ExtJS Combobox 如何设置默认和取值问题

学习ExtJs，如何给combobox选中某一行， 取值问题。

///following is for stores combobox

        var readerLocation = new Ext.data.WCFJsonReader({

            root: 'data', id: 'Store', totalProperty: 'totalCount',

            fields: [

                    { name: 'Store', type: 'int' },

                    { name: 'StoreName', type: 'string' }

                     /*0:'All', '1':'ShangHai', '2':'BeiJing' */ 

                ]

        });

        var storeStore = new Ext.data.Store({

            proxy: new Ext.data.WCFHttpProxy({

                url: '/*.svc/GetStoresList'

            }),

            reader: readerLocation,

            remoteSort: true,

            listeners: {

                load: function() {

                    comboLocation.setValue(comboLocation.getValue());

                }

            }

        });

        var comboLocation = new Ext.form.ComboBox({

            store: storeStore,

            displayField: 'StoreName',

            valueField: 'Store',

            id: 'comboboxStore',

            typeAhead: true,

            mode: 'local',

            triggerAction: 'all',

            selectOnFocus: true,

            value: '',

            width: 180,

            applyTo: 'ddlStore'

        });

        storeStore.load();

        storeStore.on("load", onstoreStoreLoad, storeStore, true);

        function onstoreStoreLoad() {

            if (storeStore.getTotalCount() > 0)

                comboLocation.setValue(storeStore.getAt(0).data.Store);

        }

        /*** above is for location ***/ 

点击按钮 取值like：

Ext.get("btnFilter").on("click", displayGrid); 

function displayGrid() {

            var storeFilter = comboLocation.getValue();

});