onReady: function () {

  console.log('onReady')

  let that = this;

  wx.canvasToTempFilePath({

    canvasId: 'qrcode',

      success: function (res) {

        that.setData({shareImg: res.tempFilePath})

      },

      fail: function (res) {

          wx.showToast({title: '图片生成失败'});

          console.log("图片生成失败error", res)

      }

  },that)

},

电脑上没问题，手机上就会显示canvasToTempFilePath: fail canvas is empty"，求大佬指点！