javascript - iView中 tree 树控件 的方法调用有错

官网的说明
Tree methods

方法名	说明	参数
getCheckedNodes	获取被勾选的节点	无
getSelectedNodes	获取被选中的节点	无

自己撸的代码----------------------------------

<button @click='getCheckedNodes'>获取被选中的节点</button>
<button @click="getSelectedNodes">获取被勾选的节点</button>

自己撸的代码---------------------------

getCheckedNodes(){
    console.log(this.$refs.Tree.getCheckedNodes());
},
getSelectedNodes(){
    console.log(this.$refs.tree.getSelectedNodes);
}

报错

App.vue?2573:105 Uncaught TypeError: Cannot read property 'getCheckedNodes' of undefined

at VueComponent.getCheckedNodes (eval at <anonymous> (app.js:967), <anonymous>:92:40)
at boundFn (eval at <anonymous> (app.js:723), <anonymous>:125:14)
at HTMLButtonElement.invoker (eval at <anonymous> (app.js:723), <anonymous>:1659:18)