微信小程序 mqtt我想在APP.JS连接服务器，在子页面选择topic，如何拆分?

主要是APP.js无法调用子页面的方法，
库paho-mqtt-min.js

 doConnect: function () {
        var that = this;
        if (that.data.client && that.data.client.isConnected()) {
            wx.showToast({
                title: '不要重复连接'
            })
            return
        }
        var client = new Client('wss://ixxxxxxm/mqtt', that.randomString());

        client.connect({
            useSSL: true,
            cleanSession: true,
            keepAliveInterval: 5,
            onSuccess: function () {
                that.data.client = client
                that.onConnectSuccess(client)//在子页面
                //收到信息并处理//在子页面
                client.onMessageArrived = function (msg) {
                    that.onMessageArrived(msg)}
                //异常//在子页面
                client.onConnectionLost = function (responseObject) {
                   that.onConnectionLost(responseObject)
                }
            },
            onFailure(res){
                wx.showModal({
                  title:'连接失败',
                  content:'code:'+res.errorCode+',message:' +res.errorMessage,
                  confirmText:'重连',
                  success(res){
                      if(res.confirm)
                      that.doConnect()
                  }
                })
            }
        });
    },