Ext4.0 tree为啥不会向后台发送请求，求解

Ext4.0 tree为什么不会向后台发送请求，求解

JScript code

    var store = Ext.create('Ext.data.TreeStore', {        proxy: {            type: 'ajax',            url: 'getTreeMenuAction'        }    });    var tree = Ext.create('Ext.tree.Panel', {        title: 'Simple Tree',        width: 250,        height: 600,        store: store        //rootVisible: true    });