正常流程对蓝牙进行连接并且使用wx.setBLEMTU对mtu设置为了247，但onBLECharacteristicValueChange没有任何数据返回

  initBlue() {
    wx.showLoading({
      title: '连接蓝牙',
    })
    let thet = this
    // 先关闭蓝牙适配器
    wx.closeBluetoothAdapter({
      success: (res) => {
      },
    })
    // 打开适配器
    wx.openBluetoothAdapter({
      mode: 'central', // ios必须要带这个参数
      success: (res) => {
        console.log(res);
        // adapter建立成功，可以通过观察者模式告诉页面
        // 开启适配器状态的监听
        wx.onBluetoothAdapterStateChange((result) => {
          console.log(result);
          if (!result.available) { // 适配器不可用，可能用户关闭了手机的蓝牙
            wx.showModal({
              title: '提示',
              content: '适配器不可用',
              showCancel: false,
              success(res) {
                if (res.confirm) {
                  wx.navigateBack()
                }
              }
            })
            return
          }
        })
        console.log('连接蓝牙');
        // 连接蓝牙
        thet.joinBLE()
      },
      fail(err) { // 蓝牙适配器未开启
        if (err.errCode === BLUETOOTH_NOT_OPEN) {
          // 手机蓝牙未开启
          wx.showModal({
            title: '提示',
            content: '手机蓝牙未开启',
            showCancel: false,
            success(res) {
              if (res.confirm) {
                wx.navigateBack()
              }
            }
          })
        } else {
          // 可能未授权小程序蓝牙权限
          wx.showModal({
            title: '提示',
            content: '请授权手机蓝牙',
            showCancel: false,
            success(res) {
              if (res.confirm) {
                wx.navigateBack()
              }
            }
          })
        }
      }
    })
  },
  joinBLE() {
    wx.createBLEConnection({
      deviceId: this.data.mac,
      success: (res) => {
        // 开启连接状态的监听
        console.log(res);
        wx.showToast({
          title: '蓝牙连接成功',
          icon: 'none',
          duration: 2000
        })
        // 获取可用的服务
        wx.getBLEDeviceServices({
          deviceId: this.data.mac,
          success: (res) => {
            // 全部的服务UUID
            let services = res.services
            console.log(services, '全部的服务UUID');
            this.getBLEDeviceCharacteristics(services[1].uuid)
          },
          fail(res) {
            console.log(res);
          }
        })


      },
      fail(res) {
        // 连接失败, 关闭适配器
        wx.showModal({
          title: '提示',
          content: '蓝牙连接失败',
          showCancel: false,
          success(res) {
            if (res.confirm) {
              wx.navigateBack()
            }
          }
        })
      }
    })


  },
  getBLEDeviceCharacteristics(serviceId) {
    wx.getBLEDeviceCharacteristics({
      deviceId: this.data.mac,
      serviceId,
      success: (res) => {
        // 当前服务UUID下的特征值列表
        let characteristics = res.characteristics;
        console.log(characteristics, '当前服务UUID下的特征值列表');
        this.data.BLEData.characteristicId = characteristics[0].uuid
        this.data.BLEData.deviceId = this.data.mac
        this.data.BLEData.serviceId = serviceId
        this.setData({
          BLEData: this.data.BLEData
        })
        // 开启监听数据接收的特征值        
        wx.notifyBLECharacteristicValueChange({
          characteristicId: characteristics[0].uuid,
          state: true, // 启用 notify 功能
          deviceId: this.data.mac,
          type: "notification",
          serviceId,
          success: (res) => {
            console.log(res, "notify创建连接，发送指令")
            this.exchange_mtu()
          },
          fail: (res) => {
            // ios有可能取消匹配，也会调用这里
          },
          complete: (res) => {
            this.onble()
          }
        })
      },
      fail(res) {
        // 断开连接，关闭适配器
      }
    })


  },
  exchange_mtu() {
    wx.setBLEMTU({
      deviceId: this.data.mac,
      mtu: 247,
      success: (res) => {
        console.log(res, "mtu update success")
        // this.writeBLECharacteristicValue(this.data.BLEsetData.step1)
        this.writeBLECharacteristicValue('D1D1000232005AE8')
      },
      fail: function (res) {
        console.log(res, "mtu update failed")
      },
      complete: function (res) {
        console.log(res, "mtu update complete")
      }
    })
  },
  onble() {
    // 开启数据接收的监听器
    console.log('开启数据接收的监听器');
    function ab2hex(buffer) {
      let hexArr = Array.prototype.map.call(
        new Uint8Array(buffer),
        function (bit) {
          return ('00' + bit.toString(16)).slice(-2)
        }
      )
      return hexArr.join('');
    }
    wx.onBLECharacteristicValueChange(function (res) {
      console.log(`characteristic ${res.characteristicId} has changed, now is ${res.value}`)
      console.log(ab2hex(res.value))
    })
  },
  writeBLECharacteristicValue(value) {
    let buffer = this.hex2buffer(value)
    wx.writeBLECharacteristicValue({
      characteristicId: this.data.BLEData.characteristicId,
      deviceId: this.data.BLEData.deviceId,
      writeType: 'writeNoResponse',
      serviceId: this.data.BLEData.serviceId,
      value: buffer,
      success: (res) => {
        console.log(res);
      }, // 回调函数
      fail: (err) => {
        console.log(err);
      }, // 回调函数
      complete: (result) => {
        setTimeout(() => {


        }, 1000)
      },
    })
  },
  hex2buffer(str) {
    console.info("写入的数据====" + str);
    //字符串 转 十六进制
    var val = "";
    for (var i = 0; i < str.length; i++) {
      if (val == "")
        val = str.charCodeAt(i).toString(16);
      else
        val += "," + str.charCodeAt(i).toString(16);
    }
    //十六进制 转 ArrayBuffer
    var buffer = new Uint8Array(val.match(/[\da-f]{2}/gi).map(function (h) {
      return parseInt(h, 16)
    })).buffer;


    return buffer;
  },