如何把上传图片时候的文件对象转换为图片的url !

 getObjectURL(file) {
      var url = null;
      if (window.createObjectURL != undefined) {
        url = window.createObjectURL(file);
      } else if (window.URL != undefined) {
        url = window.URL.createObjectURL(file);
      } else if (window.webkitURL != undefined) {
        url = window.webkitURL.createObjectURL(file);
      }
      return url;
    },