Extjs中的combobox的动态加载问题

var RecordDef = Ext.data.Record.create([   
            {name: 'sn'},{name: 'groupName'}                  
        ]);
        var groupStore=new Ext.data.Store({   
            //设定读取的地址
            proxy: new Ext.data.HttpProxy({url: 'getGroupStore.action'}),   
            //设定读取的格式   
            reader: new Ext.data.JsonReader({   
                id:"sn",totalProperty:'totalCount',root:'groupList'
            }, RecordDef),   
            remoteSort: true  
        });
        var groupCombo=new Ext.form.ComboBox({
            selectOnFocus:true,
            valueField:'sn',
            hiddenName:'groupCombo',
            displayField:'groupName',
            fieldLabel: '会员权限组',
            blankText:'请选择权限组',
            emptyText:'请选择权限组',
            editable:false,
            anchor:'90%',
            forceSelection:true,
            triggerAction:'all',
            allowBlank:false,
            store:groupStore,
            typeAhead: true
        });