Combox中，数据无法显示，使用store包含reader，proxy，求指导！

/* var cities = [
[1, "长沙市"],
[2, "株洲市"],
[3, "湘潭市"],
[4, "邵阳市"]
];
var proxy = new Ext.data.MemoryProxy(cities);
var reader = new Ext.data.ArrayReader({}, [
{name: "cid", type: "int", mapping: 0},
{name: "cname", type: "string", mapping: 1}
]);
var store = new Ext.data.Store({
proxy: proxy,
reader: reader
});
store.load();
var combobox = new Ext.form.ComboBox({
renderTo: Ext.getBody(),
triggerAction: "all",
store: store,
displayField: "cname",
valueField: "cid",
mode: "local",
emptyText: "请选择湖南城市"
});
var btn = new Ext.Button({
text: "列表框的值",
renderTo: Ext.getBody(),
handler: function(){
Ext.Msg.alert("值", "实际值：" + combobox.getValue() + "；显示值：" + combobox.getRawValue())
}
})*/