Rolling forward https://codereview.chromium.org/1155683008/ which was

third_party/polymer/v0_8/components-chromium/iron-selector/iron-selection-extracted.js

-
-
-  /**
-   * @param {!Function} selectCallback
-   * @constructor
-   */
-  Polymer.IronSelection = function(selectCallback) {
-    this.selection = [];
-    this.selectCallback = selectCallback;
-  };
-
-  Polymer.IronSelection.prototype = {
-
-    /**
-     * Retrieves the selected item(s).
-     *
-     * @method get
-     * @returns Returns the selected item(s). If the multi property is true,
-     * `get` will return an array, otherwise it will return
-     * the selected item or undefined if there is no selection.
-     */
-    get: function() {
-      return this.multi ? this.selection : this.selection[0];
-    },
-
-    /**
-     * Clears all the selection except the ones indicated.
-     *
-     * @method clear
-     * @param {Array} excludes items to be excluded.
-     */
-    clear: function(excludes) {
-      this.selection.slice().forEach(function(item) {
-        if (!excludes || excludes.indexOf(item) < 0) {
-          this.setItemSelected(item, false);
-        }
-      }, this);
-    },
-
-    /**
-     * Indicates if a given item is selected.
-     *
-     * @method isSelected
-     * @param {*} item The item whose selection state should be checked.
-     * @returns Returns true if `item` is selected.
-     */
-    isSelected: function(item) {
-      return this.selection.indexOf(item) >= 0;
-    },
-
-    /**
-     * Sets the selection state for a given item to either selected or deselected.
-     *
-     * @method setItemSelected
-     * @param {*} item The item to select.
-     * @param {boolean} isSelected True for selected, false for deselected.
-     */
-    setItemSelected: function(item, isSelected) {
-      if (item != null) {
-        if (isSelected) {
-          this.selection.push(item);
-        } else {
-          var i = this.selection.indexOf(item);
-          if (i >= 0) {
-            this.selection.splice(i, 1);
-          }
-        }
-        if (this.selectCallback) {
-          this.selectCallback(item, isSelected);
-        }
-      }
-    },
-
-    /**
-     * Sets the selection state for a given item. If the `multi` property
-     * is true, then the selected state of `item` will be toggled; otherwise
-     * the `item` will be selected.
-     *
-     * @method select
-     * @param {*} item The item to select.
-     */
-    select: function(item) {
-      if (this.multi) {
-        this.toggle(item);
-      } else if (this.get() !== item) {
-        this.setItemSelected(this.get(), false);
-        this.setItemSelected(item, true);
-      }
-    },
-
-    /**
-     * Toggles the selection state for `item`.
-     *
-     * @method toggle
-     * @param {*} item The item to toggle.
-     */
-    toggle: function(item) {
-      this.setItemSelected(item, !this.isSelected(item));
-    }
-
-  };
-