EXTJS:如何把后台返回的json数据中的值赋给text

listeners: {
afterrender: function (o) {
var me = this;
Ext.Ajax.request({
url: "ASHX/UserSession.ashx",
method: "POST",
success: function (response, options) {
var res = Ext.decode(response.responseText);

me.down('textfield').setValue(res.UserName);
}
});
}
},

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