iron-dropdown-scroll-manager.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">

<script>
  (function() {
    'use strict';
    /**
     * Used to calculate the scroll direction during touch events.
     * @type {!Object}
     */
    var lastTouchPosition = {
      pageX: 0,
      pageY: 0
    };
    /**
     * Used to avoid computing event.path and filter scrollable nodes (better perf).
     * @type {?EventTarget}
     */
    var lastRootTarget = null;
    /**
     * @type {!Array<Node>}
     */
    var lastScrollableNodes = [];

    var scrollEvents = [
      // Modern `wheel` event for mouse wheel scrolling:
      'wheel',
      // Older, non-standard `mousewheel` event for some FF:
      'mousewheel',
      // IE:
      'DOMMouseScroll',
      // Touch enabled devices
      'touchstart',
      'touchmove'
    ];

    /**
     * The IronDropdownScrollManager is intended to provide a central source
     * of authority and control over which elements in a document are currently
     * allowed to scroll.
     */

    Polymer.IronDropdownScrollManager = {

      /**
       * The current element that defines the DOM boundaries of the
       * scroll lock. This is always the most recently locking element.
       */
      get currentLockingElement() {
        return this._lockingElements[this._lockingElements.length - 1];
      },

      /**
       * Returns true if the provided element is "scroll locked", which is to
       * say that it cannot be scrolled via pointer or keyboard interactions.
       *
       * @param {HTMLElement} element An HTML element instance which may or may
       * not be scroll locked.
       */
      elementIsScrollLocked: function(element) {
        var currentLockingElement = this.currentLockingElement;

        if (currentLockingElement === undefined)
          return false;

        var scrollLocked;

        if (this._hasCachedLockedElement(element)) {
          return true;
        }

        if (this._hasCachedUnlockedElement(element)) {
          return false;
        }

        scrollLocked = !!currentLockingElement &&
          currentLockingElement !== element &&
          !this._composedTreeContains(currentLockingElement, element);

        if (scrollLocked) {
          this._lockedElementCache.push(element);
        } else {
          this._unlockedElementCache.push(element);
        }

        return scrollLocked;
      },

      /**
       * Push an element onto the current scroll lock stack. The most recently
       * pushed element and its children will be considered scrollable. All
       * other elements will not be scrollable.
       *
       * Scroll locking is implemented as a stack so that cases such as
       * dropdowns within dropdowns are handled well.
       *
       * @param {HTMLElement} element The element that should lock scroll.
       */
      pushScrollLock: function(element) {
        // Prevent pushing the same element twice
        if (this._lockingElements.indexOf(element) >= 0) {
          return;
        }

        if (this._lockingElements.length === 0) {
          this._lockScrollInteractions();
        }

        this._lockingElements.push(element);

        this._lockedElementCache = [];
        this._unlockedElementCache = [];
      },

      /**
       * Remove an element from the scroll lock stack. The element being
       * removed does not need to be the most recently pushed element. However,
       * the scroll lock constraints only change when the most recently pushed
       * element is removed.
       *
       * @param {HTMLElement} element The element to remove from the scroll
       * lock stack.
       */
      removeScrollLock: function(element) {
        var index = this._lockingElements.indexOf(element);

        if (index === -1) {
          return;
        }

        this._lockingElements.splice(index, 1);

        this._lockedElementCache = [];
        this._unlockedElementCache = [];

        if (this._lockingElements.length === 0) {
          this._unlockScrollInteractions();
        }
      },

      _lockingElements: [],

      _lockedElementCache: null,

      _unlockedElementCache: null,

      _hasCachedLockedElement: function(element) {
        return this._lockedElementCache.indexOf(element) > -1;
      },

      _hasCachedUnlockedElement: function(element) {
        return this._unlockedElementCache.indexOf(element) > -1;
      },

      _composedTreeContains: function(element, child) {
        // NOTE(cdata): This method iterates over content elements and their
        // corresponding distributed nodes to implement a contains-like method
        // that pierces through the composed tree of the ShadowDOM. Results of
        // this operation are cached (elsewhere) on a per-scroll-lock basis, to
        // guard against potentially expensive lookups happening repeatedly as
        // a user scrolls / touchmoves.
        var contentElements;
        var distributedNodes;
        var contentIndex;
        var nodeIndex;

        if (element.contains(child)) {
          return true;
        }

        contentElements = Polymer.dom(element).querySelectorAll('content,slot');

        for (contentIndex = 0; contentIndex < contentElements.length; ++contentIndex) {

          distributedNodes = Polymer.dom(contentElements[contentIndex]).getDistributedNodes();

          for (nodeIndex = 0; nodeIndex < distributedNodes.length; ++nodeIndex) {
            // Polymer 2.x returns slot.assignedNodes which can contain text nodes.
            if (distributedNodes[nodeIndex].nodeType !== Node.ELEMENT_NODE) continue;

            if (this._composedTreeContains(distributedNodes[nodeIndex], child)) {
              return true;
            }
          }
        }

        return false;
      },

      _scrollInteractionHandler: function(event) {
        // Avoid canceling an event with cancelable=false, e.g. scrolling is in
        // progress and cannot be interrupted.
        if (event.cancelable && this._shouldPreventScrolling(event)) {
          event.preventDefault();
        }
        // If event has targetTouches (touch event), update last touch position.
        if (event.targetTouches) {
          var touch = event.targetTouches[0];
          lastTouchPosition.pageX = touch.pageX;
          lastTouchPosition.pageY = touch.pageY;
        }
      },

      _lockScrollInteractions: function() {
        this._boundScrollHandler = this._boundScrollHandler ||
          this._scrollInteractionHandler.bind(this);
        for (var i = 0, l = scrollEvents.length; i < l; i++) {
          // NOTE: browsers that don't support objects as third arg will
          // interpret it as boolean, hence useCapture = true in this case. 
          document.addEventListener(scrollEvents[i], this._boundScrollHandler, {
            capture: true,
            passive: false
          });
        }
      },

      _unlockScrollInteractions: function() {
        for (var i = 0, l = scrollEvents.length; i < l; i++) {
          // NOTE: browsers that don't support objects as third arg will
          // interpret it as boolean, hence useCapture = true in this case.
          document.removeEventListener(scrollEvents[i], this._boundScrollHandler, {
            capture: true,
            passive: false
          });
        }
      },

      /**
       * Returns true if the event causes scroll outside the current locking
       * element, e.g. pointer/keyboard interactions, or scroll "leaking"
       * outside the locking element when it is already at its scroll boundaries.
       * @param {!Event} event
       * @return {boolean}
       * @private
       */
      _shouldPreventScrolling: function(event) {

        // Update if root target changed. For touch events, ensure we don't
        // update during touchmove.
        var target = Polymer.dom(event).rootTarget;
        if (event.type !== 'touchmove' && lastRootTarget !== target) {
          lastRootTarget = target;
          lastScrollableNodes = this._getScrollableNodes(Polymer.dom(event).path);
        }

        // Prevent event if no scrollable nodes.
        if (!lastScrollableNodes.length) {
          return true;
        }
        // Don't prevent touchstart event inside the locking element when it has
        // scrollable nodes.
        if (event.type === 'touchstart') {
          return false;
        }
        // Get deltaX/Y.
        var info = this._getScrollInfo(event);
        // Prevent if there is no child that can scroll.
        return !this._getScrollingNode(lastScrollableNodes, info.deltaX, info.deltaY);
      },

      /**
       * Returns an array of scrollable nodes up to the current locking element,
       * which is included too if scrollable.
       * @param {!Array<Node>} nodes
       * @return {Array<Node>} scrollables
       * @private
       */
      _getScrollableNodes: function(nodes) {
        var scrollables = [];
        var lockingIndex = nodes.indexOf(this.currentLockingElement);
        // Loop from root target to locking element (included).
        for (var i = 0; i <= lockingIndex; i++) {
          // Skip non-Element nodes.
          if (nodes[i].nodeType !== Node.ELEMENT_NODE) {
            continue;
          }
          var node = /** @type {!Element} */ (nodes[i]);
          // Check inline style before checking computed style.
          var style = node.style;
          if (style.overflow !== 'scroll' && style.overflow !== 'auto') {
            style = window.getComputedStyle(node);
          }
          if (style.overflow === 'scroll' || style.overflow === 'auto') {
            scrollables.push(node);
          }
        }
        return scrollables;
      },

      /**
       * Returns the node that is scrolling. If there is no scrolling,
       * returns undefined.
       * @param {!Array<Node>} nodes
       * @param {number} deltaX Scroll delta on the x-axis
       * @param {number} deltaY Scroll delta on the y-axis
       * @return {Node|undefined}
       * @private
       */
      _getScrollingNode: function(nodes, deltaX, deltaY) {
        // No scroll.
        if (!deltaX && !deltaY) {
          return;
        }
        // Check only one axis according to where there is more scroll.
        // Prefer vertical to horizontal.
        var verticalScroll = Math.abs(deltaY) >= Math.abs(deltaX);
        for (var i = 0; i < nodes.length; i++) {
          var node = nodes[i];
          var canScroll = false;
          if (verticalScroll) {
            // delta < 0 is scroll up, delta > 0 is scroll down.
            canScroll = deltaY < 0 ? node.scrollTop > 0 :
              node.scrollTop < node.scrollHeight - node.clientHeight;
          } else {
            // delta < 0 is scroll left, delta > 0 is scroll right.
            canScroll = deltaX < 0 ? node.scrollLeft > 0 :
              node.scrollLeft < node.scrollWidth - node.clientWidth;
          }
          if (canScroll) {
            return node;
          }
        }
      },

      /**
       * Returns scroll `deltaX` and `deltaY`.
       * @param {!Event} event The scroll event
       * @return {{deltaX: number, deltaY: number}} Object containing the
       * x-axis scroll delta (positive: scroll right, negative: scroll left,
       * 0: no scroll), and the y-axis scroll delta (positive: scroll down,
       * negative: scroll up, 0: no scroll).
       * @private
       */
      _getScrollInfo: function(event) {
        var info = {
          deltaX: event.deltaX,
          deltaY: event.deltaY
        };
        // Already available.
        if ('deltaX' in event) {
          // do nothing, values are already good.
        }
        // Safari has scroll info in `wheelDeltaX/Y`.
        else if ('wheelDeltaX' in event) {
          info.deltaX = -event.wheelDeltaX;
          info.deltaY = -event.wheelDeltaY;
        }
        // Firefox has scroll info in `detail` and `axis`.
        else if ('axis' in event) {
          info.deltaX = event.axis === 1 ? event.detail : 0;
          info.deltaY = event.axis === 2 ? event.detail : 0;
        }
        // On mobile devices, calculate scroll direction.
        else if (event.targetTouches) {
          var touch = event.targetTouches[0];
          // Touch moves from right to left => scrolling goes right.
          info.deltaX = lastTouchPosition.pageX - touch.pageX;
          // Touch moves from down to up => scrolling goes down.
          info.deltaY = lastTouchPosition.pageY - touch.pageY;
        }
        return info;
      }
    };
  })();
</script>