path: root/chrome/browser/resources/ntp4/tile_page.js

// Copyright (c) 2011 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

cr.define('ntp4', function() {
  'use strict';

  // We can't pass the currently dragging tile via dataTransfer because of
  // http://crbug.com/31037
  var currentlyDraggingTile = null;
  function getCurrentlyDraggingTile() {
    return currentlyDraggingTile;
  }
  function setCurrentlyDraggingTile(tile) {
    currentlyDraggingTile = tile;
    if (tile)
      ntp4.enterRearrangeMode();
    else
      ntp4.leaveRearrangeMode();
  }

  /**
   * Creates a new Tile object. Tiles wrap content on a TilePage, providing
   * some styling and drag functionality.
   * @constructor
   * @extends {HTMLDivElement}
   */
  function Tile(contents) {
    var tile = cr.doc.createElement('div');
    tile.__proto__ = Tile.prototype;
    tile.initialize(contents);

    return tile;
  }

  Tile.prototype = {
    __proto__: HTMLDivElement.prototype,

    initialize: function(contents) {
      // 'real' as opposed to doppleganger.
      this.className = 'tile real';
      this.appendChild(contents);
      contents.tile = this;

      this.addEventListener('dragstart', this.onDragStart_);
      this.addEventListener('drag', this.onDragMove_);
      this.addEventListener('dragend', this.onDragEnd_);

      this.eventTracker = new EventTracker();
    },

    get index() {
      return Array.prototype.indexOf.call(this.parentNode.children, this);
    },

    get tilePage() {
      return findAncestorByClass(this, 'tile-page');
    },

    /**
     * Position the tile at |x, y|, and store this as the grid location, i.e.
     * where the tile 'belongs' when it's not being dragged.
     * @param {number} x The x coordinate, in pixels.
     * @param {number} y The y coordinate, in pixels.
     */
    setGridPosition: function(x, y) {
      this.gridX = x;
      this.gridY = y;
      this.moveTo(x, y);
    },

    /**
     * Position the tile at |x, y|.
     * @param {number} x The x coordinate, in pixels.
     * @param {number} y The y coordinate, in pixels.
     */
    moveTo: function(x, y) {
      this.style.left = x + 'px';
      this.style.top = y + 'px';
    },

    /**
     * The handler for dragstart events fired on |this|.
     * @param {Event} e The event for the drag.
     * @private
     */
    onDragStart_: function(e) {
      // The user may start dragging again during a previous drag's finishing
      // animation.
      if (this.classList.contains('dragging'))
        this.finalizeDrag_();

      setCurrentlyDraggingTile(this);

      e.dataTransfer.effectAllowed = 'copyMove';
      // TODO(estade): fill this in.
      e.dataTransfer.setData('text/plain', 'foo');

      // The drag clone is the node we use as a representation during the drag.
      // It's attached to the top level document element so that it floats above
      // image masks.
      this.dragClone = this.cloneNode(true);
      this.dragClone.classList.add('drag-representation');
      this.ownerDocument.body.appendChild(this.dragClone);
      this.eventTracker.add(this.dragClone, 'webkitTransitionEnd',
                            this.onDragCloneTransitionEnd_.bind(this));

      this.classList.add('dragging');
      this.dragOffsetX = e.x - this.offsetLeft - this.parentNode.offsetLeft;
      this.dragOffsetY = e.y - this.offsetTop -
          // Unlike offsetTop, this value takes scroll position into account.
          this.parentNode.getBoundingClientRect().top;

      this.onDragMove_(e);
    },

    /**
     * The handler for drag events fired on |this|.
     * @param {Event} e The event for the drag.
     * @private
     */
    onDragMove_: function(e) {
      if (e.view != window || (e.x == 0 && e.y == 0)) {
        // attribute hidden seems to be overridden by display.
        this.dragClone.classList.add('hidden');
        return;
      }

      this.dragClone.classList.remove('hidden');
      this.dragClone.style.left = (e.x - this.dragOffsetX) + 'px';
      this.dragClone.style.top = (e.y - this.dragOffsetY) + 'px';
    },

    /**
     * The handler for dragend events fired on |this|.
     * @param {Event} e The event for the drag.
     * @private
     */
    onDragEnd_: function(e) {
      // The drag clone can still be hidden from the last drag move event.
      this.dragClone.classList.remove('hidden');
      this.dragClone.classList.add('placing');

      setCurrentlyDraggingTile(null);
      this.tilePage.positionTile_(this.index);

      if (this.tilePage.selected) {
        // The tile's contents may have moved following the respositioning;
        // adjust for that.
        var contentDiffX = this.dragClone.firstChild.offsetLeft -
            this.firstChild.offsetLeft;
        var contentDiffY = this.dragClone.firstChild.offsetTop -
            this.firstChild.offsetTop;
        this.dragClone.style.left = (this.gridX + this.parentNode.offsetLeft -
            contentDiffX) + 'px';
        this.dragClone.style.top =
            (this.gridY + this.parentNode.getBoundingClientRect().top -
            contentDiffY) + 'px';
      } else {
        // When we're showing another page and a drag fails or gets cancelled,
        // the tile shrinks to a dot.
        this.dragClone.firstChild.style.webkitTransform = 'scale(0)';
      }
    },

    /**
     * Creates a clone of this node offset by the coordinates. Used for the
     * dragging effect where a tile appears to float off one side of the grid
     * and re-appear on the other.
     * @param {number} x x-axis offset, in pixels.
     * @param {number} y y-axis offset, in pixels.
     */
    showDoppleganger: function(x, y) {
      // We always have to clear the previous doppleganger to make sure we get
      // style updates for the contents of this tile.
      this.clearDoppleganger();

      var clone = this.cloneNode(true);
      clone.classList.remove('real');
      clone.classList.add('doppleganger');
      var clonelets = clone.querySelectorAll('.real');
      for (var i = 0; i < clonelets.length; i++) {
        clonelets[i].classList.remove('real');
      }

      this.appendChild(clone);
      this.doppleganger_ = clone;

      this.doppleganger_.style.WebkitTransform = 'translate(' + x + 'px, ' +
                                                                y + 'px)';
    },

    /**
     * Destroys the current doppleganger.
     */
    clearDoppleganger: function() {
      if (this.doppleganger_) {
        this.removeChild(this.doppleganger_);
        this.doppleganger_ = null;
      }
    },

    /**
     * Returns status of doppleganger.
     * @return {boolean} True if there is a doppleganger showing for |this|.
     */
    hasDoppleganger: function() {
      return !!this.doppleganger_;
    },

    /**
     * Cleans up after the drag is over. This is either called when the
     * drag representation finishes animating to the final position, or when
     * the next drag starts (if the user starts a 2nd drag very quickly).
     * @private
     */
    finalizeDrag_: function() {
      assert(this.classList.contains('dragging'));

      var clone = this.dragClone;
      this.dragClone = null;

      clone.parentNode.removeChild(clone);
      this.eventTracker.remove(clone, 'webkitTransitionEnd');
      this.classList.remove('dragging');
    },

    /**
     * Called when the drag representation node is done migrating to its final
     * resting spot.
     * @param {Event} e The transition end event.
     */
    onDragCloneTransitionEnd_: function(e) {
      if (this.classList.contains('dragging') &&
          (e.propertyName == 'left' || e.propertyName == 'top' ||
           e.propertyName == '-webkit-transform')) {
        this.finalizeDrag_();
      }
    },
  };

  /**
   * Gives the proportion of the row width that is devoted to a single icon.
   * @param {number} rowTileCount The number of tiles in a row.
   * @return {number} The ratio between icon width and row width.
   */
  function tileWidthFraction(rowTileCount) {
    return rowTileCount +
        (rowTileCount - 1) * TILE_SPACING_FRACTION;
  }

  /**
   * Calculates an assortment of tile-related values for a grid with the
   * given dimensions.
   * @param {number} width The pixel width of the grid.
   * @param {number} numRowTiles The number of tiles in a row.
   * @return {Object} A mapping of pixel values.
   */
  function tileValuesForGrid(width, numRowTiles) {
    var tileWidth = width / tileWidthFraction(numRowTiles);
    var offsetX = tileWidth * (1 + TILE_SPACING_FRACTION);
    var interTileSpacing = offsetX - tileWidth;

    return {
      tileWidth: tileWidth,
      offsetX: offsetX,
      interTileSpacing: interTileSpacing,
    };
  }

  // The proportion of the tile width which will be used as spacing between
  // tiles.
  var TILE_SPACING_FRACTION = 1 / 8;

  // The smallest amount of horizontal blank space to display on the sides when
  // displaying a wide arrangement. There is an additional 26px of margin from
  // the tile page padding.
  var MIN_WIDE_MARGIN = 18;

  /**
   * Creates a new TilePage object. This object contains tiles and controls
   * their layout.
   * @param {string} name The display name for the page.
   * @param {Object} gridValues Pixel values that define the size and layout
   *     of the tile grid.
   * @constructor
   * @extends {HTMLDivElement}
   */
  function TilePage(name, gridValues) {
    var el = cr.doc.createElement('div');
    el.pageName = name;
    el.gridValues_ = gridValues;
    el.__proto__ = TilePage.prototype;
    el.initialize();

    return el;
  }

  /**
   * Takes a collection of grid layout pixel values and updates them with
   * additional tiling values that are calculated from TilePage constants.
   * @param {Object} grid The grid layout pixel values to update.
   */
  TilePage.initGridValues = function(grid) {
    // The amount of space we need to display a narrow grid (all narrow grids
    // are this size).
    grid.narrowWidth =
        grid.minTileWidth * tileWidthFraction(grid.minColCount);
    // The minimum amount of space we need to display a wide grid.
    grid.minWideWidth =
        grid.minTileWidth * tileWidthFraction(grid.maxColCount);
    // The largest we will ever display a wide grid.
    grid.maxWideWidth =
        grid.maxTileWidth * tileWidthFraction(grid.maxColCount);
    // Tile-related pixel values for the narrow display.
    grid.narrowTileValues = tileValuesForGrid(grid.narrowWidth,
                                              grid.minColCount);
    // Tile-related pixel values for the minimum narrow display.
    grid.wideTileValues = tileValuesForGrid(grid.minWideWidth,
                                            grid.maxColCount);
  };

  TilePage.prototype = {
    __proto__: HTMLDivElement.prototype,

    initialize: function() {
      this.className = 'tile-page';

      var title = this.ownerDocument.createElement('span');
      title.textContent = this.pageName;
      title.className = 'tile-page-title';
      this.appendChild(title);

      // Div that sets the vertical position of the tile grid.
      this.topMargin_ = this.ownerDocument.createElement('div');
      this.topMargin_.className = 'top-margin';
      this.appendChild(this.topMargin_);

      // Div that holds the tiles.
      this.tileGrid_ = this.ownerDocument.createElement('div');
      this.tileGrid_.className = 'tile-grid';
      this.appendChild(this.tileGrid_);

      // Ordered list of our tiles.
      this.tileElements_ = this.tileGrid_.getElementsByClassName('tile real');

      // These are properties used in updateTopMargin.
      this.animatedTopMarginPx_ = 0;
      this.topMarginPx_ = 0;

      this.eventTracker = new EventTracker();
      this.eventTracker.add(window, 'resize', this.onResize_.bind(this));

      this.addEventListener('DOMNodeInsertedIntoDocument',
                            this.onNodeInsertedIntoDocument_);

      this.tileGrid_.addEventListener('dragenter',
                                      this.onDragEnter_.bind(this));
      this.tileGrid_.addEventListener('dragover', this.onDragOver_.bind(this));
      this.tileGrid_.addEventListener('drop', this.onDrop_.bind(this));
      this.tileGrid_.addEventListener('dragleave',
                                      this.onDragLeave_.bind(this));

    },

    get tileCount() {
      return this.tileElements_.length;
    },

    get selected() {
      return Array.prototype.indexOf.call(this.parentNode.children, this) ==
          ntp4.getCardSlider().currentCard;
    },

    /**
     * Cleans up resources that are no longer needed after this TilePage
     * instance is removed from the DOM.
     */
    tearDown: function() {
      this.eventTracker.removeAll();
    },

    /**
     * @protected
     */
    appendTile: function(tileElement) {
      this.addTileAt(tileElement, this.tileElements_.length);
    },

    /**
     * Adds the given element to the tile grid.
     * @param {Node} tileElement The tile object/node to insert.
     * @param {number} index The location in the tile grid to insert it at.
     * @protected
     */
    addTileAt: function(tileElement, index) {
      var wrapperDiv = new Tile(tileElement);
      if (index == this.tileElements_.length) {
        this.tileGrid_.appendChild(wrapperDiv);
      } else {
        this.tileGrid_.insertBefore(wrapperDiv,
                                    this.tileElements_[index]);
      }
      this.calculateLayoutValues_();

      this.positionTile_(index);
      this.classList.remove('animating-tile-page');
    },

    /**
     * Makes some calculations for tile layout. These change depending on
     * height, width, and the number of tiles.
     * @private
     */
    calculateLayoutValues_: function() {
      var grid = this.gridValues_;
      var availableSpace = this.tileGrid_.clientWidth - 2 * MIN_WIDE_MARGIN;
      var wide = availableSpace >= grid.minWideWidth;
      var numRowTiles = wide ? grid.maxColCount : grid.minColCount;

      var effectiveGridWidth = wide ?
          Math.min(Math.max(availableSpace, grid.minWideWidth),
                   grid.maxWideWidth) :
          grid.narrowWidth;
      var realTileValues = tileValuesForGrid(effectiveGridWidth, numRowTiles);

      // leftMargin centers the grid within the avaiable space.
      var minMargin = wide ? MIN_WIDE_MARGIN : 0;
      var leftMargin =
          Math.max(minMargin,
                   (this.tileGrid_.clientWidth - effectiveGridWidth) / 2);

      var rowHeight = this.heightForWidth(realTileValues.tileWidth) +
          realTileValues.interTileSpacing;

      this.layoutValues_ = {
        numRowTiles: numRowTiles,
        leftMargin: leftMargin,
        colWidth: realTileValues.offsetX,
        rowHeight: rowHeight,
        tileWidth: realTileValues.tileWidth,
        wide: wide,
      };

      // We need to update the top margin as well.
      this.updateTopMargin_();
    },

    /**
     * Calculates the x/y coordinates for an element and moves it there.
     * @param {number} index The index of the element to be positioned.
     * @param {number} indexOffset If provided, this is added to |index| when
     *     positioning the tile. The effect is that the tile will be positioned
     *     in a non-default location.
     * @private
     */
    positionTile_: function(index, indexOffset) {
      var grid = this.gridValues_;
      var layout = this.layoutValues_;

      indexOffset = typeof indexOffset != 'undefined' ? indexOffset : 0;
      // Add the offset _after_ the modulus division. We might want to show the
      // tile off the side of the grid.
      var col = index % layout.numRowTiles + indexOffset;
      var row = Math.floor(index / layout.numRowTiles);
      // Calculate the final on-screen position for the tile.
      var realX = col * layout.colWidth + layout.leftMargin;
      var realY = row * layout.rowHeight;

      // Calculate the portion of the tile's position that should be animated.
      var animatedTileValues = layout.wide ?
          grid.wideTileValues : grid.narrowTileValues;
      // Animate the difference between three-wide and six-wide.
      var animatedLeftMargin = layout.wide ?
          0 : (grid.minWideWidth - MIN_WIDE_MARGIN - grid.narrowWidth) / 2;
      var animatedX = col * animatedTileValues.offsetX + animatedLeftMargin;
      var animatedY = row * (this.heightForWidth(animatedTileValues.tileWidth) +
                             animatedTileValues.interTileSpacing);

      var tile = this.tileElements_[index];
      tile.setGridPosition(animatedX, animatedY);
      tile.firstChild.setBounds(layout.tileWidth,
                                realX - animatedX,
                                realY - animatedY);

      // This code calculates whether the tile needs to show a clone of itself
      // wrapped around the other side of the tile grid.
      var offTheRight = col == layout.numRowTiles ||
          (col == layout.numRowTiles - 1 && tile.hasDoppleganger());
      var offTheLeft = col == -1 || (col == 0 && tile.hasDoppleganger());
      if (this.isCurrentDragTarget_ && (offTheRight || offTheLeft)) {
        var sign = offTheRight ? 1 : -1;
        tile.showDoppleganger(-layout.numRowTiles * layout.colWidth * sign,
                              layout.rowHeight * sign);
      } else {
        tile.clearDoppleganger();
      }

      if (index == this.tileElements_.length - 1)
        this.tileGrid_.style.height = (realY + layout.rowHeight) + 'px';
    },

    /**
     * Gets the index of the tile that should occupy coordinate (x, y). Note
     * that this function doesn't care where the tiles actually are, and will
     * return an index even for the space between two tiles. This function is
     * effectively the inverse of |positionTile_|.
     * @param {number} x The x coordinate, in pixels, relative to the top left
     *     of tileGrid_.
     * @param {number} y The y coordinate.
     * @private
     */
    getWouldBeIndexForPoint_: function(x, y) {
      var grid = this.gridValues_;
      var layout = this.layoutValues_;

      var col = Math.floor((x - layout.leftMargin) / layout.colWidth);
      if (col < 0 || col >= layout.numRowTiles)
        return -1;

      var row = Math.floor(
          (y - this.tileGrid_.getBoundingClientRect().top) / layout.rowHeight);
      return row * layout.numRowTiles + col;
    },

    /**
     * Window resize event handler. Window resizes may trigger re-layouts.
     * @param {Object} e The resize event.
     */
    onResize_: function(e) {
      if (this.lastWidth_ == this.clientWidth &&
          this.lastHeight_ == this.clientHeight) {
        return;
      }

      this.calculateLayoutValues_();

      this.lastWidth_ = this.clientWidth;
      this.lastHeight_ = this.clientHeight;
      this.classList.add('animating-tile-page');
      this.heightChanged_();

      for (var i = 0; i < this.tileElements_.length; i++) {
        this.positionTile_(i);
      }
    },

    /**
     * The tile grid has an image mask which fades at the edges. We only show
     * the mask when there is an active drag; it obscures doppleganger tiles
     * as they enter or exit the grid.
     * @private
     */
    updateMask_: function() {
      if (!this.isCurrentDragTarget_) {
        this.tileGrid_.style.WebkitMaskBoxImage = '';
        return;
      }

      var leftMargin = this.layoutValues_.leftMargin;
      var fadeDistance = Math.min(leftMargin, 20);
      var gradient =
          '-webkit-linear-gradient(left,' +
              'transparent, ' +
              'transparent ' + (leftMargin - fadeDistance) + 'px, ' +
              'black ' + leftMargin + 'px, ' +
              'black ' + (this.tileGrid_.clientWidth - leftMargin) + 'px, ' +
              'transparent ' + (this.tileGrid_.clientWidth - leftMargin +
                                fadeDistance) + 'px, ' +
              'transparent)';
      this.tileGrid_.style.WebkitMaskBoxImage = gradient;
    },

    updateTopMargin_: function() {
      var layout = this.layoutValues_;

      // The top margin is set so that the vertical midpoint of the grid will
      // be 1/3 down the page.
      var numTiles = this.tileCount +
          (this.isCurrentDragTarget_ && !this.withinPageDrag_ ? 1 : 0);
      var numRows = Math.ceil(numTiles / layout.numRowTiles);
      var usedHeight = layout.rowHeight * numRows;
      // 100 matches the top padding of tile-page.
      var newMargin = document.documentElement.clientHeight / 3 -
          usedHeight / 2 - 100;
      newMargin = Math.max(newMargin, 0);

      // |newMargin| is the final margin we actually want to show. However,
      // part of that should be animated and part should not (for the same
      // reason as with leftMargin). The approach is to consider differences
      // when the layout changes from wide to narrow or vice versa as
      // 'animatable'. These differences accumulate in animatedTopMarginPx_,
      // while topMarginPx_ caches the real (total) margin. Either of these
      // calculations may come out to be negative, so we use margins as the
      // css property.

      if (typeof this.topMarginIsForWide_ == 'undefined')
        this.topMarginIsForWide_ = layout.wide;
      if (this.topMarginIsForWide_ != layout.wide) {
        this.animatedTopMarginPx_ += newMargin - this.topMarginPx_;
        this.topMargin_.style.marginBottom =
            this.animatedTopMarginPx_ + 'px';
      }

      this.topMarginIsForWide_ = layout.wide;
      this.topMarginPx_ = newMargin;
      this.topMargin_.style.marginTop =
          (this.topMarginPx_ - this.animatedTopMarginPx_) + 'px';
    },

    /**
     * Handles final setup that can only happen after |this| is inserted into
     * the page.
     */
    onNodeInsertedIntoDocument_: function(e) {
      this.calculateLayoutValues_();
      this.heightChanged_();
    },

    /**
     * Called when the height of |this| has changed: update the size of
     * tileGrid.
     */
    heightChanged_: function() {
      // The tile grid will expand to the bottom footer, or enough to hold all
      // the tiles, whichever is greater. It would be nicer if tilePage were
      // a flex box, and the tile grid could be box-flex: 1, but this exposes a
      // bug where repositioning tiles will cause the scroll position to reset.
      this.tileGrid_.style.minHeight = (this.clientHeight -
          this.tileGrid_.offsetTop) + 'px';
    },

    /**
     * Get the height for a tile of a certain width. Override this function to
     * get non-square tiles.
     * @param {number} width The pixel width of a tile.
     * @return {number} The height for |width|.
     */
    heightForWidth: function(width) {
      return width;
    },

    /** Dragging **/

    /**
     * The number of un-paired dragenter events that have fired on |this|. This
     * is incremented by |onDragEnter_| and decremented by |onDragLeave_|. This
     * is necessary because dragging over child widgets will fire additional
     * enter and leave events on |this|. A non-zero value does not necessarily
     * indicate that |isCurrentDragTarget_| is true.
     * @type {number}
     * @private
     */
    dragEnters_: 0,

    /**
     * Whether the tile page is currently being dragged over with data it can
     * accept.
     * @type {boolean}
     * @private
     */
    isCurrentDragTarget_: false,

    /**
     * Handler for dragenter events fired on |tileGrid_|.
     * @param {Event} e A MouseEvent for the drag.
     * @private
     */
    onDragEnter_: function(e) {
      if (++this.dragEnters_ == 1) {
        this.doDragEnter_(e);
      } else {
        // Sometimes we'll get an enter event over a child element without an
        // over event following it. In this case we have to still call the
        // drag over handler so that we make the necessary updates (one visible
        // symptom of not doing this is that the cursor's drag state will
        // flicker during drags).
        this.onDragOver_(e);
      }
    },

    /**
     * Thunk for dragover events fired on |tileGrid_|.
     * @param {Event} e A MouseEvent for the drag.
     * @private
     */
    onDragOver_: function(e) {
      if (!this.isCurrentDragTarget_)
        return;
      this.doDragOver_(e);
    },

    /**
     * Thunk for drop events fired on |tileGrid_|.
     * @param {Event} e A MouseEvent for the drag.
     * @private
     */
    onDrop_: function(e) {
      this.dragEnters_ = 0;
      if (!this.isCurrentDragTarget_)
        return;
      this.doDrop_(e);
    },

    /**
     * Thunk for dragleave events fired on |tileGrid_|.
     * @param {Event} e A MouseEvent for the drag.
     * @private
     */
    onDragLeave_: function(e) {
      if (--this.dragEnters_ > 0)
        return;

      this.isCurrentDragTarget_ = false;
      this.cleanUpDrag_();
    },

    /**
     * Performs all actions necessary when a drag enters the tile page.
     * @param {Event} e A mouseover event for the drag enter.
     * @private
     */
    doDragEnter_: function(e) {
      if (!this.shouldAcceptDrag(e.dataTransfer))
        return;

      this.isCurrentDragTarget_ = true;

      // Applies the mask so doppleganger tiles disappear into the fog.
      this.updateMask_();

      this.classList.add('animating-tile-page');
      this.withinPageDrag_ = this.contains(currentlyDraggingTile);
      this.dragItemIndex_ = this.withinPageDrag_ ?
          currentlyDraggingTile.index : this.tileElements_.length;
      this.currentDropIndex_ = this.dragItemIndex_;

      // The new tile may change the number of rows, hence the top margin
      // will change.
      if (!this.withinPageDrag_)
        this.updateTopMargin_();

      this.doDragOver_(e);
    },

    /**
     * Performs all actions necessary when the user moves the cursor during
     * a drag over the tile page.
     * @param {Event} e A mouseover event for the drag over.
     * @private
     */
    doDragOver_: function(e) {
      e.preventDefault();

      if (currentlyDraggingTile)
        e.dataTransfer.dropEffect = 'move';
      else
        e.dataTransfer.dropEffect = 'copy';

      var newDragIndex = this.getWouldBeIndexForPoint_(e.pageX, e.pageY);
      if (newDragIndex < 0 || newDragIndex >= this.tileElements_.length)
        newDragIndex = this.dragItemIndex_;
      this.updateDropIndicator_(newDragIndex);
    },

    /**
     * Performs all actions necessary when the user completes a drop.
     * @param {Event} e A mouseover event for the drag drop.
     * @private
     */
    doDrop_: function(e) {
      e.stopPropagation();
      this.isCurrentDragTarget_ = false;

      var index = this.currentDropIndex_;
      // Only change data if this was not a 'null drag'.
      if (!((index == this.dragItemIndex_) && this.withinPageDrag_)) {
        var adjustedIndex = this.currentDropIndex_ +
            (index > this.dragItemIndex_ ? 1 : 0);
        if (currentlyDraggingTile) {
          var originalPage = currentlyDraggingTile.tilePage;
          this.tileGrid_.insertBefore(
              currentlyDraggingTile,
              this.tileElements_[adjustedIndex]);

          if (originalPage != this)
            originalPage.cleanUpDrag_();
          this.tileMoved(currentlyDraggingTile);
        } else {
          this.addOutsideData(e.dataTransfer, adjustedIndex);
        }
      }

      this.classList.remove('animating-tile-page');
      this.cleanUpDrag_();
    },

    /**
     * Makes sure all the tiles are in the right place after a drag is over.
     * @private
     */
    cleanUpDrag_: function() {
      for (var i = 0; i < this.tileElements_.length; i++) {
        // The current drag tile will be positioned in its dragend handler.
        if (this.tileElements_[i] == currentlyDraggingTile)
          continue;
        this.positionTile_(i);
      }

      // Remove the drag mask.
      this.updateMask_();
    },

    /**
     * Updates the visual indicator for the drop location for the active drag.
     * @param {Event} e A MouseEvent for the drag.
     * @private
     */
    updateDropIndicator_: function(newDragIndex) {
      var oldDragIndex = this.currentDropIndex_;
      if (newDragIndex == oldDragIndex)
        return;

      var repositionStart = Math.min(newDragIndex, oldDragIndex);
      var repositionEnd = Math.max(newDragIndex, oldDragIndex);

      for (var i = repositionStart; i <= repositionEnd; i++) {
        if (i == this.dragItemIndex_)
          continue;
        else if (i > this.dragItemIndex_)
          var adjustment = i <= newDragIndex ? -1 : 0;
        else
          var adjustment = i >= newDragIndex ? 1 : 0;

        this.positionTile_(i, adjustment);
      }
      this.currentDropIndex_ = newDragIndex;
    },

    /**
     * Checks if a page can accept a drag with the given data.
     * @param {Object} dataTransfer The dataTransfer object, if the drag object
     *     is not a tile (e.g. it is a link).
     * @return {boolean} True if this page can handle the drag.
     */
    shouldAcceptDrag: function(dataTransfer) {
      return false;
    },

    /**
     * Called to accept a drag drop.
     * @param {Object} dataTransfer The data transfer object that holds the drop
     *     data.
     * @param {number} index The tile index at which the drop occurred.
     */
    addOutsideData: function(dataTransfer, index) {
      // This should not get called unless there is a non-default
      // implementation.
      assert(false);
    },

    /**
     * Called when a tile has been moved (via dragging). Override this to make
     * backend updates.
     * @param {Node} draggedTile The tile that was dropped.
     */
    tileMoved: function(draggedTile) {
    },
  };

  return {
    getCurrentlyDraggingTile: getCurrentlyDraggingTile,
    TilePage: TilePage,
  };
});