// 该方法安卓上无效
subCanvas(){
      this.ctx.draw(true, (() => {
      	uni.canvasToTempFilePath({
      		canvasId: 'handWriting',
      		fileType: 'png',
      		quality: 1, //图片质量
      		success(res) {
      	    console.log(res.tempFilePath, 'canvas生成图片地址');
            uni.setStorageSync('signPic',res.tempFilePath)
      			uni.navigateBack({
      			  delta:1
      			})
      		}
      	});
      })());
    }

// 需将ctx.draw()里边的回调函数写成立即执行函数
subCanvas(){
      this.ctx.draw(true, (() => {......})());
    }