    /**
     * @class Ext.util.Sorter
     * @extends Object

    Represents a single sorter that can be applied to a Store. The sorter is used
    to compare two values against each other for the purpose of ordering them. Ordering
    is achieved by specifying either:
    - {@link #property A sorting property}
    - {@link #sorterFn A sorting function}

    As a contrived example, we can specify a custom sorter that sorts by rank:

        Ext.define('Person', {
            extend: 'Ext.data.Model',
            fields: ['name', 'rank']
        });

        Ext.create('Ext.data.Store', {
            model: 'Person',
            proxy: 'memory',
            sorters: [{
                sorterFn: function(o1, o2){
                    var getRank = function(o){
                        var name = o.get('rank');
                        if (name === 'first') {
                            return 1;
                        } else if (name === 'second') {
                            return 2;
                        } else {
                            return 3;
                        }
                    },
                    rank1 = getRank(o1),
                    rank2 = getRank(o2);

                    if (rank1 === rank2) {
                        return 0;
                    }

                    return rank1 < rank2 ? -1 : 1;
                }
            }],
            data: [{
                name: 'Person1',
                rank: 'second'
            }, {
                name: 'Person2',
                rank: 'third'
            }, {
                name: 'Person3',
                rank: 'first'
            }]
        });

     * @markdown
     */
    Ext.define('Ext.util.Sorter', {

        /**
         * @cfg {String} property The property to sort by. Required unless {@link #sorterFn} is provided.
         * The property is extracted from the object directly and compared for sorting using the built in
         * comparison operators.
         */

        /**
         * @cfg {Function} sorterFn A specific sorter function to execute. Can be passed instead of {@link #property}.
         * This sorter function allows for any kind of custom/complex comparisons.
         * The sorterFn receives two arguments, the objects being compared. The function should return:
         * <ul>
         * <li>-1 if o1 is "less than" o2</li>
         * <li>0 if o1 is "equal" to o2</li>
         * <li>1 if o1 is "greater than" o2</li>
         * </ul>
         */

        /**
         * @cfg {String} root Optional root property. This is mostly useful when sorting a Store, in which case we set the
         * root to 'data' to make the filter pull the {@link #property} out of the data object of each item
         */

        /**
         * @cfg {Function} transform A function that will be run on each value before
         * it is compared in the sorter. The function will receive a single argument,
         * the value.
         */

        /**
         * @cfg {String} direction The direction to sort by. Defaults to ASC
         */
        direction: "ASC",

        constructor: function(config) {
            var me = this;

            Ext.apply(me, config);

            //<debug>
            if (me.property === undefined && me.sorterFn === undefined) {
                Ext.Error.raise("A Sorter requires either a property or a sorter function");
            }
            //</debug>

            me.updateSortFunction();
        },

        /**
         * @private
         * Creates and returns a function which sorts an array by the given property and direction
         * @return {Function} A function which sorts by the property/direction combination provided
         */
        createSortFunction: function(sorterFn) {
            var me        = this,
                property  = me.property,
                direction = me.direction || "ASC",
                modifier  = direction.toUpperCase() == "DESC" ? -1 : 1;

            //create a comparison function. Takes 2 objects, returns 1 if object 1 is greater,
            //-1 if object 2 is greater or 0 if they are equal
            return function(o1, o2) {
                return modifier * sorterFn.call(me, o1, o2);
            };
        },

        /**
         * @private
         * Basic default sorter function that just compares the defined property of each object
         */
        defaultSorterFn: function(o1, o2) {
            var me = this,
                transform = me.transform,
                v1 = me.getRoot(o1)[me.property],
                v2 = me.getRoot(o2)[me.property];

            if (transform) {
                v1 = transform(v1);
                v2 = transform(v2);
            }

            return v1 > v2 ? 1 : (v1 < v2 ? -1 : 0);
        },

        /**
         * @private
         * Returns the root property of the given item, based on the configured {@link #root} property
         * @param {Object} item The item
         * @return {Object} The root property of the object
         */
        getRoot: function(item) {
            return this.root === undefined ? item : item[this.root];
        },

        /**
         * Set the sorting direction for this sorter.
         * @param {String} direction The direction to sort in. Should be either 'ASC' or 'DESC'.
         */
        setDirection: function(direction) {
            var me = this;
            me.direction = direction;
            me.updateSortFunction();
        },

        /**
         * Toggles the sorting direction for this sorter.
         */
        toggle: function() {
            var me = this;
            me.direction = Ext.String.toggle(me.direction, "ASC", "DESC");
            me.updateSortFunction();
        },

        /**
         * Update the sort function for this sorter.
         * @param {Function} fn (Optional) A new sorter function for this sorter. If not specified it will use the
         * default sorting function.
         */
        updateSortFunction: function(fn) {
            var me = this;
            fn = fn || me.sorterFn || me.defaultSorterFn;
            me.sort = me.createSortFunction(fn);
        }
    });