const downloadImage = (imageFormat) =>{
    //图片下载
    let image = new Image();
    // 解决跨域 Canvas 污染问题
    image.setAttribute('crossOrigin', 'anonymous');
    image.onload = function () {
      let canvas = document.createElement('canvas');
      canvas.width = image.width;
      canvas.height = image.height;
      const context = canvas.getContext('2d');
      context && context.drawImage(image, 0, 0, image.width, image.height);
      const url = canvas.toDataURL('image/png'); //得到图片的base64编码数据
      const a = document.createElement('a'); // 生成一个a元素
      const event = new MouseEvent('click'); // 创建一个单击事件
      a.download = `平面四色图.${imageFormat}`; // 设置图片名称
      a.href = url; // 将生成的URL设置为a.href属性
      a.dispatchEvent(event); // 触发a的单击事件
    };
    image.src = this.fourColorChartUrl;
  };