EXT的ComboBox怎么动态加载数据

EXT的ComboBox如何动态加载数据？
如题，如下代码，

JScript code

items: [{    xtype: "combo",    name: "site_id",    triggerAction: 'all',    lazyRender: false,    selectOnFocus: true,    forceSelection: true,    editable: false,    allowBlank: false,    mode: 'local',    fieldLabel: '请选择',    width: 180,    store:new Ext.data.Store({           proxy: new Ext.data.HttpProxy({            url: '/mis/expert?method=site_list'        }),           reader: new Ext.data.JsonReader({            root: 'site_info'        },[            { name: 'site_id', mapping: 'site_id' },            { name: 'site_url', mapping: 'site_url'}        ])    }),    valueField: 'sie_id',    displayField: 'site_url'}]