1、在@/utils下新增一个名为waterMark.js的脚本

具体水印样式可以在代码里自行调节style

参数 - 水印内容, 加水印的容器, 是否显示时间

let watermark = {};

function getCurrentDateTime() {
  const now = new Date();

  const year = now.getFullYear();
  const month = String(now.getMonth() + 1).padStart(2, "0");
  const day = String(now.getDate()).padStart(2, "0");

  const hours = String(now.getHours()).padStart(2, "0");
  const minutes = String(now.getMinutes()).padStart(2, "0");
  const seconds = String(now.getSeconds()).padStart(2, "0");

  const dateTime = `${year}-${month}-${day}`;
  return dateTime;
}

const currentDateTime = getCurrentDateTime();
let setWatermark = (text, sourceBody, isShowTime) => {
  if (isShowTime) {
    let time = getCurrentDateTime();
    text = `${text}\n${time}`;
  }
  //   console.log("水印文本", text);
  let id =
    Math.random() * 10000 +
    "-" +
    Math.random() * 10000 +
    "/" +
    Math.random() * 10000;

  if (document.getElementById(id) !== null) {
    document.body.removeChild(document.getElementById(id));
  }

  let can = document.createElement("canvas");
  can.width = 180;
  can.height = 75;

  let cans = can.getContext("2d");
  cans.rotate((-20 * Math.PI) / 180);
  cans.font = "15px Vedana";
  cans.fillStyle = "rgba(0, 0, 0, .5)";
  cans.textAlign = "left";
  cans.textBaseline = "Middle";
  //   let textLines = text.split("\n");
  //   textLines.forEach((line, index) => {
  //     cans.fillText(line, can.width / 20, can.height * (index + 1)); // 调整行高
  //   });
  cans.fillText(text, can.width / 20, can.height);

  let water_div = document.createElement("div");
  water_div.id = id;
  water_div.style.pointerEvents = "none";
  water_div.style.background =
    "url(" + can.toDataURL("image/png") + ") left top repeat";
  water_div.style.zIndex = "100000";
  water_div.style.whiteSpace = "pre";
  water_div.style.opacity = "0.5";
  if (sourceBody) {
    water_div.style.width = "100%";
    water_div.style.height = "100%";
    water_div.style.position = "absolute";
    water_div.style.top = "3px";
    water_div.style.left = "0px";
    sourceBody.appendChild(water_div);
  } else {
    water_div.style.top = "3px";
    water_div.style.left = "0px";
    water_div.style.position = "fixed";
    water_div.style.width = document.documentElement.clientWidth + "px";
    water_div.style.height = document.documentElement.clientHeight + "px";
    document.body.appendChild(water_div);
  }

  return id;
};

/**
 *  该方法只允许调用一次
 *  @param:
 *  @text == 水印内容
 *  @sourceBody == 水印添加在哪里，不传就是body
 *  @isShowTime == 是否显示时间
 * */
watermark.set = (text, sourceBody, isShowTime) => {
  let id = setWatermark(text, sourceBody, isShowTime);
  setInterval(() => {
    if (document.getElementById(id) === null) {
      id = setWatermark(text, sourceBody, isShowTime);
    }
  }, 1000);
  window.onresize = () => {
    setWatermark(text, sourceBody, isShowTime);
  };
};

export default watermark;

2、在main.js下进行全局注册

import watermark from '@/utils/waterMark.js'
Vue.prototype.$watermark = watermark

3、在vue组件中使用

<el-dialog :visible.sync="openFile">
    <div id="fileDialog" ref="fileDialog" style="height:100%;width:100%;"></div>
</el-dialog>

handleOpenDialog(){
    this.openFile = true; //打开对话框
    this.$nextTick(()=>{
        let nickName = "admin";
        // 参数 - 水印内容, 加水印的容器, 是否显示时间
        this.$watermark.set(nickName, this.$refs.fileDialog, true);
    })
}

4、效果