showImage(canvas) {
      wx.canvasToTempFilePath({
        x:0,
        y:0,
        width: 200,
        height: 200,
        destWidth:100,
        destHeight:100,
        canvas: this.data.canvas,
        quality: 1,
        success: res => {
          console.log(res);
          this.setData({
            imgShow: res.tempFilePath
          })
            })
 }

用的2D,所以没有用draw()，用了会报错