The source code

    /**
     * @class Ext.list.Sorter
     * @extends Ext.util.Observable
     * <p>Supporting Class for Ext.list.ListView</p>
     * @constructor
     * @param {Object} config
     */
    Ext.list.Sorter = Ext.extend(Ext.util.Observable, {
        /**
         * @cfg {Array} sortClasses
         * The CSS classes applied to a header when it is sorted. (defaults to <tt>["sort-asc", "sort-desc"]</tt>)
         */
        sortClasses : ["sort-asc", "sort-desc"],

        constructor: function(config){
            Ext.apply(this, config);
            Ext.list.Sorter.superclass.constructor.call(this);
        },

        init : function(listView){
            this.view = listView;
            listView.on('render', this.initEvents, this);
        },

        initEvents : function(view){
            view.mon(view.innerHd, 'click', this.onHdClick, this);
            view.innerHd.setStyle('cursor', 'pointer');
            view.mon(view.store, 'datachanged', this.updateSortState, this);
            this.updateSortState.defer(10, this, [view.store]);
        },

        updateSortState : function(store){
            var state = store.getSortState();
            if(!state){
                return;
            }
            this.sortState = state;
            var cs = this.view.columns, sortColumn = -1;
            for(var i = 0, len = cs.length; i < len; i++){
                if(cs[i].dataIndex == state.field){
                    sortColumn = i;
                    break;
                }
            }
            if(sortColumn != -1){
                var sortDir = state.direction;
                this.updateSortIcon(sortColumn, sortDir);
            }
        },

        updateSortIcon : function(col, dir){
            var sc = this.sortClasses;
            var hds = this.view.innerHd.select('em').removeClass(sc);
            hds.item(col).addClass(sc[dir == "DESC" ? 1 : 0]);
        },

        onHdClick : function(e){
            var hd = e.getTarget('em', 3);
            if(hd && !this.view.disableHeaders){
                var index = this.view.findHeaderIndex(hd);
                this.view.store.sort(this.view.columns[index].dataIndex);
            }
        }
    });

    // Backwards compatibility alias
    Ext.ListView.Sorter = Ext.list.Sorter;