samples-dev/swarm/swarm_ui_lib/touch/Scroller.dart - sdk - Git at Google

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 // @dart = 2.9

 part of touch;

 /**
  * Implementation of a custom scrolling behavior.
  * This behavior overrides native scrolling for an area. This area can be a
  * single defined part of a page, the entire page, or several different parts
  * of a page.
  *
  * To use this scrolling behavior you need to define a frame and the content.
  * The frame defines the area that the content will scroll within. The frame and
  * content must both be HTML Elements, with the content being a direct child of
  * the frame. Usually the frame is smaller in size than the content. This is
  * not necessary though, if the content is smaller then bouncing will occur to
  * provide feedback that you are past the scrollable area.
  *
  * The scrolling behavior works using the webkit translate3d transformation,
  * which means browsers that do not have hardware accelerated transformations
  * will not perform as well using this. Simple scrolling should be fine even
  * without hardware acceleration, but animating momentum and deceleration is
  * unacceptably slow without it. There is also the option to use relative
  * positioning (setting the left and top styles).
  *
  * For this to work properly you need to set -webkit-text-size-adjust to 'none'
  * on an ancestor element of the frame, or on the frame itself. If you forget
  * this you may see the text content of the scrollable area changing size as it
  * moves.
  *
  * The behavior is intended to support vertical and horizontal scrolling, and
  * scrolling with momentum when a touch gesture flicks with enough velocity.
  */
 typedef void Callback();

 // Helper method to await the completion of 2 futures.
 void joinFutures(List<Future> futures, Callback callback) {
   int count = 0;
   int len = futures.length;
   void helper(value) {
     count++;
     if (count == len) {
       callback();
     }
   }

   for (Future p in futures) {
     p.then(helper);
   }
 }

 class Scroller implements Draggable, MomentumDelegate {
   /** Pixels to move each time an arrow key is pressed. */
   static const ARROW_KEY_DELTA = 30;
   static const SCROLL_WHEEL_VELOCITY = 0.01;
   static const FAST_SNAP_DECELERATION_FACTOR = 0.84;
   static const PAGE_KEY_SCROLL_FRACTION = .85;

   // TODO(jacobr): remove this static variable.
   static bool _dragInProgress = false;

   /** The node that will actually scroll. */
   Element _element;

   /**
    * Frame is the node that will serve as the container for the scrolling
    * content.
    */
   Element _frame;

   /** Touch manager to track the events on the scrollable area. */
   TouchHandler _touchHandler;

   Momentum _momentum;

   StreamController<Event> _onScrollerStart;
   Stream<Event> _onScrollerStartStream;
   StreamController<Event> _onScrollerEnd;
   Stream<Event> _onScrollerEndStream;
   StreamController<Event> _onScrollerDragEnd;
   Stream<Event> _onScrollerDragEndStream;
   StreamController<Event> _onContentMoved;
   Stream<Event> _onContentMovedStream;
   StreamController<Event> _onDecelStart;
   Stream<Event> _onDecelStartStream;

   /** Set if vertical scrolling should be enabled. */
   bool verticalEnabled;

   /** Set if horizontal scrolling should be enabled. */
   bool horizontalEnabled;

   /**
    * Set if momentum should be enabled.
    */
   bool _momentumEnabled;

   /** Set which type of scrolling translation technique should be used. */
   int _scrollTechnique;

   /**
    * The maximum coordinate that the left upper corner of the content can scroll
    * to.
    */
   Coordinate _maxPoint;

   /**
    * An offset to subtract from the maximum coordinate that the left upper
    * corner of the content can scroll to.
    */
   Coordinate _maxOffset;

   /**
    * An offset to add to the minimum coordinate that the left upper corner of
    * the content can scroll to.
    */
   Coordinate _minOffset;

   /** Initialize the current content offset. */
   Coordinate _contentOffset;

   // TODO(jacobr): the function type is
   // [:Function(Element, num, num)->void:].
   /**
    * The function to use that will actually translate the scrollable node.
    */
   Function _setOffsetFunction;
   /**
    * Function that returns the content size that can be specified instead of
    * querying the DOM.
    */
   Function _lookupContentSizeDelegate;

   Size _scrollSize;
   Size _contentSize;
   Coordinate _minPoint;
   bool _isStopping = false;
   Coordinate _contentStartOffset;
   bool _started = false;
   bool _activeGesture = false;
   ScrollWatcher _scrollWatcher;

   Scroller(Element scrollableElem,
       [this.verticalEnabled = false,
       this.horizontalEnabled = false,
       momentumEnabled = true,
       lookupContentSizeDelegate = null,
       num defaultDecelerationFactor = 1,
       int scrollTechnique = null,
       bool capture = false])
       : _momentumEnabled = momentumEnabled,
         _lookupContentSizeDelegate = lookupContentSizeDelegate,
         _element = scrollableElem,
         _frame = scrollableElem.parent,
         _scrollTechnique = scrollTechnique != null
             ? scrollTechnique
             : ScrollerScrollTechnique.TRANSFORM_3D,
         _minPoint = new Coordinate(0, 0),
         _maxPoint = new Coordinate(0, 0),
         _maxOffset = new Coordinate(0, 0),
         _minOffset = new Coordinate(0, 0),
         _contentOffset = new Coordinate(0, 0) {
     _touchHandler = new TouchHandler(this, scrollableElem.parent);
     _momentum = new Momentum(this, defaultDecelerationFactor);

     Element parentElem = scrollableElem.parent;
     assert(parentElem != null);
     _setOffsetFunction = _getOffsetFunction(_scrollTechnique);
     _touchHandler.setDraggable(this);
     _touchHandler.enable(capture);

     _frame.onMouseWheel.listen((e) {
       if (e.deltaY != 0 && verticalEnabled ||
           e.deltaX != 0 && horizontalEnabled) {
         num x = horizontalEnabled ? e.deltaX : 0;
         num y = verticalEnabled ? e.deltaY : 0;
         throwDelta(x, y, FAST_SNAP_DECELERATION_FACTOR);
         e.preventDefault();
       }
     });

     _frame.onKeyDown.listen((KeyboardEvent e) {
       bool handled = false;
       // We ignore key events where further scrolling in that direction
       // would have no impact which matches default browser behavior with
       // nested scrollable areas.

       switch (e.keyCode) {
         case 33: // page-up
           throwDelta(0, _scrollSize.height * PAGE_KEY_SCROLL_FRACTION);
           handled = true;
           break;
         case 34: // page-down
           throwDelta(0, -_scrollSize.height * PAGE_KEY_SCROLL_FRACTION);
           handled = true;
           break;
         case 35: // End
           throwTo(_maxPoint.x, _minPoint.y, FAST_SNAP_DECELERATION_FACTOR);
           handled = true;
           break;
         case 36: // Home
           throwTo(_maxPoint.x, _maxPoint.y, FAST_SNAP_DECELERATION_FACTOR);
           handled = true;
           break;
 /* TODO(jacobr): enable arrow keys when the don't conflict with other
    application keyboard shortcuts.
           case 38: // up
             handled = throwDelta(
                 0,
                 ARROW_KEY_DELTA,
                 FAST_SNAP_DECELERATION_FACTOR);
             break;
           case 40: // down
             handled = throwDelta(
                 0, -ARROW_KEY_DELTA,
                 FAST_SNAP_DECELERATION_FACTOR);
             break;
           case 37: // left
             handled = throwDelta(
                 ARROW_KEY_DELTA, 0,
                 FAST_SNAP_DECELERATION_FACTOR);
             break;
           case 39: // right
             handled = throwDelta(
                 -ARROW_KEY_DELTA,
                 0,
                 FAST_SNAP_DECELERATION_FACTOR);
             break;
             */
       }
       if (handled) {
         e.preventDefault();
       }
     });
     // The scrollable element must be relatively positioned.
     // TODO(jacobr): this assert fires asynchronously which could be confusing.
     if (_scrollTechnique == ScrollerScrollTechnique.RELATIVE_POSITIONING) {
       assert(_element.getComputedStyle().position != "static");
     }

     _initLayer();
   }

   Stream<Event> get onScrollerStart {
     if (_onScrollerStart == null) {
       _onScrollerStart = new StreamController<Event>.broadcast(sync: true);
       _onScrollerStartStream = _onScrollerStart.stream;
     }
     return _onScrollerStartStream;
   }

   Stream<Event> get onScrollerEnd {
     if (_onScrollerEnd == null) {
       _onScrollerEnd = new StreamController<Event>.broadcast(sync: true);
       _onScrollerEndStream = _onScrollerEnd.stream;
     }
     return _onScrollerEndStream;
   }

   Stream<Event> get onScrollerDragEnd {
     if (_onScrollerDragEnd == null) {
       _onScrollerDragEnd = new StreamController<Event>.broadcast(sync: true);
       _onScrollerDragEndStream = _onScrollerDragEnd.stream;
     }
     return _onScrollerDragEndStream;
   }

   Stream<Event> get onContentMoved {
     if (_onContentMoved == null) {
       _onContentMoved = new StreamController<Event>.broadcast(sync: true);
       _onContentMovedStream = _onContentMoved.stream;
     }
     return _onContentMovedStream;
   }

   Stream<Event> get onDecelStart {
     if (_onDecelStart == null) {
       _onDecelStart = new StreamController<Event>.broadcast(sync: true);
       _onDecelStartStream = _onDecelStart.stream;
     }
     return _onDecelStartStream;
   }

   /**
    * Add a scroll listener. This allows other classes to subscribe to scroll
    * notifications from this scroller.
    */
   void addScrollListener(ScrollListener listener) {
     if (_scrollWatcher == null) {
       _scrollWatcher = new ScrollWatcher(this);
       _scrollWatcher.initialize();
     }
     _scrollWatcher.addListener(listener);
   }

   /**
    * Adjust the new calculated scroll position based on the minimum allowed
    * position and returns the adjusted scroll value.
    */
   num _adjustValue(num newPosition, num minPosition, num maxPosition) {
     assert(minPosition <= maxPosition);

     if (newPosition < minPosition) {
       newPosition -= (newPosition - minPosition) / 2;
     } else {
       if (newPosition > maxPosition) {
         newPosition -= (newPosition - maxPosition) / 2;
       }
     }
     return newPosition;
   }

   /**
    * Coordinate we would end up at if we did nothing.
    */
   Coordinate get currentTarget {
     Coordinate end = _momentum.destination;
     if (end == null) {
       end = _contentOffset;
     }
     return end;
   }

   Coordinate get contentOffset => _contentOffset;

   /**
    * Animate the position of the scroller to the specified [x], [y] coordinates
    * by applying the throw gesture with the correct velocity to end at that
    * location.
    */
   void throwTo(num x, num y, [num decelerationFactor = null]) {
     reconfigure(() {
       final snappedTarget = _snapToBounds(x, y);
       // If a deceleration factor is not specified, use the existing
       // deceleration factor specified by the momentum simulator.
       if (decelerationFactor == null) {
         decelerationFactor = _momentum.decelerationFactor;
       }

       if (snappedTarget != currentTarget) {
         _momentum.abort();

         _startDeceleration(
             _momentum.calculateVelocity(
                 _contentOffset, snappedTarget, decelerationFactor),
             decelerationFactor);
         if (_onDecelStart != null) {
           _onDecelStart.add(new Event(ScrollerEventType.DECEL_START));
         }
       }
     });
   }

   void throwDelta(num deltaX, num deltaY, [num decelerationFactor = null]) {
     Coordinate start = _contentOffset;
     Coordinate end = currentTarget;
     int x = end.x.toInt();
     int y = end.y.toInt();
     // If we are throwing in the opposite direction of the existing momentum,
     // cancel the current momentum.
     if (deltaX != 0 && deltaX.isNegative != (end.x - start.x).isNegative) {
       x = start.x;
     }
     if (deltaY != 0 && deltaY.isNegative != (end.y - start.y).isNegative) {
       y = start.y;
     }
     x += deltaX.toInt();
     y += deltaY.toInt();
     throwTo(x, y, decelerationFactor);
   }

   void setPosition(num x, num y) {
     _momentum.abort();
     _contentOffset.x = x;
     _contentOffset.y = y;
     _snapContentOffsetToBounds();
     _setContentOffset(_contentOffset.x, _contentOffset.y);
   }

   /**
    * Adjusted content size is a size with the combined largest height and width
    * of both the content and the frame.
    */
   Size _getAdjustedContentSize() {
     return new Size(Math.max(_scrollSize.width, _contentSize.width),
         Math.max(_scrollSize.height, _contentSize.height));
   }

   // TODO(jmesserly): these should be properties instead of get* methods
   num getDefaultVerticalOffset() => _maxPoint.y;
   Element getElement() => _element;
   Element getFrame() => _frame;
   num getHorizontalOffset() => _contentOffset.x;

   /**
    * [x] Value to use as reference for percent measurement. If
    *      none is provided then the content's current x offset will be used.
    * Returns the percent of the page scrolled horizontally.
    */
   num getHorizontalScrollPercent([num x = null]) {
     x = x != null ? x : _contentOffset.x;
     return (x - _minPoint.x) / (_maxPoint.x - _minPoint.x);
   }

   num getMaxPointY() => _maxPoint.y;
   num getMinPointY() => _minPoint.y;
   Momentum get momentum => _momentum;

   /**
    * Provide access to the touch handler that the scroller created to manage
    * touch events.
    */
   TouchHandler getTouchHandler() => _touchHandler;
   num getVerticalOffset() => _contentOffset.y;

   /**
    * [y] value is used as reference for percent measurement. If
    * none is provided then the content's current y offset will be used.
    */
   num getVerticalScrollPercent([num y = null]) {
     y = y != null ? y : _contentOffset.y;
     return (y - _minPoint.y) / Math.max(1, _maxPoint.y - _minPoint.y);
   }

   /**
    * Initialize the dom elements necessary for the scrolling to work.
    */
   void _initLayer() {
     // The scrollable node provided to Scroller must be a direct child
     // of the scrollable frame.
     // TODO(jacobr): Figure out why this is failing on dartium.
     // assert(_element.parent == _frame);
     _setContentOffset(_maxPoint.x, _maxPoint.y);
   }

   void onDecelerate(num x, num y) {
     _setContentOffset(x, y);
   }

   void onDecelerationEnd() {
     if (_onScrollerEnd != null) {
       _onScrollerEnd.add(new Event(ScrollerEventType.SCROLLER_END));
     }
     _started = false;
   }

   void onDragEnd() {
     _dragInProgress = false;

     bool decelerating = false;
     if (_activeGesture) {
       if (_momentumEnabled) {
         decelerating = _startDeceleration(_touchHandler.getEndVelocity());
       }
     }

     if (_onScrollerDragEnd != null) {
       _onScrollerDragEnd.add(new Event(ScrollerEventType.DRAG_END));
     }

     if (!decelerating) {
       _snapContentOffsetToBounds();
       if (_onScrollerEnd != null) {
         _onScrollerEnd.add(new Event(ScrollerEventType.SCROLLER_END));
       }
       _started = false;
     } else {
       if (_onDecelStart != null) {
         _onDecelStart.add(new Event(ScrollerEventType.DECEL_START));
       }
     }
     _activeGesture = false;
   }

   void onDragMove() {
     if (_isStopping || (!_activeGesture && _dragInProgress)) {
       return;
     }

     assert(_contentStartOffset != null); // Content start not set
     Coordinate contentStart = _contentStartOffset;
     num newX = contentStart.x + _touchHandler.getDragDeltaX();
     num newY = contentStart.y + _touchHandler.getDragDeltaY();
     newY = _shouldScrollVertically()
         ? _adjustValue(newY, _minPoint.y, _maxPoint.y)
         : 0;
     newX = _shouldScrollHorizontally()
         ? _adjustValue(newX, _minPoint.x, _maxPoint.x)
         : 0;
     if (!_activeGesture) {
       _activeGesture = true;
       _dragInProgress = true;
     }
     if (!_started) {
       _started = true;
       if (_onScrollerStart != null) {
         _onScrollerStart.add(new Event(ScrollerEventType.SCROLLER_START));
       }
     }
     _setContentOffset(newX, newY);
   }

   bool onDragStart(TouchEvent e) {
     if (e.touches.length > 1) {
       return false;
     }
     bool shouldHorizontal = _shouldScrollHorizontally();
     bool shouldVertical = _shouldScrollVertically();
     bool verticalish = _touchHandler.getDragDeltaY().abs() >
         _touchHandler.getDragDeltaX().abs();
     return !!(shouldVertical || shouldHorizontal && !verticalish);
   }

   void onTouchEnd() {}

   /**
    * Prepare the scrollable area for possible movement.
    */
   bool onTouchStart(TouchEvent e) {
     reconfigure(() {
       final touch = e.touches[0];
       if (_momentum.decelerating) {
         e.preventDefault();
         e.stopPropagation();
         stop();
       }
       _contentStartOffset = _contentOffset.clone();
       _snapContentOffsetToBounds();
     });
     return true;
   }

   /**
    * Recalculate dimensions of the frame and the content. Adjust the minPoint
    * and maxPoint allowed for scrolling and scroll to a valid position. Call
    * this method if you know the frame or content has been updated. Called
    * internally on every touchstart event the frame receives.
    */
   void reconfigure(Callback callback) {
     _resize(() {
       _snapContentOffsetToBounds();
       callback();
     });
   }

   void reset() {
     stop();
     _touchHandler.reset();
     _maxOffset.x = 0;
     _maxOffset.y = 0;
     _minOffset.x = 0;
     _minOffset.y = 0;
     reconfigure(() => _setContentOffset(_maxPoint.x, _maxPoint.y));
   }

   /**
    * Recalculate dimensions of the frame and the content. Adjust the minPoint
    * and maxPoint allowed for scrolling.
    */
   void _resize(Callback callback) {
     scheduleMicrotask(() {
       if (_lookupContentSizeDelegate != null) {
         _contentSize = _lookupContentSizeDelegate();
       } else {
         _contentSize = new Size(_element.scrollWidth, _element.scrollHeight);
       }

       _scrollSize = new Size(_frame.offset.width, _frame.offset.height);
       Size adjusted = _getAdjustedContentSize();
       _maxPoint = new Coordinate(-_maxOffset.x, -_maxOffset.y);
       _minPoint = new Coordinate(
           Math.min(
               _scrollSize.width - adjusted.width + _minOffset.x, _maxPoint.x),
           Math.min(_scrollSize.height - adjusted.height + _minOffset.y,
               _maxPoint.y));
       callback();
     });
   }

   Coordinate _snapToBounds(num x, num y) {
     num clampX = GoogleMath.clamp(_minPoint.x, x, _maxPoint.x);
     num clampY = GoogleMath.clamp(_minPoint.y, y, _maxPoint.y);
     return new Coordinate(clampX, clampY);
   }

   /**
    * Translate the content to a new position specified in px.
    */
   void _setContentOffset(num x, num y) {
     _contentOffset.x = x;
     _contentOffset.y = y;
     _setOffsetFunction(_element, x, y);
     if (_onContentMoved != null) {
       _onContentMoved.add(new Event(ScrollerEventType.CONTENT_MOVED));
     }
   }

   /**
    * Enable or disable momentum.
    */
   void setMomentum(bool enable) {
     _momentumEnabled = enable;
   }

   /**
    * Sets the vertical scrolled offset of the element where [y] is the amount
    * of vertical space to be scrolled, in pixels.
    */
   void setVerticalOffset(num y) {
     _setContentOffset(_contentOffset.x, y);
   }

   /**
    * Whether the scrollable area should scroll horizontally. Only
    * returns true if the client has enabled horizontal scrolling, and the
    * content is wider than the frame.
    */
   bool _shouldScrollHorizontally() {
     return horizontalEnabled && _scrollSize.width < _contentSize.width;
   }

   /**
    * Whether the scrollable area should scroll vertically. Only
    * returns true if the client has enabled vertical scrolling.
    * Vertical bouncing will occur even if frame is taller than content, because
    * this is what iPhone web apps tend to do. If this is not the desired
    * behavior, either disable vertical scrolling for this scroller or add a
    * 'bouncing' parameter to this interface.
    */
   bool _shouldScrollVertically() {
     return verticalEnabled;
   }

   /**
    * In the event that the content is currently beyond the bounds of
    * the frame, snap it back in to place.
    */
   void _snapContentOffsetToBounds() {
     num clampX = GoogleMath.clamp(_minPoint.x, _contentOffset.x, _maxPoint.x);
     num clampY = GoogleMath.clamp(_minPoint.y, _contentOffset.y, _maxPoint.y);
     if (_contentOffset.x != clampX || _contentOffset.y != clampY) {
       _setContentOffset(clampX, clampY);
     }
   }

   /**
    * Initiate the deceleration behavior given a flick [velocity].
    * Returns true if deceleration has been initiated.
    */
   bool _startDeceleration(Coordinate velocity,
       [num decelerationFactor = null]) {
     if (!_shouldScrollHorizontally()) {
       velocity.x = 0;
     }
     if (!_shouldScrollVertically()) {
       velocity.y = 0;
     }
     assert(_minPoint != null); // Min point is not set
     assert(_maxPoint != null); // Max point is not set
     return _momentum.start(
         velocity, _minPoint, _maxPoint, _contentOffset, decelerationFactor);
   }

   Coordinate stop() {
     return _momentum.stop();
   }

   /**
    * Stop the deceleration of the scrollable content given a new position in px.
    */
   void _stopDecelerating(num x, num y) {
     _momentum.stop();
     _setContentOffset(x, y);
   }

   static Function _getOffsetFunction(int scrollTechnique) {
     return scrollTechnique == ScrollerScrollTechnique.TRANSFORM_3D
         ? (el, x, y) {
             FxUtil.setTranslate(el, x, y, 0);
           }
         : (el, x, y) {
             FxUtil.setLeftAndTop(el, x, y);
           };
   }
 }

 // TODO(jacobr): cleanup this class of enum constants.
 class ScrollerEventType {
   static const SCROLLER_START = "scroller:scroll_start";
   static const SCROLLER_END = "scroller:scroll_end";
   static const DRAG_END = "scroller:drag_end";
   static const CONTENT_MOVED = "scroller:content_moved";
   static const DECEL_START = "scroller:decel_start";
 }

 class ScrollerScrollTechnique {
   static const TRANSFORM_3D = 1;
   static const RELATIVE_POSITIONING = 2;
 }
	// Copyright (c) 2011, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	// @dart = 2.9

	part of touch;

	/**
	* Implementation of a custom scrolling behavior.
	* This behavior overrides native scrolling for an area. This area can be a
	* single defined part of a page, the entire page, or several different parts
	* of a page.
	*
	* To use this scrolling behavior you need to define a frame and the content.
	* The frame defines the area that the content will scroll within. The frame and
	* content must both be HTML Elements, with the content being a direct child of
	* the frame. Usually the frame is smaller in size than the content. This is
	* not necessary though, if the content is smaller then bouncing will occur to
	* provide feedback that you are past the scrollable area.
	*
	* The scrolling behavior works using the webkit translate3d transformation,
	* which means browsers that do not have hardware accelerated transformations
	* will not perform as well using this. Simple scrolling should be fine even
	* without hardware acceleration, but animating momentum and deceleration is
	* unacceptably slow without it. There is also the option to use relative
	* positioning (setting the left and top styles).
	*
	* For this to work properly you need to set -webkit-text-size-adjust to 'none'
	* on an ancestor element of the frame, or on the frame itself. If you forget
	* this you may see the text content of the scrollable area changing size as it
	* moves.
	*
	* The behavior is intended to support vertical and horizontal scrolling, and
	* scrolling with momentum when a touch gesture flicks with enough velocity.
	*/
	typedef void Callback();

	// Helper method to await the completion of 2 futures.
	void joinFutures(List<Future> futures, Callback callback) {
	int count = 0;
	int len = futures.length;
	void helper(value) {
	count++;
	if (count == len) {
	callback();
	}
	}

	for (Future p in futures) {
	p.then(helper);
	}
	}

	class Scroller implements Draggable, MomentumDelegate {
	/** Pixels to move each time an arrow key is pressed. */
	static const ARROW_KEY_DELTA = 30;
	static const SCROLL_WHEEL_VELOCITY = 0.01;
	static const FAST_SNAP_DECELERATION_FACTOR = 0.84;
	static const PAGE_KEY_SCROLL_FRACTION = .85;

	// TODO(jacobr): remove this static variable.
	static bool _dragInProgress = false;

	/** The node that will actually scroll. */
	Element _element;

	/**
	* Frame is the node that will serve as the container for the scrolling
	* content.
	*/
	Element _frame;

	/** Touch manager to track the events on the scrollable area. */
	TouchHandler _touchHandler;

	Momentum _momentum;

	StreamController<Event> _onScrollerStart;
	Stream<Event> _onScrollerStartStream;
	StreamController<Event> _onScrollerEnd;
	Stream<Event> _onScrollerEndStream;
	StreamController<Event> _onScrollerDragEnd;
	Stream<Event> _onScrollerDragEndStream;
	StreamController<Event> _onContentMoved;
	Stream<Event> _onContentMovedStream;
	StreamController<Event> _onDecelStart;
	Stream<Event> _onDecelStartStream;

	/** Set if vertical scrolling should be enabled. */
	bool verticalEnabled;

	/** Set if horizontal scrolling should be enabled. */
	bool horizontalEnabled;

	/**
	* Set if momentum should be enabled.
	*/
	bool _momentumEnabled;

	/** Set which type of scrolling translation technique should be used. */
	int _scrollTechnique;

	/**
	* The maximum coordinate that the left upper corner of the content can scroll
	* to.
	*/
	Coordinate _maxPoint;

	/**
	* An offset to subtract from the maximum coordinate that the left upper
	* corner of the content can scroll to.
	*/
	Coordinate _maxOffset;

	/**
	* An offset to add to the minimum coordinate that the left upper corner of
	* the content can scroll to.
	*/
	Coordinate _minOffset;

	/** Initialize the current content offset. */
	Coordinate _contentOffset;

	// TODO(jacobr): the function type is
	// [:Function(Element, num, num)->void:].
	/**
	* The function to use that will actually translate the scrollable node.
	*/
	Function _setOffsetFunction;
	/**
	* Function that returns the content size that can be specified instead of
	* querying the DOM.
	*/
	Function _lookupContentSizeDelegate;

	Size _scrollSize;
	Size _contentSize;
	Coordinate _minPoint;
	bool _isStopping = false;
	Coordinate _contentStartOffset;
	bool _started = false;
	bool _activeGesture = false;
	ScrollWatcher _scrollWatcher;

	Scroller(Element scrollableElem,
	[this.verticalEnabled = false,
	this.horizontalEnabled = false,
	momentumEnabled = true,
	lookupContentSizeDelegate = null,
	num defaultDecelerationFactor = 1,
	int scrollTechnique = null,
	bool capture = false])
	: _momentumEnabled = momentumEnabled,
	_lookupContentSizeDelegate = lookupContentSizeDelegate,
	_element = scrollableElem,
	_frame = scrollableElem.parent,
	_scrollTechnique = scrollTechnique != null
	? scrollTechnique
	: ScrollerScrollTechnique.TRANSFORM_3D,
	_minPoint = new Coordinate(0, 0),
	_maxPoint = new Coordinate(0, 0),
	_maxOffset = new Coordinate(0, 0),
	_minOffset = new Coordinate(0, 0),
	_contentOffset = new Coordinate(0, 0) {
	_touchHandler = new TouchHandler(this, scrollableElem.parent);
	_momentum = new Momentum(this, defaultDecelerationFactor);

	Element parentElem = scrollableElem.parent;
	assert(parentElem != null);
	_setOffsetFunction = _getOffsetFunction(_scrollTechnique);
	_touchHandler.setDraggable(this);
	_touchHandler.enable(capture);

	_frame.onMouseWheel.listen((e) {
	if (e.deltaY != 0 && verticalEnabled \|\|
	e.deltaX != 0 && horizontalEnabled) {
	num x = horizontalEnabled ? e.deltaX : 0;
	num y = verticalEnabled ? e.deltaY : 0;
	throwDelta(x, y, FAST_SNAP_DECELERATION_FACTOR);
	e.preventDefault();
	}
	});

	_frame.onKeyDown.listen((KeyboardEvent e) {
	bool handled = false;
	// We ignore key events where further scrolling in that direction
	// would have no impact which matches default browser behavior with
	// nested scrollable areas.

	switch (e.keyCode) {
	case 33: // page-up
	throwDelta(0, _scrollSize.height * PAGE_KEY_SCROLL_FRACTION);
	handled = true;
	break;
	case 34: // page-down
	throwDelta(0, -_scrollSize.height * PAGE_KEY_SCROLL_FRACTION);
	handled = true;
	break;
	case 35: // End
	throwTo(_maxPoint.x, _minPoint.y, FAST_SNAP_DECELERATION_FACTOR);
	handled = true;
	break;
	case 36: // Home
	throwTo(_maxPoint.x, _maxPoint.y, FAST_SNAP_DECELERATION_FACTOR);
	handled = true;
	break;
	/* TODO(jacobr): enable arrow keys when the don't conflict with other
	application keyboard shortcuts.
	case 38: // up
	handled = throwDelta(
	0,
	ARROW_KEY_DELTA,
	FAST_SNAP_DECELERATION_FACTOR);
	break;
	case 40: // down
	handled = throwDelta(
	0, -ARROW_KEY_DELTA,
	FAST_SNAP_DECELERATION_FACTOR);
	break;
	case 37: // left
	handled = throwDelta(
	ARROW_KEY_DELTA, 0,
	FAST_SNAP_DECELERATION_FACTOR);
	break;
	case 39: // right
	handled = throwDelta(
	-ARROW_KEY_DELTA,
	0,
	FAST_SNAP_DECELERATION_FACTOR);
	break;
	*/
	}
	if (handled) {
	e.preventDefault();
	}
	});
	// The scrollable element must be relatively positioned.
	// TODO(jacobr): this assert fires asynchronously which could be confusing.
	if (_scrollTechnique == ScrollerScrollTechnique.RELATIVE_POSITIONING) {
	assert(_element.getComputedStyle().position != "static");
	}

	_initLayer();
	}

	Stream<Event> get onScrollerStart {
	if (_onScrollerStart == null) {
	_onScrollerStart = new StreamController<Event>.broadcast(sync: true);
	_onScrollerStartStream = _onScrollerStart.stream;
	}
	return _onScrollerStartStream;
	}

	Stream<Event> get onScrollerEnd {
	if (_onScrollerEnd == null) {
	_onScrollerEnd = new StreamController<Event>.broadcast(sync: true);
	_onScrollerEndStream = _onScrollerEnd.stream;
	}
	return _onScrollerEndStream;
	}

	Stream<Event> get onScrollerDragEnd {
	if (_onScrollerDragEnd == null) {
	_onScrollerDragEnd = new StreamController<Event>.broadcast(sync: true);
	_onScrollerDragEndStream = _onScrollerDragEnd.stream;
	}
	return _onScrollerDragEndStream;
	}

	Stream<Event> get onContentMoved {
	if (_onContentMoved == null) {
	_onContentMoved = new StreamController<Event>.broadcast(sync: true);
	_onContentMovedStream = _onContentMoved.stream;
	}
	return _onContentMovedStream;
	}

	Stream<Event> get onDecelStart {
	if (_onDecelStart == null) {
	_onDecelStart = new StreamController<Event>.broadcast(sync: true);
	_onDecelStartStream = _onDecelStart.stream;
	}
	return _onDecelStartStream;
	}

	/**
	* Add a scroll listener. This allows other classes to subscribe to scroll
	* notifications from this scroller.
	*/
	void addScrollListener(ScrollListener listener) {
	if (_scrollWatcher == null) {
	_scrollWatcher = new ScrollWatcher(this);
	_scrollWatcher.initialize();
	}
	_scrollWatcher.addListener(listener);
	}

	/**
	* Adjust the new calculated scroll position based on the minimum allowed
	* position and returns the adjusted scroll value.
	*/
	num _adjustValue(num newPosition, num minPosition, num maxPosition) {
	assert(minPosition <= maxPosition);

	if (newPosition < minPosition) {
	newPosition -= (newPosition - minPosition) / 2;
	} else {
	if (newPosition > maxPosition) {
	newPosition -= (newPosition - maxPosition) / 2;
	}
	}
	return newPosition;
	}

	/**
	* Coordinate we would end up at if we did nothing.
	*/
	Coordinate get currentTarget {
	Coordinate end = _momentum.destination;
	if (end == null) {
	end = _contentOffset;
	}
	return end;
	}

	Coordinate get contentOffset => _contentOffset;

	/**
	* Animate the position of the scroller to the specified [x], [y] coordinates
	* by applying the throw gesture with the correct velocity to end at that
	* location.
	*/
	void throwTo(num x, num y, [num decelerationFactor = null]) {
	reconfigure(() {
	final snappedTarget = _snapToBounds(x, y);
	// If a deceleration factor is not specified, use the existing
	// deceleration factor specified by the momentum simulator.
	if (decelerationFactor == null) {
	decelerationFactor = _momentum.decelerationFactor;
	}

	if (snappedTarget != currentTarget) {
	_momentum.abort();

	_startDeceleration(
	_momentum.calculateVelocity(
	_contentOffset, snappedTarget, decelerationFactor),
	decelerationFactor);
	if (_onDecelStart != null) {
	_onDecelStart.add(new Event(ScrollerEventType.DECEL_START));
	}
	}
	});
	}

	void throwDelta(num deltaX, num deltaY, [num decelerationFactor = null]) {
	Coordinate start = _contentOffset;
	Coordinate end = currentTarget;
	int x = end.x.toInt();
	int y = end.y.toInt();
	// If we are throwing in the opposite direction of the existing momentum,
	// cancel the current momentum.
	if (deltaX != 0 && deltaX.isNegative != (end.x - start.x).isNegative) {
	x = start.x;
	}
	if (deltaY != 0 && deltaY.isNegative != (end.y - start.y).isNegative) {
	y = start.y;
	}
	x += deltaX.toInt();
	y += deltaY.toInt();
	throwTo(x, y, decelerationFactor);
	}

	void setPosition(num x, num y) {
	_momentum.abort();
	_contentOffset.x = x;
	_contentOffset.y = y;
	_snapContentOffsetToBounds();
	_setContentOffset(_contentOffset.x, _contentOffset.y);
	}

	/**
	* Adjusted content size is a size with the combined largest height and width
	* of both the content and the frame.
	*/
	Size _getAdjustedContentSize() {
	return new Size(Math.max(_scrollSize.width, _contentSize.width),
	Math.max(_scrollSize.height, _contentSize.height));
	}

	// TODO(jmesserly): these should be properties instead of get* methods
	num getDefaultVerticalOffset() => _maxPoint.y;
	Element getElement() => _element;
	Element getFrame() => _frame;
	num getHorizontalOffset() => _contentOffset.x;

	/**
	* [x] Value to use as reference for percent measurement. If
	* none is provided then the content's current x offset will be used.
	* Returns the percent of the page scrolled horizontally.
	*/
	num getHorizontalScrollPercent([num x = null]) {
	x = x != null ? x : _contentOffset.x;
	return (x - _minPoint.x) / (_maxPoint.x - _minPoint.x);
	}

	num getMaxPointY() => _maxPoint.y;
	num getMinPointY() => _minPoint.y;
	Momentum get momentum => _momentum;

	/**
	* Provide access to the touch handler that the scroller created to manage
	* touch events.
	*/
	TouchHandler getTouchHandler() => _touchHandler;
	num getVerticalOffset() => _contentOffset.y;

	/**
	* [y] value is used as reference for percent measurement. If
	* none is provided then the content's current y offset will be used.
	*/
	num getVerticalScrollPercent([num y = null]) {
	y = y != null ? y : _contentOffset.y;
	return (y - _minPoint.y) / Math.max(1, _maxPoint.y - _minPoint.y);
	}

	/**
	* Initialize the dom elements necessary for the scrolling to work.
	*/
	void _initLayer() {
	// The scrollable node provided to Scroller must be a direct child
	// of the scrollable frame.
	// TODO(jacobr): Figure out why this is failing on dartium.
	// assert(_element.parent == _frame);
	_setContentOffset(_maxPoint.x, _maxPoint.y);
	}

	void onDecelerate(num x, num y) {
	_setContentOffset(x, y);
	}

	void onDecelerationEnd() {
	if (_onScrollerEnd != null) {
	_onScrollerEnd.add(new Event(ScrollerEventType.SCROLLER_END));
	}
	_started = false;
	}

	void onDragEnd() {
	_dragInProgress = false;

	bool decelerating = false;
	if (_activeGesture) {
	if (_momentumEnabled) {
	decelerating = _startDeceleration(_touchHandler.getEndVelocity());
	}
	}

	if (_onScrollerDragEnd != null) {
	_onScrollerDragEnd.add(new Event(ScrollerEventType.DRAG_END));
	}

	if (!decelerating) {
	_snapContentOffsetToBounds();
	if (_onScrollerEnd != null) {
	_onScrollerEnd.add(new Event(ScrollerEventType.SCROLLER_END));
	}
	_started = false;
	} else {
	if (_onDecelStart != null) {
	_onDecelStart.add(new Event(ScrollerEventType.DECEL_START));
	}
	}
	_activeGesture = false;
	}

	void onDragMove() {
	if (_isStopping \|\| (!_activeGesture && _dragInProgress)) {
	return;
	}

	assert(_contentStartOffset != null); // Content start not set
	Coordinate contentStart = _contentStartOffset;
	num newX = contentStart.x + _touchHandler.getDragDeltaX();
	num newY = contentStart.y + _touchHandler.getDragDeltaY();
	newY = _shouldScrollVertically()
	? _adjustValue(newY, _minPoint.y, _maxPoint.y)
	: 0;
	newX = _shouldScrollHorizontally()
	? _adjustValue(newX, _minPoint.x, _maxPoint.x)
	: 0;
	if (!_activeGesture) {
	_activeGesture = true;
	_dragInProgress = true;
	}
	if (!_started) {
	_started = true;
	if (_onScrollerStart != null) {
	_onScrollerStart.add(new Event(ScrollerEventType.SCROLLER_START));
	}
	}
	_setContentOffset(newX, newY);
	}

	bool onDragStart(TouchEvent e) {
	if (e.touches.length > 1) {
	return false;
	}
	bool shouldHorizontal = _shouldScrollHorizontally();
	bool shouldVertical = _shouldScrollVertically();
	bool verticalish = _touchHandler.getDragDeltaY().abs() >
	_touchHandler.getDragDeltaX().abs();
	return !!(shouldVertical \|\| shouldHorizontal && !verticalish);
	}

	void onTouchEnd() {}

	/**
	* Prepare the scrollable area for possible movement.
	*/
	bool onTouchStart(TouchEvent e) {
	reconfigure(() {
	final touch = e.touches[0];
	if (_momentum.decelerating) {
	e.preventDefault();
	e.stopPropagation();
	stop();
	}
	_contentStartOffset = _contentOffset.clone();
	_snapContentOffsetToBounds();
	});
	return true;
	}

	/**
	* Recalculate dimensions of the frame and the content. Adjust the minPoint
	* and maxPoint allowed for scrolling and scroll to a valid position. Call
	* this method if you know the frame or content has been updated. Called
	* internally on every touchstart event the frame receives.
	*/
	void reconfigure(Callback callback) {
	_resize(() {
	_snapContentOffsetToBounds();
	callback();
	});
	}

	void reset() {
	stop();
	_touchHandler.reset();
	_maxOffset.x = 0;
	_maxOffset.y = 0;
	_minOffset.x = 0;
	_minOffset.y = 0;
	reconfigure(() => _setContentOffset(_maxPoint.x, _maxPoint.y));
	}

	/**
	* Recalculate dimensions of the frame and the content. Adjust the minPoint
	* and maxPoint allowed for scrolling.
	*/
	void _resize(Callback callback) {
	scheduleMicrotask(() {
	if (_lookupContentSizeDelegate != null) {
	_contentSize = _lookupContentSizeDelegate();
	} else {
	_contentSize = new Size(_element.scrollWidth, _element.scrollHeight);
	}

	_scrollSize = new Size(_frame.offset.width, _frame.offset.height);
	Size adjusted = _getAdjustedContentSize();
	_maxPoint = new Coordinate(-_maxOffset.x, -_maxOffset.y);
	_minPoint = new Coordinate(
	Math.min(
	_scrollSize.width - adjusted.width + _minOffset.x, _maxPoint.x),
	Math.min(_scrollSize.height - adjusted.height + _minOffset.y,
	_maxPoint.y));
	callback();
	});
	}

	Coordinate _snapToBounds(num x, num y) {
	num clampX = GoogleMath.clamp(_minPoint.x, x, _maxPoint.x);
	num clampY = GoogleMath.clamp(_minPoint.y, y, _maxPoint.y);
	return new Coordinate(clampX, clampY);
	}

	/**
	* Translate the content to a new position specified in px.
	*/
	void _setContentOffset(num x, num y) {
	_contentOffset.x = x;
	_contentOffset.y = y;
	_setOffsetFunction(_element, x, y);
	if (_onContentMoved != null) {
	_onContentMoved.add(new Event(ScrollerEventType.CONTENT_MOVED));
	}
	}

	/**
	* Enable or disable momentum.
	*/
	void setMomentum(bool enable) {
	_momentumEnabled = enable;
	}

	/**
	* Sets the vertical scrolled offset of the element where [y] is the amount
	* of vertical space to be scrolled, in pixels.
	*/
	void setVerticalOffset(num y) {
	_setContentOffset(_contentOffset.x, y);
	}

	/**
	* Whether the scrollable area should scroll horizontally. Only
	* returns true if the client has enabled horizontal scrolling, and the
	* content is wider than the frame.
	*/
	bool _shouldScrollHorizontally() {
	return horizontalEnabled && _scrollSize.width < _contentSize.width;
	}

	/**
	* Whether the scrollable area should scroll vertically. Only
	* returns true if the client has enabled vertical scrolling.
	* Vertical bouncing will occur even if frame is taller than content, because
	* this is what iPhone web apps tend to do. If this is not the desired
	* behavior, either disable vertical scrolling for this scroller or add a
	* 'bouncing' parameter to this interface.
	*/
	bool _shouldScrollVertically() {
	return verticalEnabled;
	}

	/**
	* In the event that the content is currently beyond the bounds of
	* the frame, snap it back in to place.
	*/
	void _snapContentOffsetToBounds() {
	num clampX = GoogleMath.clamp(_minPoint.x, _contentOffset.x, _maxPoint.x);
	num clampY = GoogleMath.clamp(_minPoint.y, _contentOffset.y, _maxPoint.y);
	if (_contentOffset.x != clampX \|\| _contentOffset.y != clampY) {
	_setContentOffset(clampX, clampY);
	}
	}

	/**
	* Initiate the deceleration behavior given a flick [velocity].
	* Returns true if deceleration has been initiated.
	*/
	bool _startDeceleration(Coordinate velocity,
	[num decelerationFactor = null]) {
	if (!_shouldScrollHorizontally()) {
	velocity.x = 0;
	}
	if (!_shouldScrollVertically()) {
	velocity.y = 0;
	}
	assert(_minPoint != null); // Min point is not set
	assert(_maxPoint != null); // Max point is not set
	return _momentum.start(
	velocity, _minPoint, _maxPoint, _contentOffset, decelerationFactor);
	}

	Coordinate stop() {
	return _momentum.stop();
	}

	/**
	* Stop the deceleration of the scrollable content given a new position in px.
	*/
	void _stopDecelerating(num x, num y) {
	_momentum.stop();
	_setContentOffset(x, y);
	}

	static Function _getOffsetFunction(int scrollTechnique) {
	return scrollTechnique == ScrollerScrollTechnique.TRANSFORM_3D
	? (el, x, y) {
	FxUtil.setTranslate(el, x, y, 0);
	}
	: (el, x, y) {
	FxUtil.setLeftAndTop(el, x, y);
	};
	}
	}

	// TODO(jacobr): cleanup this class of enum constants.
	class ScrollerEventType {
	static const SCROLLER_START = "scroller:scroll_start";
	static const SCROLLER_END = "scroller:scroll_end";
	static const DRAG_END = "scroller:drag_end";
	static const CONTENT_MOVED = "scroller:content_moved";
	static const DECEL_START = "scroller:decel_start";
	}

	class ScrollerScrollTechnique {
	static const TRANSFORM_3D = 1;
	static const RELATIVE_POSITIONING = 2;
	}