WebCompDev / COMPONENTS

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<link rel="import" href="../polymer/polymer.html">
<link rel="import" href="../iron-selector/iron-multi-selectable.html">
<link rel="import" href="../iron-a11y-keys-behavior/iron-a11y-keys-behavior.html">

<script>

  /**
   * `Polymer.IronMenuBehavior` implements accessible menu behavior.
   *
   * @demo demo/index.html
   * @polymerBehavior Polymer.IronMenuBehavior
   */
  Polymer.IronMenuBehaviorImpl = {

    properties: {

      /**
       * Returns the currently focused item.
       * @type {?Object}
       */
      focusedItem: {
        observer: '_focusedItemChanged',
        readOnly: true,
        type: Object
      },

      /**
       * The attribute to use on menu items to look up the item title. Typing the first
       * letter of an item when the menu is open focuses that item. If unset, `textContent`
       * will be used.
       */
      attrForItemTitle: {
        type: String
      }
    },

    hostAttributes: {
      'role': 'menu',
      'tabindex': '0'
    },

    observers: [
      '_updateMultiselectable(multi)'
    ],

    listeners: {
      'focus': '_onFocus',
      'keydown': '_onKeydown',
      'iron-items-changed': '_onIronItemsChanged'
    },

    keyBindings: {
      'up': '_onUpKey',
      'down': '_onDownKey',
      'esc': '_onEscKey',
      'shift+tab:keydown': '_onShiftTabDown'
    },

    attached: function() {
      this._resetTabindices();
    },

    /**
     * Selects the given value. If the `multi` property is true, then the selected state of the
     * `value` will be toggled; otherwise the `value` will be selected.
     *
     * @param {string} value the value to select.
     */
    select: function(value) {
      if (this._defaultFocusAsync) {
        this.cancelAsync(this._defaultFocusAsync);
        this._defaultFocusAsync = null;
      }
      var item = this._valueToItem(value);
      if (item && item.hasAttribute('disabled')) return;
      this._setFocusedItem(item);
      Polymer.IronMultiSelectableBehaviorImpl.select.apply(this, arguments);
    },

    /**
     * Resets all tabindex attributes to the appropriate value based on the
     * current selection state. The appropriate value is `0` (focusable) for
     * the default selected item, and `-1` (not keyboard focusable) for all
     * other items.
     */
    _resetTabindices: function() {
      var selectedItem = this.multi ? (this.selectedItems && this.selectedItems[0]) : this.selectedItem;

      this.items.forEach(function(item) {
        item.setAttribute('tabindex', item === selectedItem ? '0' : '-1');
      }, this);
    },

    /**
     * Sets appropriate ARIA based on whether or not the menu is meant to be
     * multi-selectable.
     *
     * @param {boolean} multi True if the menu should be multi-selectable.
     */
    _updateMultiselectable: function(multi) {
      if (multi) {
        this.setAttribute('aria-multiselectable', 'true');
      } else {
        this.removeAttribute('aria-multiselectable');
      }
    },

    /**
     * Given a KeyboardEvent, this method will focus the appropriate item in the
     * menu (if there is a relevant item, and it is possible to focus it).
     *
     * @param {KeyboardEvent} event A KeyboardEvent.
     */
    _focusWithKeyboardEvent: function(event) {
      for (var i = 0, item; item = this.items[i]; i++) {
        var attr = this.attrForItemTitle || 'textContent';
        var title = item[attr] || item.getAttribute(attr);

        if (title && title.trim().charAt(0).toLowerCase() === String.fromCharCode(event.keyCode).toLowerCase()) {
          this._setFocusedItem(item);
          break;
        }
      }
    },

    /**
     * Focuses the previous item (relative to the currently focused item) in the
     * menu.
     */
    _focusPrevious: function() {
      var length = this.items.length;
      var index = (Number(this.indexOf(this.focusedItem)) - 1 + length) % length;
      this._setFocusedItem(this.items[index]);
    },

    /**
     * Focuses the next item (relative to the currently focused item) in the
     * menu.
     */
    _focusNext: function() {
      var index = (Number(this.indexOf(this.focusedItem)) + 1) % this.items.length;
      this._setFocusedItem(this.items[index]);
    },

    /**
     * Mutates items in the menu based on provided selection details, so that
     * all items correctly reflect selection state.
     *
     * @param {Element} item An item in the menu.
     * @param {boolean} isSelected True if the item should be shown in a
     * selected state, otherwise false.
     */
    _applySelection: function(item, isSelected) {
      if (isSelected) {
        item.setAttribute('aria-selected', 'true');
      } else {
        item.removeAttribute('aria-selected');
      }
      Polymer.IronSelectableBehavior._applySelection.apply(this, arguments);
    },

    /**
     * Discretely updates tabindex values among menu items as the focused item
     * changes.
     *
     * @param {Element} focusedItem The element that is currently focused.
     * @param {?Element} old The last element that was considered focused, if
     * applicable.
     */
    _focusedItemChanged: function(focusedItem, old) {
      old && old.setAttribute('tabindex', '-1');
      if (focusedItem) {
        focusedItem.setAttribute('tabindex', '0');
        focusedItem.focus();
      }
    },

    /**
     * A handler that responds to mutation changes related to the list of items
     * in the menu.
     *
     * @param {CustomEvent} event An event containing mutation records as its
     * detail.
     */
    _onIronItemsChanged: function(event) {
      var mutations = event.detail;
      var mutation;
      var index;

      for (index = 0; index < mutations.length; ++index) {
        mutation = mutations[index];

        if (mutation.addedNodes.length) {
          this._resetTabindices();
          break;
        }
      }
    },

    /**
     * Handler that is called when a shift+tab keypress is detected by the menu.
     *
     * @param {CustomEvent} event A key combination event.
     */
    _onShiftTabDown: function(event) {
      var oldTabIndex = this.getAttribute('tabindex');

      Polymer.IronMenuBehaviorImpl._shiftTabPressed = true;

      this._setFocusedItem(null);

      this.setAttribute('tabindex', '-1');

      this.async(function() {
        this.setAttribute('tabindex', oldTabIndex);
        Polymer.IronMenuBehaviorImpl._shiftTabPressed = false;
        // NOTE(cdata): polymer/polymer#1305
      }, 1);
    },

    /**
     * Handler that is called when the menu receives focus.
     *
     * @param {FocusEvent} event A focus event.
     */
    _onFocus: function(event) {
      if (Polymer.IronMenuBehaviorImpl._shiftTabPressed) {
        // do not focus the menu itself
        return;
      }

      this.blur();

      // clear the cached focus item
      this._defaultFocusAsync = this.async(function() {
        // focus the selected item when the menu receives focus, or the first item
        // if no item is selected
        var selectedItem = this.multi ? (this.selectedItems && this.selectedItems[0]) : this.selectedItem;

        this._setFocusedItem(null);

        if (selectedItem) {
          this._setFocusedItem(selectedItem);
        } else {
          this._setFocusedItem(this.items[0]);
        }
      // async 1ms to wait for `select` to get called from `_itemActivate`
      }, 1);
    },

    /**
     * Handler that is called when the up key is pressed.
     *
     * @param {CustomEvent} event A key combination event.
     */
    _onUpKey: function(event) {
      // up and down arrows moves the focus
      this._focusPrevious();
    },

    /**
     * Handler that is called when the down key is pressed.
     *
     * @param {CustomEvent} event A key combination event.
     */
    _onDownKey: function(event) {
      this._focusNext();
    },

    /**
     * Handler that is called when the esc key is pressed.
     *
     * @param {CustomEvent} event A key combination event.
     */
    _onEscKey: function(event) {
      // esc blurs the control
      this.focusedItem.blur();
    },

    /**
     * Handler that is called when a keydown event is detected.
     *
     * @param {KeyboardEvent} event A keyboard event.
     */
    _onKeydown: function(event) {
      if (!this.keyboardEventMatchesKeys(event, 'up down esc')) {
        // all other keys focus the menu item starting with that character
        this._focusWithKeyboardEvent(event);
      }
      event.stopPropagation();
    },

    // override _activateHandler
    _activateHandler: function(event) {
      Polymer.IronSelectableBehavior._activateHandler.call(this, event);
      event.stopPropagation();
    }
  };

  Polymer.IronMenuBehaviorImpl._shiftTabPressed = false;

  /** @polymerBehavior Polymer.IronMenuBehavior */
  Polymer.IronMenuBehavior = [
    Polymer.IronMultiSelectableBehavior,
    Polymer.IronA11yKeysBehavior,
    Polymer.IronMenuBehaviorImpl
  ];

</script>