iron-list.html

<!--
@license
Copyright (c) 2015 The Polymer Project Authors. All rights reserved.
This code may only be used under the BSD style license found at http://polymer.github.io/LICENSE.txt
The complete set of authors may be found at http://polymer.github.io/AUTHORS.txt
The complete set of contributors may be found at http://polymer.github.io/CONTRIBUTORS.txt
Code distributed by Google as part of the polymer project is also
subject to an additional IP rights grant found at http://polymer.github.io/PATENTS.txt
-->

<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="../iron-resizable-behavior/iron-resizable-behavior.html">

<!--

`iron-list` displays a virtual, 'infinite' list. The template inside
the iron-list element represents the DOM to create for each list item.
The `items` property specifies an array of list item data.

For performance reasons, not every item in the list is rendered at once;
instead a small subset of actual template elements *(enough to fill the viewport)*
are rendered and reused as the user scrolls. As such, it is important that all
state of the list template be bound to the model driving it, since the view may
be reused with a new model at any time. Particularly, any state that may change
as the result of a user interaction with the list item must be bound to the model
to avoid view state inconsistency.

__Important:__ `iron-list` must either be explicitly sized, or delegate scrolling to an
explicitly sized parent. By "explicitly sized", we mean it either has an explicit
CSS `height` property set via a class or inline style, or else is sized by other
layout means (e.g. the `flex` or `fit` classes).

### Template model

List item templates should bind to template models of the following structure:

    {
      index: 0,     // data index for this item
      item: {       // user data corresponding to items[index]
        /* user item data  */
      }
    }

Alternatively, you can change the property name used as data index by changing the
`indexAs` property. The `as` property defines the name of the variable to add to the binding
scope for the array.

For example, given the following `data` array:

##### data.json

    [
      {"name": "Bob"},
      {"name": "Tim"},
      {"name": "Mike"}
    ]

The following code would render the list (note the name and checked properties are
bound from the model object provided to the template scope):

    <template is="dom-bind">
      <iron-ajax url="data.json" last-response="{{data}}" auto></iron-ajax>
      <iron-list items="[[data]]" as="item">
        <template>
          <div>
            Name: <span>[[item.name]]</span>
          </div>
        </template>
      </iron-list>
    </template>

### Styling

Use the `--iron-list-items-container` mixin to style the container of items, e.g.

    iron-list {
     --iron-list-items-container: {
        margin: auto;
      };
    }

### Resizing

`iron-list` lays out the items when it receives a notification via the `iron-resize` event.
This event is fired by any element that implements `IronResizableBehavior`.

By default, elements such as `iron-pages`, `paper-tabs` or `paper-dialog` will trigger
this event automatically. If you hide the list manually (e.g. you use `display: none`)
you might want to implement `IronResizableBehavior` or fire this event manually right
after the list became visible again. e.g.

    document.querySelector('iron-list').fire('iron-resize');

### When should `<iron-list>` be used?

`iron-list` should be used when a page has significantly more DOM nodes than the ones
visible on the screen. e.g. the page has 500 nodes, but only 20 are visible at the time.
This is why we refer to it as a `virtual` list. In this case, a `dom-repeat` will still
create 500 nodes which could slow down the web app, but `iron-list` will only create 20.

However, having an `iron-list` does not mean that you can load all the data at once. 
Say, you have a million records in the database, you want to split the data into pages
so you can bring a page at the time. The page could contain 500 items, and iron-list
will only render 20.

@group Iron Element
@element iron-list
@demo demo/index.html Simple list
@demo demo/selection.html Selection of items
@demo demo/collapse.html Collapsable items
-->

<dom-module id="iron-list">
  <template>
    <style>
      :host {
        display: block;
      }

      :host(.has-scroller) {
        overflow: auto;
      }

      :host(:not(.has-scroller)) {
        position: relative;
      }

      #items {
        @apply(--iron-list-items-container);
        position: relative;
      }

      #items > ::content > * {
        width: 100%;
        box-sizing: border-box;
        position: absolute;
        top: 0;
        will-change: transform;
      }
    </style>

    <array-selector id="selector" items="{{items}}"
      selected="{{selectedItems}}" selected-item="{{selectedItem}}">
    </array-selector>

    <div id="items">
      <content></content>
    </div>

  </template>
</dom-module>

<script>

(function() {

  var IOS = navigator.userAgent.match(/iP(?:hone|ad;(?: U;)? CPU) OS (\d+)/);
  var IOS_TOUCH_SCROLLING = IOS && IOS[1] >= 8;
  var DEFAULT_PHYSICAL_COUNT = 20;
  var MAX_PHYSICAL_COUNT = 500;

  Polymer({

    is: 'iron-list',

    properties: {

      /**
       * An array containing items determining how many instances of the template
       * to stamp and that that each template instance should bind to.
       */
      items: {
        type: Array
      },

      /**
       * The name of the variable to add to the binding scope for the array
       * element associated with a given template instance.
       */
      as: {
        type: String,
        value: 'item'
      },

      /**
       * The name of the variable to add to the binding scope with the index
       * for the row.
       */
      indexAs: {
        type: String,
        value: 'index'
      },

      /**
       * The name of the variable to add to the binding scope to indicate
       * if the row is selected.
       */
      selectedAs: {
        type: String,
        value: 'selected'
      },

      /**
       * When true, tapping a row will select the item, placing its data model
       * in the set of selected items retrievable via the selection property.
       *
       * Note that tapping focusable elements within the list item will not
       * result in selection, since they are presumed to have their * own action.
       */
      selectionEnabled: {
        type: Boolean,
        value: false
      },

      /**
       * When `multiSelection` is false, this is the currently selected item, or `null`
       * if no item is selected.
       */
      selectedItem: {
        type: Object,
        notify: true
      },

      /**
       * When `multiSelection` is true, this is an array that contains the selected items.
       */
      selectedItems: {
        type: Object,
        notify: true
      },

      /**
       * When `true`, multiple items may be selected at once (in this case,
       * `selected` is an array of currently selected items).  When `false`,
       * only one item may be selected at a time.
       */
      multiSelection: {
        type: Boolean,
        value: false
      }
    },

    observers: [
      '_itemsChanged(items.*)',
      '_selectionEnabledChanged(selectionEnabled)',
      '_multiSelectionChanged(multiSelection)'
    ],

    behaviors: [
      Polymer.Templatizer,
      Polymer.IronResizableBehavior
    ],

    listeners: {
      'iron-resize': '_resizeHandler'
    },

    /**
     * The ratio of hidden tiles that should remain in the scroll direction.
     * Recommended value ~0.5, so it will distribute tiles evely in both directions.
     */
    _ratio: 0.5,

    /**
     * The element that controls the scroll
     * @type {?Element}
     */
    _scroller: null,

    /**
     * The padding-top value of the `scroller` element
     */
    _scrollerPaddingTop: 0,

    /**
     * This value is the same as `scrollTop`.
     */
    _scrollPosition: 0,

    /**
     * The number of tiles in the DOM.
     */
    _physicalCount: 0,

    /**
     * The k-th tile that is at the top of the scrolling list.
     */
    _physicalStart: 0,

    /**
     * The k-th tile that is at the bottom of the scrolling list.
     */
    _physicalEnd: 0,

    /**
     * The sum of the heights of all the tiles in the DOM.
     */
    _physicalSize: 0,

    /**
     * The average `offsetHeight` of the tiles observed till now.
     */
    _physicalAverage: 0,

    /**
     * The number of tiles which `offsetHeight` > 0 observed until now.
     */
    _physicalAverageCount: 0,

    /**
     * The Y position of the item rendered in the `_physicalStart`
     * tile relative to the scrolling list.
     */
    _physicalTop: 0,

    /**
     * The number of items in the list.
     */
    _virtualCount: 0,

    /**
     * The n-th item rendered in the `_physicalStart` tile.
     */
    _virtualStartVal: 0,

    /**
     * A map between an item key and its physical item index
     */
    _physicalIndexForKey: null,

    /**
     * The estimated scroll height based on `_physicalAverage`
     */
    _estScrollHeight: 0,

    /**
     * The scroll height of the dom node
     */
    _scrollHeight: 0,

    /**
     * The size of the viewport
     */
    _viewportSize: 0,

    /**
     * An array of DOM nodes that are currently in the tree
     * @type {?Array<!TemplatizerNode>}
     */
    _physicalItems: null,

    /**
     * An array of heights for each item in `_physicalItems`
     * @type {?Array<number>}
     */
    _physicalSizes: null,

    /**
     * A cached value for the visible index.
     * See `firstVisibleIndex`
     * @type {?number}
     */
    _firstVisibleIndexVal: null,

    /**
     * A Polymer collection for the items.
     * @type {?Polymer.Collection}
     */
    _collection: null,

    /**
     * True if the current item list was rendered for the first time
     * after attached.
     */
    _itemsRendered: false,

    /**
     * The bottom of the physical content.
     */
    get _physicalBottom() {
      return this._physicalTop + this._physicalSize;
    },

    /**
     * The bottom of the scroll.
     */
    get _scrollBottom() {
      return this._scrollPosition + this._viewportSize;
    },

    /**
     * The n-th item rendered in the last physical item.
     */
    get _virtualEnd() {
      return this._virtualStartVal + this._physicalCount - 1;
    },

    /**
     * The lowest n-th value for an item such that it can be rendered in `_physicalStart`.
     */
    _minVirtualStart: 0,

    /**
     * The largest n-th value for an item such that it can be rendered in `_physicalStart`.
     */
    get _maxVirtualStart() {
      return Math.max(0, this._virtualCount - this._physicalCount);
    },

    /**
     * The height of the physical content that isn't on the screen.
     */
    get _hiddenContentSize() {
      return this._physicalSize - this._viewportSize;
    },

    /**
     * The maximum scroll top value.
     */
    get _maxScrollTop() {
      return this._estScrollHeight - this._viewportSize;
    },

    /**
     * Sets the n-th item rendered in `_physicalStart`
     */
    set _virtualStart(val) {
      // clamp the value so that _minVirtualStart <= val <= _maxVirtualStart
      this._virtualStartVal = Math.min(this._maxVirtualStart, Math.max(this._minVirtualStart, val));
      this._physicalStart = this._virtualStartVal % this._physicalCount;
      this._physicalEnd = (this._physicalStart + this._physicalCount - 1) % this._physicalCount;
    },

    /**
     * Gets the n-th item rendered in `_physicalStart`
     */
    get _virtualStart() {
      return this._virtualStartVal;
    },

    /**
     * An optimal physical size such that we will have enough physical items
     * to fill up the viewport and recycle when the user scrolls.
     *
     * This default value assumes that we will at least have the equivalent
     * to a viewport of physical items above and below the user's viewport.
     */
    get _optPhysicalSize() {
      return this._viewportSize * 3;
    },

   /**
    * True if the current list is visible.
    */
    get _isVisible() {
      return this._scroller && Boolean(this._scroller.offsetWidth || this._scroller.offsetHeight);
    },

    /**
     * Gets the index of the first visible item in the viewport.
     *
     * @type {number}
     */
    get firstVisibleIndex() {
      var physicalOffset;

      if (this._firstVisibleIndexVal === null) {
        physicalOffset = this._physicalTop;

        this._firstVisibleIndexVal = this._iterateItems(
          function(pidx, vidx) {
            physicalOffset += this._physicalSizes[pidx];

            if (physicalOffset > this._scrollPosition) {
              return vidx;
            }
          }) || 0;
      }

      return this._firstVisibleIndexVal;
    },

    ready: function() {
      if (IOS_TOUCH_SCROLLING) {
        this._scrollListener = function() {
          requestAnimationFrame(this._scrollHandler.bind(this));
        }.bind(this);
      } else {
        this._scrollListener = this._scrollHandler.bind(this);
      }
    },

    /**
     * When the element has been attached to the DOM tree.
     */
    attached: function() {
      // delegate to the parent's scroller
      // e.g. paper-scroll-header-panel
      var el = Polymer.dom(this);

      var parentNode = /** @type {?{scroller: ?Element}} */ (el.parentNode);
      if (parentNode && parentNode.scroller) {
        this._scroller = parentNode.scroller;
      } else {
        this._scroller = this;
        this.classList.add('has-scroller');
      }

      if (IOS_TOUCH_SCROLLING) {
        this._scroller.style.webkitOverflowScrolling = 'touch';
      }

      this._scroller.addEventListener('scroll', this._scrollListener);

      this.updateViewportBoundaries();
      this._render();
    },

    /**
     * When the element has been removed from the DOM tree.
     */
    detached: function() {
      this._itemsRendered = false;
      if (this._scroller) {
        this._scroller.removeEventListener('scroll', this._scrollListener);
      }
    },

    /**
     * Invoke this method if you dynamically update the viewport's
     * size or CSS padding.
     *
     * @method updateViewportBoundaries
     */
    updateViewportBoundaries: function() {
      var scrollerStyle = window.getComputedStyle(this._scroller);
      this._scrollerPaddingTop = parseInt(scrollerStyle['padding-top'], 10);
      this._viewportSize = this._scroller.offsetHeight;
    },

    /**
     * Update the models, the position of the
     * items in the viewport and recycle tiles as needed.
     */
    _refresh: function() {
      // clamp the `scrollTop` value
      // IE 10|11 scrollTop may go above `_maxScrollTop`
      // iOS `scrollTop` may go below 0 and above `_maxScrollTop`
      var scrollTop = Math.max(0, Math.min(this._maxScrollTop, this._scroller.scrollTop));
      var tileHeight, tileTop, kth, recycledTileSet, scrollBottom;
      var ratio = this._ratio;
      var delta = scrollTop - this._scrollPosition;
      var recycledTiles = 0;
      var hiddenContentSize = this._hiddenContentSize;
      var currentRatio = ratio;
      var movingUp = [];

      // track the last `scrollTop`
      this._scrollPosition = scrollTop;

      // clear cached visible index
      this._firstVisibleIndexVal = null;

      scrollBottom = this._scrollBottom;

      // random access
      if (Math.abs(delta) > this._physicalSize) {
        this._physicalTop += delta;
        recycledTiles =  Math.round(delta / this._physicalAverage);
      }
      // scroll up
      else if (delta < 0) {
        var topSpace = scrollTop - this._physicalTop;
        var virtualStart = this._virtualStart;
        var physicalBottom = this._physicalBottom;

        recycledTileSet = [];

        kth = this._physicalEnd;
        currentRatio = topSpace / hiddenContentSize;

        // move tiles from bottom to top
        while (
            // approximate `currentRatio` to `ratio`
            currentRatio < ratio &&
            // recycle less physical items than the total
            recycledTiles < this._physicalCount &&
            // ensure that these recycled tiles are needed
            virtualStart - recycledTiles > 0 &&
            // ensure that the tile is not visible
            physicalBottom - this._physicalSizes[kth] > scrollBottom
        ) {

          tileHeight = this._physicalSizes[kth];
          currentRatio += tileHeight / hiddenContentSize;
          physicalBottom -= tileHeight;
          recycledTileSet.push(kth);
          recycledTiles++;
          kth = (kth === 0) ? this._physicalCount - 1 : kth - 1;
        }

        movingUp = recycledTileSet;
        recycledTiles = -recycledTiles;
      }
      // scroll down
      else if (delta > 0) {
        var bottomSpace = this._physicalBottom - scrollBottom;
        var virtualEnd = this._virtualEnd;
        var lastVirtualItemIndex = this._virtualCount-1;

        recycledTileSet = [];

        kth = this._physicalStart;
        currentRatio = bottomSpace / hiddenContentSize;

        // move tiles from top to bottom
        while (
            // approximate `currentRatio` to `ratio`
            currentRatio < ratio &&
            // recycle less physical items than the total
            recycledTiles < this._physicalCount &&
            // ensure that these recycled tiles are needed
            virtualEnd + recycledTiles < lastVirtualItemIndex &&
            // ensure that the tile is not visible
            this._physicalTop + this._physicalSizes[kth] < scrollTop
          ) {

          tileHeight = this._physicalSizes[kth];
          currentRatio += tileHeight / hiddenContentSize;

          this._physicalTop += tileHeight;
          recycledTileSet.push(kth);
          recycledTiles++;
          kth = (kth + 1) % this._physicalCount;
        }
      }

      if (recycledTiles === 0) {
        // If the list ever reach this case, the physical average is not significant enough
        // to create all the items needed to cover the entire viewport.
        // e.g. A few items have a height that differs from the average by serveral order of magnitude.
        if (this._increasePoolIfNeeded()) {
          // yield and set models to the new items
          this.async(this._update);
        }
      } else {
        this._virtualStart = this._virtualStart + recycledTiles;
        this._update(recycledTileSet, movingUp);
      }
    },

    /**
     * Update the list of items, starting from the `_virtualStartVal` item.
     * @param {!Array<number>=} itemSet
     * @param {!Array<number>=} movingUp
     */
    _update: function(itemSet, movingUp) {
      // update models
      this._assignModels(itemSet);

      // measure heights
      this._updateMetrics(itemSet);

      // adjust offset after measuring
      if (movingUp) {
        while (movingUp.length) {
          this._physicalTop -= this._physicalSizes[movingUp.pop()];
        }
      }
      // update the position of the items
      this._positionItems();

      // set the scroller size
      this._updateScrollerSize();

      // increase the pool of physical items if needed
      if (this._increasePoolIfNeeded()) {
        // yield set models to the new items
        this.async(this._update);
      }
    },

    /**
     * Creates a pool of DOM elements and attaches them to the local dom.
     */
    _createPool: function(size) {
      var physicalItems = new Array(size);

      this._ensureTemplatized();

      for (var i = 0; i < size; i++) {
        var inst = this.stamp(null);

        // First element child is item; Safari doesn't support children[0]
        // on a doc fragment
        physicalItems[i] = inst.root.querySelector('*');
        Polymer.dom(this).appendChild(inst.root);
      }

      return physicalItems;
    },

    /**
     * Increases the pool of physical items only if needed.
     * This function will allocate additional physical items
     * (limited by `MAX_PHYSICAL_COUNT`) if the content size is shorter than
     * `_optPhysicalSize`
     *
     * @return boolean
     */
    _increasePoolIfNeeded: function() {
      if (this._physicalAverage === 0) {
        return false;
      }
      if (this._physicalBottom < this._scrollBottom || this._physicalTop > this._scrollPosition) {
        return this._increasePool(1);
      }
      if (this._physicalSize < this._optPhysicalSize) {
        return this._increasePool(Math.round((this._optPhysicalSize - this._physicalSize) * 1.2 / this._physicalAverage));
      }
      return false;
    },

    /**
     * Increases the pool size.
     */
    _increasePool: function(missingItems) {
      // limit the size
      var nextPhysicalCount = Math.min(
          this._physicalCount + missingItems,
          this._virtualCount,
          MAX_PHYSICAL_COUNT
        );

      var prevPhysicalCount = this._physicalCount;
      var delta = nextPhysicalCount - prevPhysicalCount;

      if (delta <= 0) {
        return false;
      }

      [].push.apply(this._physicalItems, this._createPool(delta));
      [].push.apply(this._physicalSizes, new Array(delta));

      this._physicalCount = prevPhysicalCount + delta;

      return true;
    },

    /**
     * Render a new list of items. This method does exactly the same as `update`,
     * but it also ensures that only one `update` cycle is created.
     */
    _render: function() {
      var requiresUpdate = this._virtualCount > 0 || this._physicalCount > 0;

      if (this.isAttached && !this._itemsRendered && this._isVisible && requiresUpdate)  {
        this._update();
        this._itemsRendered = true;
      }
    },

    /**
     * Templetizes the user template.
     */
    _ensureTemplatized: function() {
      if (!this.ctor) {
        // Template instance props that should be excluded from forwarding
        var props = {};

        props.__key__ = true;
        props[this.as] = true;
        props[this.indexAs] = true;
        props[this.selectedAs] = true;

        this._instanceProps = props;
        this._userTemplate = Polymer.dom(this).querySelector('template');

        if (this._userTemplate) {
          this.templatize(this._userTemplate);
        } else {
          console.warn('iron-list requires a template to be provided in light-dom');
        }
      }
    },

    /**
     * Implements extension point from Templatizer mixin.
     */
    _getStampedChildren: function() {
      return this._physicalItems;
    },

    /**
     * Implements extension point from Templatizer
     * Called as a side effect of a template instance path change, responsible
     * for notifying items.<key-for-instance>.<path> change up to host.
     */
    _forwardInstancePath: function(inst, path, value) {
      if (path.indexOf(this.as + '.') === 0) {
        this.notifyPath('items.' + inst.__key__ + '.' +
          path.slice(this.as.length + 1), value);
      }
    },

    /**
     * Implements extension point from Templatizer mixin
     * Called as side-effect of a host property change, responsible for
     * notifying parent path change on each row.
     */
    _forwardParentProp: function(prop, value) {
      if (this._physicalItems) {
        this._physicalItems.forEach(function(item) {
          item._templateInstance[prop] = value;
        }, this);
      }
    },

    /**
     * Implements extension point from Templatizer
     * Called as side-effect of a host path change, responsible for
     * notifying parent.<path> path change on each row.
     */
    _forwardParentPath: function(path, value) {
      if (this._physicalItems) {
        this._physicalItems.forEach(function(item) {
          item._templateInstance.notifyPath(path, value, true);
        }, this);
      }
    },

    /**
     * Called as a side effect of a host items.<key>.<path> path change,
     * responsible for notifying item.<path> changes to row for key.
     */
    _forwardItemPath: function(path, value) {
      if (this._physicalIndexForKey) {
        var dot = path.indexOf('.');
        var key = path.substring(0, dot < 0 ? path.length : dot);
        var idx = this._physicalIndexForKey[key];
        var row = this._physicalItems[idx];
        if (row) {
          var inst = row._templateInstance;
          if (dot >= 0) {
            path = this.as + '.' + path.substring(dot+1);
            inst.notifyPath(path, value, true);
          } else {
            inst[this.as] = value;
          }
        }
      }
    },

    /**
     * Called when the items have changed. That is, ressignments
     * to `items`, splices or updates to a single item.
     */
    _itemsChanged: function(change) {
      if (change.path === 'items') {
        // render the new set
        this._itemsRendered = false;

        // update the whole set
        this._virtualStartVal = 0;
        this._physicalTop = 0;
        this._virtualCount = this.items ? this.items.length : 0;
        this._collection = this.items ? Polymer.Collection.get(this.items) : null;
        this._physicalIndexForKey = {};

        // scroll to the top
        this._resetScrollPosition(0);

        // create the initial physical items
        if (!this._physicalItems) {
          this._physicalCount = Math.max(1, Math.min(DEFAULT_PHYSICAL_COUNT, this._virtualCount));
          this._physicalItems = this._createPool(this._physicalCount);
          this._physicalSizes = new Array(this._physicalCount);
        }

        this.debounce('refresh', this._render);

      } else if (change.path === 'items.splices') {
        // render the new set
        this._itemsRendered = false;

        this._adjustVirtualIndex(change.value.indexSplices);
        this._virtualCount = this.items ? this.items.length : 0;

        this.debounce('refresh', this._render);

      } else {
        // update a single item
        this._forwardItemPath(change.path.split('.').slice(1).join('.'), change.value);
      }
    },

    /**
     * @param {!Array<!PolymerSplice>} splices
     */
    _adjustVirtualIndex: function(splices) {
      var i, splice, idx;

      for (i = 0; i < splices.length; i++) {
        splice = splices[i];

        // deselect removed items
        splice.removed.forEach(this.$.selector.deselect, this.$.selector);

        idx = splice.index;
        // We only need to care about changes happening above the current position
        if (idx >= this._virtualStartVal) {
          break;
        }

        this._virtualStart = this._virtualStart +
            Math.max(splice.addedCount - splice.removed.length, idx - this._virtualStartVal);
      }
    },

    _scrollHandler: function() {
      this._refresh();
    },

    /**
     * Executes a provided function per every physical index in `itemSet`
     * `itemSet` default value is equivalent to the entire set of physical indexes.
     *
     * @param {!function(number, number)} fn
     * @param {!Array<number>=} itemSet
     */
    _iterateItems: function(fn, itemSet) {
      var pidx, vidx, rtn, i;

      if (arguments.length === 2 && itemSet) {
        for (i = 0; i < itemSet.length; i++) {
          pidx = itemSet[i];
          if (pidx >= this._physicalStart) {
            vidx = this._virtualStartVal + (pidx - this._physicalStart);
          } else {
            vidx = this._virtualStartVal + (this._physicalCount - this._physicalStart) + pidx;
          }
          if ((rtn = fn.call(this, pidx, vidx)) != null) {
            return rtn;
          }
        }
      } else {
        pidx = this._physicalStart;
        vidx = this._virtualStartVal;

        for (; pidx < this._physicalCount; pidx++, vidx++) {
          if ((rtn = fn.call(this, pidx, vidx)) != null) {
            return rtn;
          }
        }

        pidx = 0;

        for (; pidx < this._physicalStart; pidx++, vidx++) {
          if ((rtn = fn.call(this, pidx, vidx)) != null) {
            return rtn;
          }
        }
      }
    },

    /**
     * Assigns the data models to a given set of items.
     * @param {!Array<number>=} itemSet
     */
    _assignModels: function(itemSet) {
      this._iterateItems(function(pidx, vidx) {
        var el = this._physicalItems[pidx];
        var inst = el._templateInstance;
        var item = this.items && this.items[vidx];

        if (item) {
          inst[this.as] = item;
          inst.__key__ = this._collection.getKey(item);
          inst[this.selectedAs] =
            /** @type {!ArraySelectorElement} */ (this.$.selector).isSelected(item);
          inst[this.indexAs] = vidx;
          el.removeAttribute('hidden');
          this._physicalIndexForKey[inst.__key__] = pidx;
        } else {
          inst.__key__ = null;
          el.setAttribute('hidden', '');
        }

      }, itemSet);
    },

    /**
     * Updates the height for a given set of items.
     *
     * @param {!Array<number>=} itemSet
     */
     _updateMetrics: function(itemSet) {
      var newPhysicalSize = 0;
      var oldPhysicalSize = 0;
      var prevAvgCount = this._physicalAverageCount;
      var prevPhysicalAvg = this._physicalAverage;
      // Make sure we distributed all the physical items
      // so we can measure them
      Polymer.dom.flush();

      this._iterateItems(function(pidx, vidx) {
        oldPhysicalSize += this._physicalSizes[pidx] || 0;
        this._physicalSizes[pidx] = this._physicalItems[pidx].offsetHeight;
        newPhysicalSize += this._physicalSizes[pidx];
        this._physicalAverageCount += this._physicalSizes[pidx] ? 1 : 0;
      }, itemSet);

      this._physicalSize = this._physicalSize + newPhysicalSize - oldPhysicalSize;
      this._viewportSize = this._scroller.offsetHeight;

      // update the average if we measured something
      if (this._physicalAverageCount !== prevAvgCount) {
        this._physicalAverage = Math.round(
            ((prevPhysicalAvg * prevAvgCount) + newPhysicalSize) /
            this._physicalAverageCount);
      }
    },

    /**
     * Updates the position of the physical items.
     */
    _positionItems: function() {
      this._adjustScrollPosition();

      var y = this._physicalTop;

      this._iterateItems(function(pidx) {

        this.transform('translate3d(0, ' + y + 'px, 0)', this._physicalItems[pidx]);
        y += this._physicalSizes[pidx];

      });
    },

    /**
     * Adjusts the scroll position when it was overestimated.
     */
    _adjustScrollPosition: function() {
      var deltaHeight = this._virtualStartVal === 0 ? this._physicalTop :
          Math.min(this._scrollPosition + this._physicalTop, 0);

      if (deltaHeight) {
        this._physicalTop = this._physicalTop - deltaHeight;

        // juking scroll position during interial scrolling on iOS is no bueno
        if (!IOS_TOUCH_SCROLLING) {
          this._resetScrollPosition(this._scroller.scrollTop - deltaHeight);
        }
      }
    },

    /**
     * Sets the position of the scroll.
     */
    _resetScrollPosition: function(pos) {
      if (this._scroller) {
        this._scroller.scrollTop = pos;
        this._scrollPosition = this._scroller.scrollTop;
      }
    },

    /**
     * Sets the scroll height, that's the height of the content,
     *
     * @param {boolean=} forceUpdate If true, updates the height no matter what.
     */
    _updateScrollerSize: function(forceUpdate) {
      this._estScrollHeight = (this._physicalBottom +
          Math.max(this._virtualCount - this._physicalCount - this._virtualStartVal, 0) * this._physicalAverage);

      forceUpdate = forceUpdate || this._scrollHeight === 0;
      forceUpdate = forceUpdate || this._scrollPosition >= this._estScrollHeight - this._physicalSize;

      // amortize height adjustment, so it won't trigger repaints very often
      if (forceUpdate || Math.abs(this._estScrollHeight - this._scrollHeight) >= this._optPhysicalSize) {
        this.$.items.style.height = this._estScrollHeight + 'px';
        this._scrollHeight = this._estScrollHeight;
      }
    },

    /**
     * Scroll to a specific item in the virtual list regardless
     * of the physical items in the DOM tree.
     *
     * @method scrollToIndex
     * @param {number} idx The index of the item
     */
    scrollToIndex: function(idx) {
      if (typeof idx !== 'number') {
        return;
      }

      var firstVisible = this.firstVisibleIndex;

      idx = Math.min(Math.max(idx, 0), this._virtualCount-1);

      // start at the previous virtual item
      // so we have a item above the first visible item
      this._virtualStart = idx - 1;

      // assign new models
      this._assignModels();

      // measure the new sizes
      this._updateMetrics();

      // estimate new physical offset
      this._physicalTop = this._virtualStart * this._physicalAverage;

      var currentTopItem = this._physicalStart;
      var currentVirtualItem = this._virtualStart;
      var targetOffsetTop = 0;
      var hiddenContentSize = this._hiddenContentSize;

      // scroll to the item as much as we can
      while (currentVirtualItem !== idx && targetOffsetTop < hiddenContentSize) {
        targetOffsetTop = targetOffsetTop + this._physicalSizes[currentTopItem];
        currentTopItem = (currentTopItem + 1) % this._physicalCount;
        currentVirtualItem++;
      }

      // update the scroller size
      this._updateScrollerSize(true);

      // update the position of the items
      this._positionItems();

      // set the new scroll position
      this._resetScrollPosition(this._physicalTop + targetOffsetTop + 1);

      // increase the pool of physical items if needed
      if (this._increasePoolIfNeeded()) {
        // yield set models to the new items
        this.async(this._update);
      }
      // clear cached visible index
      this._firstVisibleIndexVal = null;
    },

    /**
     * Reset the physical average and the average count.
     */
    _resetAverage: function() {
      this._physicalAverage = 0;
      this._physicalAverageCount = 0;
    },

    /**
     * A handler for the `iron-resize` event triggered by `IronResizableBehavior`
     * when the element is resized.
     */
    _resizeHandler: function() {
      this.debounce('resize', function() {
        this._render();
        if (this._itemsRendered && this._physicalItems && this._isVisible) {
          this._resetAverage();
          this.updateViewportBoundaries();
          this.scrollToIndex(this.firstVisibleIndex);
        }
      });
    },

    _getModelFromItem: function(item) {
      var key = this._collection.getKey(item);
      var pidx = this._physicalIndexForKey[key];

      if (pidx !== undefined) {
        return this._physicalItems[pidx]._templateInstance;
      }
      return null;
    },

    /**
     * Gets a valid item instance from its index or the object value.
     *
     * @param {(Object|number)} item The item object or its index
     */
    _getNormalizedItem: function(item) {
      if (typeof item === 'number') {
        item = this.items[item];
        if (!item) {
          throw new RangeError('<item> not found');
        }
      } else if (this._collection.getKey(item) === undefined) {
        throw new TypeError('<item> should be a valid item');
      }
      return item;
    },

    /**
     * Select the list item at the given index.
     *
     * @method selectItem
     * @param {(Object|number)} item The item object or its index
     */
    selectItem: function(item) {
      item = this._getNormalizedItem(item);
      var model = this._getModelFromItem(item);

      if (!this.multiSelection && this.selectedItem) {
        this.deselectItem(this.selectedItem);
      }
      if (model) {
        model[this.selectedAs] = true;
      }
      this.$.selector.select(item);
    },

    /**
     * Deselects the given item list if it is already selected.
     *

     * @method deselect
     * @param {(Object|number)} item The item object or its index
     */
    deselectItem: function(item) {
      item = this._getNormalizedItem(item);
      var model = this._getModelFromItem(item);

      if (model) {
        model[this.selectedAs] = false;
      }
      this.$.selector.deselect(item);
    },

    /**
     * Select or deselect a given item depending on whether the item
     * has already been selected.
     *
     * @method toggleSelectionForItem
     * @param {(Object|number)} item The item object or its index
     */
    toggleSelectionForItem: function(item) {
      item = this._getNormalizedItem(item);
      if (/** @type {!ArraySelectorElement} */ (this.$.selector).isSelected(item)) {
        this.deselectItem(item);
      } else {
        this.selectItem(item);
      }
    },

    /**
     * Clears the current selection state of the list.
     *
     * @method clearSelection
     */
    clearSelection: function() {
      function unselect(item) {
        var model = this._getModelFromItem(item);
        if (model) {
          model[this.selectedAs] = false;
        }
      }

      if (Array.isArray(this.selectedItems)) {
        this.selectedItems.forEach(unselect, this);
      } else if (this.selectedItem) {
        unselect.call(this, this.selectedItem);
      }

      /** @type {!ArraySelectorElement} */ (this.$.selector).clearSelection();
    },

    /**
     * Add an event listener to `tap` if `selectionEnabled` is true,
     * it will remove the listener otherwise.
     */
    _selectionEnabledChanged: function(selectionEnabled) {
      if (selectionEnabled) {
        this.listen(this, 'tap', '_selectionHandler');
        this.listen(this, 'keypress', '_selectionHandler');
      } else {
        this.unlisten(this, 'tap', '_selectionHandler');
        this.unlisten(this, 'keypress', '_selectionHandler');
      }
    },

    /**
     * Select an item from an event object.
     */
    _selectionHandler: function(e) {
      if (e.type !== 'keypress' || e.keyCode === 13) {
        var model = this.modelForElement(e.target);
        if (model) {
          this.toggleSelectionForItem(model[this.as]);
        }
      }
    },

    _multiSelectionChanged: function(multiSelection) {
      this.clearSelection();
      this.$.selector.multi = multiSelection;
    },

    /**
     * Updates the size of an item.
     *
     * @method updateSizeForItem
     * @param {(Object|number)} item The item object or its index
     */
    updateSizeForItem: function(item) {
      item = this._getNormalizedItem(item);
      var key = this._collection.getKey(item);
      var pidx = this._physicalIndexForKey[key];

      if (pidx !== undefined) {
        this._updateMetrics([pidx]);
        this._positionItems();
      }
    }
  });

})();

</script>