Cannot use the same canvas during multiple render() operations. Use different canvas or ensure previ

记录一次使用pdf.js插件触发的canvas渲染类bug

业务场景：点击多个tab标签预览对应的pdf

修改前代码：

//html
<div class="canvas-scale" ref="canvasScale">
	<canvas
	   class="contract-canvas"
	   :ref="`contractCanvas${index + 1}`"
	   v-for="(_, index) in pageArray"
	   :key="index"
	 ></canvas>
</div>
//js
initPdf(url) {
     this.pdfLoading = true;
     // eslint-disable-next-line no-undef
     const loadingTask = pdfjsLib.getDocument(url);
     loadingTask.promise.then(
       async (pdf) => {
         this.allPageNum = pdf.numPages;
         this.pageNum = 1;
         this.pageArray = Array(pdf.numPages).fill({});
         this.pdfObj = pdf;
         await this.forRenderPage();
         this.pdfLoading = false;
       },
       (reason) => {
         this.pdfLoading = false;
         console.error(reason);
       },
     );
 };
async forRenderPage() {
   for (let index = 1; index < this.allPageNum + 1; index++) {
     await this.renderPage(index);
   }
 },
async renderPage(pageNumber) {
      const page = await this.pdfObj.getPage(pageNumber);
      const scale = 2;
      const viewport = page.getViewport({ scale });

      // 准备画布使用PDF页面尺寸
      const canvas = this.$refs[`contractCanvas${pageNumber}`][0];
      const context = canvas.getContext('2d');
      canvas.height = viewport.height;
      canvas.width = viewport.width;

      // 渲染PDF页到画布上下文
      const renderContext = {
        canvasContext: context,
        viewport,
      };
      const renderTask = page.render(renderContext);
      renderTask.promise.then(() => {});
    },

好了，大功告成！

但是第二天就被测试小姐姐喊去
测试小姐姐：为什么这里有页pdf是空白的呀…？
我(一脸懵逼)：这种不是后端返回的pdf就是长这样么（炒鸡自信）？
然后测试小姐姐一顿输出操作拉满，终于找到了后端接口返回的pdf，浏览器打开一看，嗯？我又懵了，果然不一样！！
go！来活了
本地调试发现控制台报以下错误~~
Uncaught (in promise) Error: Cannot use the same canvas during multiple render() operations. Use different canvas or ensure previous operations were cancelled or completed.

遇到问题不要急，先百度…然鹅，一天过去了…各种修改都无用…
最后跟大佬一起看发现原因是canvas重复渲染了，原来pdf.js里面for循环跟我的v-for渲染重复了
决定修改如下代码：

//html
//key值修改
<canvas
  class="contract-canvas"
   :ref="`contractCanvas${index + 1}`"
   v-for="(_, index) in pageArray"
   :key="`a${_.index}`"
 ></canvas>
 >//js 
 >//nextSnowflakeId是snowflakeGenerator插件封装的方法，可以随机生成唯一id。
 >this.pageArray = Array(pdf.numPages).fill({}).map(() => ({ index: nextSnowflakeId() }));

然后再运行，好啦！！不会再出现了，落班~~