The source code

    /**
     * @class Ext.grid.GroupingView
     * @extends Ext.grid.GridView
     * Adds the ability for single level grouping to the grid. A {@link Ext.data.GroupingStore GroupingStore}
     * must be used to enable grouping.  Some grouping characteristics may also be configured at the
     * {@link Ext.grid.Column Column level}
     * {@link Ext.grid.Column#emptyGroupText emptyGroupText}
     * {@link Ext.grid.Column#groupable groupable}
     * {@link Ext.grid.Column#groupName groupName}
     * {@link Ext.grid.Column#groupRender groupRender}
     *
     * Sample usage:
     *
    var grid = new Ext.grid.GridPanel({
        // A groupingStore is required for a GroupingView
        store: new {@link Ext.data.GroupingStore}({
            autoDestroy: true,
            reader: reader,
            data: xg.dummyData,
            sortInfo: {field: 'company', direction: 'ASC'},
            {@link Ext.data.GroupingStore#groupOnSort groupOnSort}: true,
            {@link Ext.data.GroupingStore#remoteGroup remoteGroup}: true,
            {@link Ext.data.GroupingStore#groupField groupField}: 'industry'
        }),
        colModel: new {@link Ext.grid.ColumnModel}({
            columns:[
                {id:'company',header: 'Company', width: 60, dataIndex: 'company'},
                // {@link Ext.grid.Column#groupable groupable}, {@link Ext.grid.Column#groupName groupName}, {@link Ext.grid.Column#groupRender groupRender} are also configurable at column level
                {header: 'Price', renderer: Ext.util.Format.usMoney, dataIndex: 'price', {@link Ext.grid.Column#groupable groupable}: false},
                {header: 'Change', dataIndex: 'change', renderer: Ext.util.Format.usMoney},
                {header: 'Industry', dataIndex: 'industry'},
                {header: 'Last Updated', renderer: Ext.util.Format.dateRenderer('m/d/Y'), dataIndex: 'lastChange'}
            ],
            defaults: {
                sortable: true,
                menuDisabled: false,
                width: 20
            }
        }),

        view: new Ext.grid.GroupingView({
            {@link Ext.grid.GridView#forceFit forceFit}: true,
            // custom grouping text template to display the number of items per group
            {@link #groupTextTpl}: '{text} ({[values.rs.length]} {[values.rs.length > 1 ? "Items" : "Item"]})'
        }),

        frame:true,
        width: 700,
        height: 450,
        collapsible: true,
        animCollapse: false,
        title: 'Grouping Example',
        iconCls: 'icon-grid',
        renderTo: document.body
    });
     *
     * @constructor
     * @param {Object} config
     */
    Ext.grid.GroupingView = Ext.extend(Ext.grid.GridView, {

        /**
         * @cfg {String} groupByText Text displayed in the grid header menu for grouping by a column
         * (defaults to 'Group By This Field').
         */
        groupByText : 'Group By This Field',
        /**
         * @cfg {String} showGroupsText Text displayed in the grid header for enabling/disabling grouping
         * (defaults to 'Show in Groups').
         */
        showGroupsText : 'Show in Groups',
        /**
         * @cfg {Boolean} hideGroupedColumn true to hide the column that is currently grouped (defaults to false)
         */
        hideGroupedColumn : false,
        /**
         * @cfg {Boolean} showGroupName If true will display a prefix plus a ': ' before the group field value
         * in the group header line.  The prefix will consist of the {@link Ext.grid.Column#groupName groupName}
         * (or the configured {@link Ext.grid.Column#header header} if not provided) configured in the
         * {@link Ext.grid.Column} for each set of grouped rows (defaults to true).
         */
        showGroupName : true,
        /**
         * @cfg {Boolean} startCollapsed true to start all groups collapsed (defaults to false)
         */
        startCollapsed : false,
        /**
         * @cfg {Boolean} enableGrouping false to disable grouping functionality (defaults to true)
         */
        enableGrouping : true,
        /**
         * @cfg {Boolean} enableGroupingMenu true to enable the grouping control in the column menu (defaults to true)
         */
        enableGroupingMenu : true,
        /**
         * @cfg {Boolean} enableNoGroups true to allow the user to turn off grouping (defaults to true)
         */
        enableNoGroups : true,
        /**
         * @cfg {String} emptyGroupText The text to display when there is an empty group value (defaults to '(None)').
         * May also be specified per column, see {@link Ext.grid.Column}.{@link Ext.grid.Column#emptyGroupText emptyGroupText}.
         */
        emptyGroupText : '(None)',
        /**
         * @cfg {Boolean} ignoreAdd true to skip refreshing the view when new rows are added (defaults to false)
         */
        ignoreAdd : false,
        /**
         * @cfg {String} groupTextTpl The template used to render the group header (defaults to '{text}').
         * This is used to format an object which contains the following properties:
         *
         * group : StringThe rendered value of the group field.
         * By default this is the unchanged value of the group field. If a {@link Ext.grid.Column#groupRenderer groupRenderer}
         * is specified, it is the result of a call to that function.
         * gvalue : ObjectThe raw value of the group field.
         * text : StringThe configured header (as described in {@link #showGroupName})
         * if {@link #showGroupName} is true) plus the rendered group field value.
         * groupId : StringA unique, generated ID which is applied to the
         * View Element which contains the group.
         * startRow : NumberThe row index of the Record which caused group change.
         * rs : ArrayContains a single element: The Record providing the data
         * for the row which caused group change.
         * cls : StringThe generated class name string to apply to the group header Element.
         * style : StringThe inline style rules to apply to the group header Element.
         *
         * See {@link Ext.XTemplate} for information on how to format data using a template. Possible usage:
    var grid = new Ext.grid.GridPanel({
        ...
        view: new Ext.grid.GroupingView({
            groupTextTpl: '{text} ({[values.rs.length]} {[values.rs.length > 1 ? "Items" : "Item"]})'
        }),
    });
         *
         */
        groupTextTpl : '{text}',
        /**
         * @cfg {Function} groupRenderer This property must be configured in the {@link Ext.grid.Column} for
         * each column.
         */

        // private
        gidSeed : 1000,

        // private
        initTemplates : function(){
            Ext.grid.GroupingView.superclass.initTemplates.call(this);
            this.state = {};

            var sm = this.grid.getSelectionModel();
            sm.on(sm.selectRow ? 'beforerowselect' : 'beforecellselect',
                    this.onBeforeRowSelect, this);

            if(!this.startGroup){
                this.startGroup = new Ext.XTemplate(
                    '',
                        '', this.groupTextTpl ,'',
                        ''
                );
            }
            this.startGroup.compile();
            this.endGroup = '';
        },

        // private
        findGroup : function(el){
            return Ext.fly(el).up('.x-grid-group', this.mainBody.dom);
        },

        // private
        getGroups : function(){
            return this.hasRows() ? this.mainBody.dom.childNodes : [];
        },

        // private
        onAdd : function(){
            if(this.enableGrouping && !this.ignoreAdd){
                var ss = this.getScrollState();
                this.refresh();
                this.restoreScroll(ss);
            }else if(!this.enableGrouping){
                Ext.grid.GroupingView.superclass.onAdd.apply(this, arguments);
            }
        },

        // private
        onRemove : function(ds, record, index, isUpdate){
            Ext.grid.GroupingView.superclass.onRemove.apply(this, arguments);
            var g = document.getElementById(record._groupId);
            if(g && g.childNodes[1].childNodes.length < 1){
                Ext.removeNode(g);
            }
            this.applyEmptyText();
        },

        // private
        refreshRow : function(record){
            if(this.ds.getCount()==1){
                this.refresh();
            }else{
                this.isUpdating = true;
                Ext.grid.GroupingView.superclass.refreshRow.apply(this, arguments);
                this.isUpdating = false;
            }
        },

        // private
        beforeMenuShow : function(){
            var item, items = this.hmenu.items, disabled = this.cm.config[this.hdCtxIndex].groupable === false;
            if((item = items.get('groupBy'))){
                item.setDisabled(disabled);
            }
            if((item = items.get('showGroups'))){
                item.setDisabled(disabled);
    		    item.setChecked(!!this.getGroupField(), true);
            }
        },

        // private
        renderUI : function(){
            Ext.grid.GroupingView.superclass.renderUI.call(this);
            this.mainBody.on('mousedown', this.interceptMouse, this);

            if(this.enableGroupingMenu && this.hmenu){
                this.hmenu.add('-',{
                    itemId:'groupBy',
                    text: this.groupByText,
                    handler: this.onGroupByClick,
                    scope: this,
                    iconCls:'x-group-by-icon'
                });
                if(this.enableNoGroups){
                    this.hmenu.add({
                        itemId:'showGroups',
                        text: this.showGroupsText,
                        checked: true,
                        checkHandler: this.onShowGroupsClick,
                        scope: this
                    });
                }
                this.hmenu.on('beforeshow', this.beforeMenuShow, this);
            }
        },

        // private
        onGroupByClick : function(){
            this.grid.store.groupBy(this.cm.getDataIndex(this.hdCtxIndex));
            this.beforeMenuShow(); // Make sure the checkboxes get properly set when changing groups
        },

        // private
        onShowGroupsClick : function(mi, checked){
            if(checked){
                this.onGroupByClick();
            }else{
                this.grid.store.clearGrouping();
            }
        },

        /**
         * Toggles the specified group if no value is passed, otherwise sets the expanded state of the group to the value passed.
         * @param {String} groupId The groupId assigned to the group (see getGroupId)
         * @param {Boolean} expanded (optional)
         */
        toggleGroup : function(group, expanded){
            this.grid.stopEditing(true);
            group = Ext.getDom(group);
            var gel = Ext.fly(group);
            expanded = expanded !== undefined ?
                    expanded : gel.hasClass('x-grid-group-collapsed');

            this.state[gel.dom.id] = expanded;
            gel[expanded ? 'removeClass' : 'addClass']('x-grid-group-collapsed');
        },

        /**
         * Toggles all groups if no value is passed, otherwise sets the expanded state of all groups to the value passed.
         * @param {Boolean} expanded (optional)
         */
        toggleAllGroups : function(expanded){
            var groups = this.getGroups();
            for(var i = 0, len = groups.length; i < len; i++){
                this.toggleGroup(groups[i], expanded);
            }
        },

        /**
         * Expands all grouped rows.
         */
        expandAllGroups : function(){
            this.toggleAllGroups(true);
        },

        /**
         * Collapses all grouped rows.
         */
        collapseAllGroups : function(){
            this.toggleAllGroups(false);
        },

        // private
        interceptMouse : function(e){
            var hd = e.getTarget('.x-grid-group-hd', this.mainBody);
            if(hd){
                e.stopEvent();
                this.toggleGroup(hd.parentNode);
            }
        },

        // private
        getGroup : function(v, r, groupRenderer, rowIndex, colIndex, ds){
            var g = groupRenderer ? groupRenderer(v, {}, r, rowIndex, colIndex, ds) : String(v);
            if(g === ''){
                g = this.cm.config[colIndex].emptyGroupText || this.emptyGroupText;
            }
            return g;
        },

        // private
        getGroupField : function(){
            return this.grid.store.getGroupState();
        },

        // private
        afterRender : function(){
            Ext.grid.GroupingView.superclass.afterRender.call(this);
            if(this.grid.deferRowRender){
                this.updateGroupWidths();
            }
        },

        // private
        renderRows : function(){
            var groupField = this.getGroupField();
            var eg = !!groupField;
            // if they turned off grouping and the last grouped field is hidden
            if(this.hideGroupedColumn) {
                var colIndex = this.cm.findColumnIndex(groupField);
                if(!eg && this.lastGroupField !== undefined) {
                    this.mainBody.update('');
                    this.cm.setHidden(this.cm.findColumnIndex(this.lastGroupField), false);
                    delete this.lastGroupField;
                }else if (eg && this.lastGroupField === undefined) {
                    this.lastGroupField = groupField;
                    this.cm.setHidden(colIndex, true);
                }else if (eg && this.lastGroupField !== undefined && groupField !== this.lastGroupField) {
                    this.mainBody.update('');
                    var oldIndex = this.cm.findColumnIndex(this.lastGroupField);
                    this.cm.setHidden(oldIndex, false);
                    this.lastGroupField = groupField;
                    this.cm.setHidden(colIndex, true);
                }
            }
            return Ext.grid.GroupingView.superclass.renderRows.apply(
                        this, arguments);
        },

        // private
        doRender : function(cs, rs, ds, startRow, colCount, stripe){
            if(rs.length < 1){
                return '';
            }
            var groupField = this.getGroupField(),
                colIndex = this.cm.findColumnIndex(groupField),
                g;

            this.enableGrouping = !!groupField;

            if(!this.enableGrouping || this.isUpdating){
                return Ext.grid.GroupingView.superclass.doRender.apply(
                        this, arguments);
            }
            var gstyle = 'width:'+this.getTotalWidth()+';';

            var gidPrefix = this.grid.getGridEl().id;
            var cfg = this.cm.config[colIndex];
            var groupRenderer = cfg.groupRenderer || cfg.renderer;
            var prefix = this.showGroupName ?
                         (cfg.groupName || cfg.header)+': ' : '';

            var groups = [], curGroup, i, len, gid;
            for(i = 0, len = rs.length; i < len; i++){
                var rowIndex = startRow + i,
                    r = rs[i],
                    gvalue = r.data[groupField];

                    g = this.getGroup(gvalue, r, groupRenderer, rowIndex, colIndex, ds);
                if(!curGroup || curGroup.group != g){
                    gid = gidPrefix + '-gp-' + groupField + '-' + Ext.util.Format.htmlEncode(g);
                   	// if state is defined use it, however state is in terms of expanded
    				// so negate it, otherwise use the default.
    				var isCollapsed  = typeof this.state[gid] !== 'undefined' ? !this.state[gid] : this.startCollapsed;
    				var gcls = isCollapsed ? 'x-grid-group-collapsed' : '';
                    curGroup = {
                        group: g,
                        gvalue: gvalue,
                        text: prefix + g,
                        groupId: gid,
                        startRow: rowIndex,
                        rs: [r],
                        cls: gcls,
                        style: gstyle
                    };
                    groups.push(curGroup);
                }else{
                    curGroup.rs.push(r);
                }
                r._groupId = gid;
            }

            var buf = [];
            for(i = 0, len = groups.length; i < len; i++){
                g = groups[i];
                this.doGroupStart(buf, g, cs, ds, colCount);
                buf[buf.length] = Ext.grid.GroupingView.superclass.doRender.call(
                        this, cs, g.rs, ds, g.startRow, colCount, stripe);

                this.doGroupEnd(buf, g, cs, ds, colCount);
            }
            return buf.join('');
        },

        /**
         * Dynamically tries to determine the groupId of a specific value
         * @param {String} value
         * @return {String} The group id
         */
        getGroupId : function(value){
            var gidPrefix = this.grid.getGridEl().id;
            var groupField = this.getGroupField();
            var colIndex = this.cm.findColumnIndex(groupField);
            var cfg = this.cm.config[colIndex];
            var groupRenderer = cfg.groupRenderer || cfg.renderer;
            var gtext = this.getGroup(value, {data:{}}, groupRenderer, 0, colIndex, this.ds);
            return gidPrefix + '-gp-' + groupField + '-' + Ext.util.Format.htmlEncode(value);
        },

        // private
        doGroupStart : function(buf, g, cs, ds, colCount){
            buf[buf.length] = this.startGroup.apply(g);
        },

        // private
        doGroupEnd : function(buf, g, cs, ds, colCount){
            buf[buf.length] = this.endGroup;
        },

        // private
        getRows : function(){
            if(!this.enableGrouping){
                return Ext.grid.GroupingView.superclass.getRows.call(this);
            }
            var r = [];
            var g, gs = this.getGroups();
            for(var i = 0, len = gs.length; i < len; i++){
                g = gs[i].childNodes[1].childNodes;
                for(var j = 0, jlen = g.length; j < jlen; j++){
                    r[r.length] = g[j];
                }
            }
            return r;
        },

        // private
        updateGroupWidths : function(){
            if(!this.enableGrouping || !this.hasRows()){
                return;
            }
            var tw = Math.max(this.cm.getTotalWidth(), this.el.dom.offsetWidth-this.scrollOffset) +'px';
            var gs = this.getGroups();
            for(var i = 0, len = gs.length; i < len; i++){
                gs[i].firstChild.style.width = tw;
            }
        },

        // private
        onColumnWidthUpdated : function(col, w, tw){
            Ext.grid.GroupingView.superclass.onColumnWidthUpdated.call(this, col, w, tw);
            this.updateGroupWidths();
        },

        // private
        onAllColumnWidthsUpdated : function(ws, tw){
            Ext.grid.GroupingView.superclass.onAllColumnWidthsUpdated.call(this, ws, tw);
            this.updateGroupWidths();
        },

        // private
        onColumnHiddenUpdated : function(col, hidden, tw){
            Ext.grid.GroupingView.superclass.onColumnHiddenUpdated.call(this, col, hidden, tw);
            this.updateGroupWidths();
        },

        // private
        onLayout : function(){
            this.updateGroupWidths();
        },

        // private
        onBeforeRowSelect : function(sm, rowIndex){
            if(!this.enableGrouping){
                return;
            }
            var row = this.getRow(rowIndex);
            if(row && !row.offsetParent){
                var g = this.findGroup(row);
                this.toggleGroup(g, true);
            }
        }
    });
    // private
    Ext.grid.GroupingView.GROUP_ID = 1000;