The source code

    Ext.ns('Ext.ux.grid');

    /**
     * @class Ext.ux.grid.RowExpander
     * @extends Ext.util.Observable
     * Plugin (ptype = 'rowexpander') that adds the ability to have a Column in a grid which enables
     * a second row body which expands/contracts.  The expand/contract behavior is configurable to react
     * on clicking of the column, double click of the row, and/or hitting enter while a row is selected.
     *
     * @ptype rowexpander
     */
    Ext.ux.grid.RowExpander = Ext.extend(Ext.util.Observable, {
        /**
         * @cfg {Boolean} expandOnEnter
         * true to toggle selected row(s) between expanded/collapsed when the enter
         * key is pressed (defaults to true).
         */
        expandOnEnter : true,
        /**
         * @cfg {Boolean} expandOnDblClick
         * true to toggle a row between expanded/collapsed when double clicked
         * (defaults to true).
         */
        expandOnDblClick : true,

        header : '',
        width : 20,
        sortable : false,
        fixed : true,
        menuDisabled : true,
        dataIndex : '',
        id : 'expander',
        lazyRender : true,
        enableCaching : true,

        constructor: function(config){
            Ext.apply(this, config);

            this.addEvents({
                /**
                 * @event beforeexpand
                 * Fires before the row expands. Have the listener return false to prevent the row from expanding.
                 * @param {Object} this RowExpander object.
                 * @param {Object} Ext.data.Record Record for the selected row.
                 * @param {Object} body body element for the secondary row.
                 * @param {Number} rowIndex The current row index.
                 */
                beforeexpand: true,
                /**
                 * @event expand
                 * Fires after the row expands.
                 * @param {Object} this RowExpander object.
                 * @param {Object} Ext.data.Record Record for the selected row.
                 * @param {Object} body body element for the secondary row.
                 * @param {Number} rowIndex The current row index.
                 */
                expand: true,
                /**
                 * @event beforecollapse
                 * Fires before the row collapses. Have the listener return false to prevent the row from collapsing.
                 * @param {Object} this RowExpander object.
                 * @param {Object} Ext.data.Record Record for the selected row.
                 * @param {Object} body body element for the secondary row.
                 * @param {Number} rowIndex The current row index.
                 */
                beforecollapse: true,
                /**
                 * @event collapse
                 * Fires after the row collapses.
                 * @param {Object} this RowExpander object.
                 * @param {Object} Ext.data.Record Record for the selected row.
                 * @param {Object} body body element for the secondary row.
                 * @param {Number} rowIndex The current row index.
                 */
                collapse: true
            });

            Ext.ux.grid.RowExpander.superclass.constructor.call(this);

            if(this.tpl){
                if(typeof this.tpl == 'string'){
                    this.tpl = new Ext.Template(this.tpl);
                }
                this.tpl.compile();
            }

            this.state = {};
            this.bodyContent = {};
        },

        getRowClass : function(record, rowIndex, p, ds){
            p.cols = p.cols-1;
            var content = this.bodyContent[record.id];
            if(!content && !this.lazyRender){
                content = this.getBodyContent(record, rowIndex);
            }
            if(content){
                p.body = content;
            }
            return this.state[record.id] ? 'x-grid3-row-expanded' : 'x-grid3-row-collapsed';
        },

        init : function(grid){
            this.grid = grid;

            var view = grid.getView();
            view.getRowClass = this.getRowClass.createDelegate(this);

            view.enableRowBody = true;

            grid.on('render', this.onRender, this);
            grid.on('destroy', this.onDestroy, this);
        },

        // @private
        onRender: function() {
            var grid = this.grid;
            var mainBody = grid.getView().mainBody;
            mainBody.on('mousedown', this.onMouseDown, this, {delegate: '.x-grid3-row-expander'});
            if (this.expandOnEnter) {
                this.keyNav = new Ext.KeyNav(this.grid.getGridEl(), {
                    'enter' : this.onEnter,
                    scope: this
                });
            }
            if (this.expandOnDblClick) {
                grid.on('rowdblclick', this.onRowDblClick, this);
            }
        },

        // @private
        onDestroy: function() {
            this.keyNav.disable();
            delete this.keyNav;
            var mainBody = this.grid.getView().mainBody;
            mainBody.un('mousedown', this.onMouseDown, this);
        },
        // @private
        onRowDblClick: function(grid, rowIdx, e) {
            this.toggleRow(rowIdx);
        },

        onEnter: function(e) {
            var g = this.grid;
            var sm = g.getSelectionModel();
            var sels = sm.getSelections();
            for (var i = 0, len = sels.length; i < len; i++) {
                var rowIdx = g.getStore().indexOf(sels[i]);
                this.toggleRow(rowIdx);
            }
        },

        getBodyContent : function(record, index){
            if(!this.enableCaching){
                return this.tpl.apply(record.data);
            }
            var content = this.bodyContent[record.id];
            if(!content){
                content = this.tpl.apply(record.data);
                this.bodyContent[record.id] = content;
            }
            return content;
        },

        onMouseDown : function(e, t){
            e.stopEvent();
            var row = e.getTarget('.x-grid3-row');
            this.toggleRow(row);
        },

        renderer : function(v, p, record){
            p.cellAttr = 'rowspan="2"';
            return ' ';
        },

        beforeExpand : function(record, body, rowIndex){
            if(this.fireEvent('beforeexpand', this, record, body, rowIndex) !== false){
                if(this.tpl && this.lazyRender){
                    body.innerHTML = this.getBodyContent(record, rowIndex);
                }
                return true;
            }else{
                return false;
            }
        },

        toggleRow : function(row){
            if(typeof row == 'number'){
                row = this.grid.view.getRow(row);
            }
            this[Ext.fly(row).hasClass('x-grid3-row-collapsed') ? 'expandRow' : 'collapseRow'](row);
        },

        expandRow : function(row){
            if(typeof row == 'number'){
                row = this.grid.view.getRow(row);
            }
            var record = this.grid.store.getAt(row.rowIndex);
            var body = Ext.DomQuery.selectNode('tr:nth(2) div.x-grid3-row-body', row);
            if(this.beforeExpand(record, body, row.rowIndex)){
                this.state[record.id] = true;
                Ext.fly(row).replaceClass('x-grid3-row-collapsed', 'x-grid3-row-expanded');
                this.fireEvent('expand', this, record, body, row.rowIndex);
            }
        },

        collapseRow : function(row){
            if(typeof row == 'number'){
                row = this.grid.view.getRow(row);
            }
            var record = this.grid.store.getAt(row.rowIndex);
            var body = Ext.fly(row).child('tr:nth(1) div.x-grid3-row-body', true);
            if(this.fireEvent('beforecollapse', this, record, body, row.rowIndex) !== false){
                this.state[record.id] = false;
                Ext.fly(row).replaceClass('x-grid3-row-expanded', 'x-grid3-row-collapsed');
                this.fireEvent('collapse', this, record, body, row.rowIndex);
            }
        }
    });

    Ext.preg('rowexpander', Ext.ux.grid.RowExpander);

    //backwards compat
    Ext.grid.RowExpander = Ext.ux.grid.RowExpander;