false;

   },

   /**

    * Starts the swipe animation and handles fast swiping (i.e. a swipe animation

    * is already in progress when a new one is initiated).

    *

    * @param aIsVerticalSwipe

    *        Whether we're dealing with a vertical swipe or not.

    */

   startAnimation: function HSA_startAnimation(aIsVerticalSwipe) {

     this._direction = aIsVerticalSwipe ? "vertical" : "horizontal";

     if (this.isAnimationRunning()) {

       // If this is a horizontal scroll, or if this is a vertical scroll that

       // was started while a horizontal scroll was still running, handle it as

       // as a fast swipe. In the case of the latter scenario, this allows us to

       // start the vertical animation without first loading the final page, or

       // taking another snapshot. If vertical scrolls are initiated repeatedly

       // without prior horizontal scroll we skip this and restart the animation

       // from 0.

       if (this._direction == "horizontal" || this._lastSwipeDir != "") {

         gBrowser.stop();

         this._lastSwipeDir = "RELOAD"; // just ensure that != ""

         this._canGoBack = this.canGoBack();

         this._canGoForward = this.canGoForward();

         this._handleFastSwiping();

       }

     }

     else {

       this._startingIndex = gBrowser.webNavigation.sessionHistory.index;

       this._historyIndex = this._startingIndex;

       this._canGoBack = this.canGoBack();

       this._canGoForward = this.canGoForward();

       if (this.active) {

         this._addBoxes();

         this._takeSnapshot();

         this._installPrevAndNextSnapshots();

         this._lastSwipeDir = "";

       }

     }

     this.updateAnimation(0);

   },

   /**

    * Stops the swipe animation.

    */

   stopAnimation: function HSA_stopAnimation() {

     gHistorySwipeAnimation._removeBoxes();

     this._historyIndex = gBrowser.webNavigation.sessionHistory.index;

   },

   /**

    * Updates the animation between two pages in history.

    *

    * @param aVal

    *        A floating point value that represents the progress of the

    *        swipe gesture.

    */

   updateAnimation: function HSA_updateAnimation(aVal) {

     if (!this.isAnimationRunning()) {

       return;

     }

     // We use the following value to decrease the bounce effect when scrolling

     // to the top or bottom of the page, or when swiping back/forward past the

     // browsing history. This value was determined experimentally.

     let dampValue = 4;

     if (this._direction == "vertical") {

       this._prevBox.collapsed = true;

       this._nextBox.collapsed = true;

       this._positionBox(this._curBox, -1 * aVal / dampValue);

     } else if ((aVal >= 0 && this.isLTR) ||

                (aVal <= 0 && !this.isLTR)) {

       let tempDampValue = 1;

       if (this._canGoBack) {

         this._prevBox.collapsed = false;

       } else {

         tempDampValue = dampValue;

         this._prevBox.collapsed = true;

       }

       // The current page is pushed to the right (LTR) or left (RTL),

       // the intention is to go back.

       // If there is a page to go back to, it should show in the background.

       this._positionBox(this._curBox, aVal / tempDampValue);

       // The forward page should be pushed offscreen all the way to the right.

       this._positionBox(this._nextBox, 1);

     } else {

       // The intention is to go forward. If there is a page to go forward to,

       // it should slide in from the right (LTR) or left (RTL).

       // Otherwise, the current page should slide to the left (LTR) or

       // right (RTL) and the backdrop should appear in the background.

       // For the backdrop to be visible in that case, the previous page needs

       // to be hidden (if it exists).

       if (this._canGoForward) {

         this._nextBox.collapsed = false;

         let offset = this.isLTR ? 1 : -1;

         this._positionBox(this._curBox, 0);

         this._positionBox(this._nextBox, offset + aVal);

       } else {

         this._prevBox.collapsed = true;

         this._positionBox(this._curBox, aVal / dampValue);

       }

     }

   },

   /**

    * Event handler for events relevant to the history swipe animation.

    *

    * @param aEvent

    *        An event to process.

    */

   handleEvent: function HSA_handleEvent(aEvent) {

     let browser = gBrowser.selectedBrowser;

     switch (aEvent.type) {

       case "TabClose":

         let browserForTab = gBrowser.getBrowserForTab(aEvent.target);

         this._removeTrackedSnapshot(-1, browserForTab);

         break;

       case "DOMModalDialogClosed":

         this.stopAnimation();

         break;

       case "pageshow":

         if (aEvent.target == browser.contentDocument) {

           this.stopAnimation();

         }

         break;

       case "popstate":

         if (aEvent.target == browser.contentDocument.defaultView) {

           this.stopAnimation();

         }

         break;

       case "pagehide":

         if (aEvent.target == browser.contentDocument) {

           // Take and compress a snapshot of a page whenever it's about to be

           // navigated away from. We already have a snapshot of the page if an

           // animation is running, so we're left with compressing it.

           if (!this.isAnimationRunning()) {

             this._takeSnapshot();

           }

           this._compressSnapshotAtCurrentIndex();

         }

         break;

     }

   },

   /**

    * Checks whether the history swipe animation is currently running or not.

    *

    * @return true if the animation is currently running, false otherwise.

    */

   isAnimationRunning: function HSA_isAnimationRunning() {

     return !!this._container;

   },

   /**

    * Process a swipe event based on the given direction.

    *

    * @param aEvent

    *        The swipe event to handle

    * @param aDir

    *        The direction for the swipe event

    */

   processSwipeEvent: function HSA_processSwipeEvent(aEvent, aDir) {

     if (aDir == "RIGHT")

       this._historyIndex += this.isLTR ? 1 : -1;

     else if (aDir == "LEFT")

       this._historyIndex += this.isLTR ? -1 : 1;

     else

       return;

     this._lastSwipeDir = aDir;

   },

   /**

    * Checks if there is a page in the browser history to go back to.

    *

    * @return true if there is a previous page in history, false otherwise.

    */

   canGoBack: function HSA_canGoBack() {

     if (this.isAnimationRunning())

       return this._doesIndexExistInHistory(this._historyIndex - 1);

     return gBrowser.webNavigation.canGoBack;

   },

   /**

    * Checks if there is a page in the browser history to go forward to.

    *

    * @return true if there is a next page in history, false otherwise.

    */

   canGoForward: function HSA_canGoForward() {

     if (this.isAnimationRunning())

       return this._doesIndexExistInHistory(this._historyIndex + 1);

     return gBrowser.webNavigation.canGoForward;

   },

   /**

    * Used to notify the history swipe animation that the OS sent a swipe end

    * event and that we should navigate to the page that the user swiped to, if

    * any. This will also result in the animation overlay to be torn down.

    */

   swipeEndEventReceived: function HSA_swipeEndEventReceived() {

     if (this._lastSwipeDir != "" && this._historyIndex != this._startingIndex)

       this._navigateToHistoryIndex();

     else

       this.stopAnimation();

   },

   /**

    * Checks whether a particular index exists in the browser history or not.

    *

    * @param aIndex

    *        The index to check for availability for in the history.

    * @return true if the index exists in the browser history, false otherwise.

    */

   _doesIndexExistInHistory: function HSA__doesIndexExistInHistory(aIndex) {

     try {

       gBrowser.webNavigation.sessionHistory.getEntryAtIndex(aIndex, false);

     }

     catch(ex) {

       return false;

     }

     return true;

   },

   /**

    * Navigates to the index in history that is currently being tracked by

    * |this|.

    */

   _navigateToHistoryIndex: function HSA__navigateToHistoryIndex() {

     if (this._doesIndexExistInHistory(this._historyIndex))

       gBrowser.webNavigation.gotoIndex(this._historyIndex);

     else

       this.stopAnimation();

   },

   /**

    * Checks to see if history swipe animations are supported by this

    * platform/configuration.

    *

    * return true if supported, false otherwise.

    */

   _isSupported: function HSA__isSupported() {

     return window.matchMedia("(-moz-swipe-animation-enabled)").matches;

   },

   /**

    * Handle fast swiping (i.e. a swipe animation is already in

    * progress when a new one is initiated). This will swap out the snapshots

    * used in the previous animation with the appropriate new ones.

    */

   _handleFastSwiping: function HSA__handleFastSwiping() {

     this._installCurrentPageSnapshot(null);

     this._installPrevAndNextSnapshots();

   },

   /**

    * Adds the boxes that contain the snapshots used during the swipe animation.

    */

   _addBoxes: function HSA__addBoxes() {

     let browserStack =

       document.getAnonymousElementByAttribute(gBrowser.getNotificationBox(),

                                               "class", "browserStack");

     this._container = this._createElement("historySwipeAnimationContainer",

                                           "stack");

     browserStack.appendChild(this._container);

     this._prevBox = this._createElement("historySwipeAnimationPreviousPage",

                                         "box");

     this._container.appendChild(this._prevBox);

     this._curBox = this._createElement("historySwipeAnimationCurrentPage",

                                        "box");

     this._container.appendChild(this._curBox);

     this._nextBox = this._createElement("historySwipeAnimationNextPage",

                                         "box");

     this._container.appendChild(this._nextBox);

     // Cache width and height.

     this._boxWidth = this._curBox.getBoundingClientRect().width;

     this._boxHeight = this._curBox.getBoundingClientRect().height;

   },

   /**

    * Removes the boxes.

    */

   _removeBoxes: function HSA__removeBoxes() {

     this._curBox = null;

     this._prevBox = null;

     this._nextBox = null;

     if (this._container)

       this._container.parentNode.removeChild(this._container);

     this._container = null;

     this._boxWidth = -1;

     this._boxHeight = -1;

   },

   /**

    * Creates an element with a given identifier and tag name.

    *

    * @param aID

    *        An identifier to create the element with.

    * @param aTagName

    *        The name of the tag to create the element for.

    * @return the newly created element.

    */

   _createElement: function HSA__createElement(aID, aTagName) {

     let XULNS = "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul";

     let element = document.createElementNS(XULNS, aTagName);

     element.id = aID;

     return element;

   },

   /**

    * Moves a given box to a given X coordinate position.

    *

    * @param aBox

    *        The box element to position.

    * @param aPosition

    *        The position (in X coordinates) to move the box element to.

    */

   _positionBox: function HSA__positionBox(aBox, aPosition) {

     let transform = "";

     if (this._direction == "vertical")

       transform = "translateY(" + this._boxHeight * aPosition + "px)";

     else

       transform = "translateX(" + this._boxWidth * aPosition + "px)";

     aBox.style.transform = transform;

   },

   /**

    * Verifies that we're ready to take snapshots based on the global pref and

    * the current index in history.

    *

    * @return true if we're ready to take snapshots, false otherwise.

    */

   _readyToTakeSnapshots: function HSA__readyToTakeSnapshots() {

     if ((this._maxSnapshots < 1) ||

         (gBrowser.webNavigation.sessionHistory.index < 0)) {

       return false;

     }

     return true;

   },

   /**

    * Takes a snapshot of the page the browser is currently on.

    */

   _takeSnapshot: function HSA__takeSnapshot() {

     if (!this._readyToTakeSnapshots()) {

       return;

     }

     let canvas = null;

     TelemetryStopwatch.start("FX_GESTURE_TAKE_SNAPSHOT_OF_PAGE");

     try {

       let browser = gBrowser.selectedBrowser;

       let r = browser.getBoundingClientRect();

       canvas = document.createElementNS("http://www.w3.org/1999/xhtml",

                                         "canvas");

       canvas.mozOpaque = true;

       let scale = window.devicePixelRatio;

       canvas.width = r.width * scale;

       canvas.height = r.height * scale;

       let ctx = canvas.getContext("2d");

       let zoom = browser.markupDocumentViewer.fullZoom * scale;

       ctx.scale(zoom, zoom);

       ctx.drawWindow(browser.contentWindow,

                      0, 0, canvas.width / zoom, canvas.height / zoom, "white",

                      ctx.DRAWWINDOW_DO_NOT_FLUSH | ctx.DRAWWINDOW_DRAW_VIEW |

                      ctx.DRAWWINDOW_ASYNC_DECODE_IMAGES |

                      ctx.DRAWWINDOW_USE_WIDGET_LAYERS);

     } finally {

       TelemetryStopwatch.finish("FX_GESTURE_TAKE_SNAPSHOT_OF_PAGE");

     }

     TelemetryStopwatch.start("FX_GESTURE_INSTALL_SNAPSHOT_OF_PAGE");

     try {

       this._installCurrentPageSnapshot(canvas);

       this._assignSnapshotToCurrentBrowser(canvas);

     } finally {

       TelemetryStopwatch.finish("FX_GESTURE_INSTALL_SNAPSHOT_OF_PAGE");

     }

   },

   /**

    * Retrieves the maximum number of snapshots that should be kept in memory.

    * This limit is a global limit and is valid across all open tabs.

    */

   _getMaxSnapshots: function HSA__getMaxSnapshots() {

     return gPrefService.getIntPref("browser.snapshots.limit");

   },

   /**

    * Adds a snapshot to the list and initiates the compression of said snapshot.

    * Once the compression is completed, it will replace the uncompressed

    * snapshot in the list.

    *

    * @param aCanvas

    *        The snapshot to add to the list and compress.

    */

   _assignSnapshotToCurrentBrowser:

   function HSA__assignSnapshotToCurrentBrowser(aCanvas) {

     let browser = gBrowser.selectedBrowser;

     let currIndex = browser.webNavigation.sessionHistory.index;

     this._removeTrackedSnapshot(currIndex, browser);

     this._addSnapshotRefToArray(currIndex, browser);

     if (!("snapshots" in browser))

       browser.snapshots = [];

     let snapshots = browser.snapshots;

     // Temporarily store the canvas as the compressed snapshot.

     // This avoids a blank page if the user swipes quickly

     // between pages before the compression could complete.

     snapshots[currIndex] = {

       image: aCanvas,

       scale: window.devicePixelRatio

     };

   },

   /**

    * Compresses the HTMLCanvasElement that's stored at the current history

    * index in the snapshot array and stores the compressed image in its place.

    */

   _compressSnapshotAtCurrentIndex:

   function HSA__compressSnapshotAtCurrentIndex() {

     if (!this._readyToTakeSnapshots()) {

       // We didn't take a snapshot earlier because we weren't ready to, so

       // there's nothing to compress.

       return;

     }

     TelemetryStopwatch.start("FX_GESTURE_COMPRESS_SNAPSHOT_OF_PAGE");

     try {

       let browser = gBrowser.selectedBrowser;

       let snapshots = browser.snapshots;

       let currIndex = browser.webNavigation.sessionHistory.index;

       // Kick off snapshot compression.

       let canvas = snapshots[currIndex].image;

       canvas.toBlob(function(aBlob) {

           if (snapshots[currIndex]) {

             snapshots[currIndex].image = aBlob;

           }

         }, "image/png"

       );

     } finally {

       TelemetryStopwatch.finish("FX_GESTURE_COMPRESS_SNAPSHOT_OF_PAGE");

     }

   },

   /**

    * Removes a snapshot identified by the browser and index in the array of

    * snapshots for that browser, if present. If no snapshot could be identified

    * the method simply returns without taking any action. If aIndex is negative,

    * all snapshots for a particular browser will be removed.

    *

    * @param aIndex

    *        The index in history of the new snapshot, or negative value if all

    *        snapshots for a browser should be removed.

    * @param aBrowser

    *        The browser the new snapshot was taken in.

    */

   _removeTrackedSnapshot: function HSA__removeTrackedSnapshot(aIndex, aBrowser) {

     let arr = this._trackedSnapshots;

     let requiresExactIndexMatch = aIndex >= 0;

     for (let i = 0; i < arr.length; i++) {

       if ((arr[i].browser == aBrowser) &&

           (aIndex < 0 || aIndex == arr[i].index)) {

         delete aBrowser.snapshots[arr[i].index];

         arr.splice(i, 1);

         if (requiresExactIndexMatch)

           return; // Found and removed the only element.

         i--; // Make sure to revisit the index that we just removed an

              // element at.

       }

     }

   },

   /**

    * Adds a new snapshot reference for a given index and browser to the array

    * of references to tracked snapshots.

    *

    * @param aIndex

    *        The index in history of the new snapshot.

    * @param aBrowser

    *        The browser the new snapshot was taken in.

    */

   _addSnapshotRefToArray:

   function HSA__addSnapshotRefToArray(aIndex, aBrowser) {

     let id = { index: aIndex,

                browser: aBrowser };

     let arr = this._trackedSnapshots;

     arr.unshift(id);

     while (arr.length > this._maxSnapshots) {

       let lastElem = arr[arr.length - 1];

       delete lastElem.browser.snapshots[lastElem.index].image;

       delete lastElem.browser.snapshots[lastElem.index];

       arr.splice(-1, 1);

     }

   },

   /**

    * Converts a compressed blob to an Image object. In some situations

    * (especially during fast swiping) aBlob may still be a canvas, not a

    * compressed blob. In this case, we simply return the canvas.

    *

    * @param aBlob

    *        The compressed blob to convert, or a canvas if a blob compression

    *        couldn't complete before this method was called.

    * @return A new Image object representing the converted blob.

    */

   _convertToImg: function HSA__convertToImg(aBlob) {

     if (!aBlob)

       return null;

     // Return aBlob if it's still a canvas and not a compressed blob yet.

     if (aBlob instanceof HTMLCanvasElement)

       return aBlob;

     let img = new Image();

     let url = "";

     try {

       url = URL.createObjectURL(aBlob);

       img.onload = function() {

         URL.revokeObjectURL(url);

       };

     }

     finally {

       img.src = url;

       return img;

     }

   },

   /**

    * Scales the background of a given box element (which uses a given snapshot

    * as background) based on a given scale factor.

    * @param aSnapshot

    *        The snapshot that is used as background of aBox.

    * @param aScale

    *        The scale factor to use.

    * @param aBox

    *        The box element that uses aSnapshot as background.

    */

   _scaleSnapshot: function HSA__scaleSnapshot(aSnapshot, aScale, aBox) {

     if (aSnapshot && aScale != 1 && aBox) {

       if (aSnapshot instanceof HTMLCanvasElement) {

         aBox.style.backgroundSize =

           aSnapshot.width / aScale + "px " + aSnapshot.height / aScale + "px";

       } else {

         // snapshot is instanceof HTMLImageElement

         aSnapshot.addEventListener("load", function() {

           aBox.style.backgroundSize =

             aSnapshot.width / aScale + "px " + aSnapshot.height / aScale + "px";

         });

       }

     }

   },

   /**

    * Sets the snapshot of the current page to the snapshot passed as parameter,

    * or to the one previously stored for the current index in history if the

    * parameter is null.

    *

    * @param aCanvas

    *        The snapshot to set the current page to. If this parameter is null,

    *        the previously stored snapshot for this index (if any) will be used.

    */

   _installCurrentPageSnapshot:

   function HSA__installCurrentPageSnapshot(aCanvas) {

     let currSnapshot = aCanvas;

     let scale = window.devicePixelRatio;

     if (!currSnapshot) {

       let snapshots = gBrowser.selectedBrowser.snapshots || {};

       let currIndex = this._historyIndex;

       if (currIndex in snapshots) {

         currSnapshot = this._convertToImg(snapshots[currIndex].image);

         scale = snapshots[currIndex].scale;

       }

     }

     this._scaleSnapshot(currSnapshot, scale, this._curBox ? this._curBox :

                                                             null);

     document.mozSetImageElement("historySwipeAnimationCurrentPageSnapshot",

                                 currSnapshot);

   },

   /**

    * Sets the snapshots of the previous and next pages to the snapshots

    * previously stored for their respective indeces.

    */

   _installPrevAndNextSnapshots:

   function HSA__installPrevAndNextSnapshots() {

     let snapshots = gBrowser.selectedBrowser.snapshots || [];

     let currIndex = this._historyIndex;

     let prevIndex = currIndex - 1;

     let prevSnapshot = null;

     if (prevIndex in snapshots) {

       prevSnapshot = this._convertToImg(snapshots[prevIndex].image);

       this._scaleSnapshot(prevSnapshot, snapshots[prevIndex].scale,

                           this._prevBox);

     }

     document.mozSetImageElement("historySwipeAnimationPreviousPageSnapshot",

                                 prevSnapshot);

     let nextIndex = currIndex + 1;

     let nextSnapshot = null;

     if (nextIndex in snapshots) {

       nextSnapshot = this._convertToImg(snapshots[nextIndex].image);

       this._scaleSnapshot(nextSnapshot, snapshots[nextIndex].scale,

                           this._nextBox);

     }

     document.mozSetImageElement("historySwipeAnimationNextPageSnapshot",

                                 nextSnapshot);

   },

 };