官网demo地址：

Layer Spy

 这篇实现了鼠标跟随望远镜效果，鼠标移动时绘制一个圆形的剪裁区剪裁上层图层。

container.addEventListener("mousemove", function (event) {
      mousePosition = map.getEventPixel(event);
      map.render();
    });

    container.addEventListener("mouseout", function () {
      mousePosition = null;
      map.render();
    });

通过map.render()触发prerender，postrender两个函数来完成裁剪，其原理和上篇的卷帘效果一样。

上篇地址： 

 完整代码：

<template>
  <div class="box">
    <h1>Layer Spy</h1>
    <div id="map" class="map"></div>
  </div>
</template>

<script>
import StadiaMaps from "ol/source/StadiaMaps.js";
import Map from "ol/Map.js";
import TileLayer from "ol/layer/Tile.js";
import View from "ol/View.js";
import XYZ from "ol/source/XYZ.js";
import { fromLonLat } from "ol/proj.js";
import { getRenderPixel } from "ol/render.js";
export default {
  name: "",
  components: {},
  data() {
    return {
      map: null,
    };
  },
  computed: {},
  created() {},
  mounted() {
    const container = document.getElementById("map");
    const layer = new TileLayer({
      source: new StadiaMaps({
        layer: "stamen_terrain_background",
      }),
      zIndex: 2,
    });

    const map = new Map({
      layers: [
        layer,
        new TileLayer({
          source: new StadiaMaps({
            layer: "outdoors",
          }),
        }),
      ],
      target: "map",
      view: new View({
        center: [0, 0],
        zoom: 2,
        projection: "EPSG:4326",
      }),
    });

    let radius = 75;
    document.addEventListener("keydown", function (evt) {
      if (evt.key === "ArrowUp") {
        radius = Math.min(radius + 5, 150);
        map.render();
        evt.preventDefault();
      } else if (evt.key === "ArrowDown") {
        radius = Math.max(radius - 5, 25);
        map.render();
        evt.preventDefault();
      }
    });

    let mousePosition = null;

    container.addEventListener("mousemove", function (event) {
      mousePosition = map.getEventPixel(event);
      map.render();
    });

    container.addEventListener("mouseout", function () {
      mousePosition = null;
      map.render();
    });

    layer.on("prerender", function (event) {
      const ctx = event.context;
      ctx.save();
      ctx.beginPath();
      if (mousePosition) {
        const pixel = getRenderPixel(event, mousePosition);
        const offset = getRenderPixel(event, [
          mousePosition[0] + radius,
          mousePosition[1],
        ]);
        const canvasRadius = Math.sqrt(
          Math.pow(offset[0] - pixel[0], 2) + Math.pow(offset[1] - pixel[1], 2)
        );
        ctx.arc(pixel[0], pixel[1], canvasRadius, 0, 2 * Math.PI);
        ctx.lineWidth = (5 * canvasRadius) / radius;
        ctx.strokeStyle = "rgba(0,0,0,0.5)";
        ctx.stroke();
      }
      ctx.clip();
    });


    layer.on("postrender", function (event) {
      const ctx = event.context;
      ctx.restore();
    });
  },
  methods: {},
};
</script>

<style lang="scss" scoped>
#map {
  width: 100%;
  height: 500px;
}
.box {
  height: 100%;
}

</style>