requestTask.onHeadersReceived 怎么使用

求大神帮忙：

项目需求：要监听 Response Headers 中的信息

报错：Cannot read property ‘onHeadersReceived’ of undefined

const requestTask = wx.request({   url: rootDocment + $api.home.porList,   data: {},   method: 'get',   header: {     'Content-Type': 'application/x-www-form-urlencoded',     Authorization: token   },   success(res) {     console.log(res.data)   } }) requestTask.onHeadersReceived((res) => {   console.log('onHeadersReceived: ', res)   console.log('获取到了header') })