原文链接及内容

原文链接：https://openlayers.org/en/latest/examples/d3.html

运行界面

该示例加载了TopoJSON几何图形，并使用d3 (d3.geo.path)将这些几何图形渲染为SVG元素。

TopoJSON库地址：https://github.com/topojson/topojson
d3库地址：https://d3js.org/

main.js代码如下：

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import Map from 'ol/Map.js';
import Stamen from 'ol/source/Stamen.js';
import View from 'ol/View.js';
import {Layer, Tile as TileLayer} from 'ol/layer.js';
import {fromLonLat, toLonLat} from 'ol/proj.js';
import {getCenter, getWidth} from 'ol/extent.js';

class CanvasLayer extends Layer {
  constructor(options) {
    super(options);

    this.features = options.features;

    this.svg = d3
      .select(document.createElement('div'))
      .append('svg')
      .style('position', 'absolute');

    this.svg.append('path').datum(this.features).attr('class', 'boundary');
  }

  getSourceState() {
    return 'ready';
  }

  render(frameState) {
    const width = frameState.size[0];
    const height = frameState.size[1];
    const projection = frameState.viewState.projection;
    const d3Projection = d3.geoMercator().scale(1).translate([0, 0]);
    let d3Path = d3.geoPath().projection(d3Projection);

    const pixelBounds = d3Path.bounds(this.features);
    const pixelBoundsWidth = pixelBounds[1][0] - pixelBounds[0][0];
    const pixelBoundsHeight = pixelBounds[1][1] - pixelBounds[0][1];

    const geoBounds = d3.geoBounds(this.features);
    const geoBoundsLeftBottom = fromLonLat(geoBounds[0], projection);
    const geoBoundsRightTop = fromLonLat(geoBounds[1], projection);
    let geoBoundsWidth = geoBoundsRightTop[0] - geoBoundsLeftBottom[0];
    if (geoBoundsWidth < 0) {
      geoBoundsWidth += getWidth(projection.getExtent());
    }
    const geoBoundsHeight = geoBoundsRightTop[1] - geoBoundsLeftBottom[1];

    const widthResolution = geoBoundsWidth / pixelBoundsWidth;
    const heightResolution = geoBoundsHeight / pixelBoundsHeight;
    const r = Math.max(widthResolution, heightResolution);
    const scale = r / frameState.viewState.resolution;

    const center = toLonLat(getCenter(frameState.extent), projection);
    const angle = (-frameState.viewState.rotation * 180) / Math.PI;

    d3Projection
      .scale(scale)
      .center(center)
      .translate([width / 2, height / 2])
      .angle(angle);

    d3Path = d3Path.projection(d3Projection);
    d3Path(this.features);

    this.svg.attr('width', width);
    this.svg.attr('height', height);

    this.svg.select('path').attr('d', d3Path);

    return this.svg.node();
  }
}

const map = new Map({
  layers: [
    new TileLayer({
      source: new Stamen({
        layer: 'watercolor',
      }),
    }),
  ],
  target: 'map',
  view: new View({
    center: fromLonLat([-97, 38]),
    zoom: 4,
  }),
});

/**
 * Load the topojson data and create an ol/layer/Image for that data.
 */
d3.json('data/topojson/us.json').then(function (us) {
  const layer = new CanvasLayer({
    features: topojson.feature(us, us.objects.counties),
  });

  map.addLayer(layer);
});

界面布局文件index.html代码如下：

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<!DOCTYPE html>
<html lang="en">
  <head>
    <meta charset="UTF-8">
    <title>d3 Integration</title>
    <link rel="stylesheet" href="node_modules/ol/ol.css">
    <style>
      .map {
        width: 100%;
        height: 400px;
      }
      path.boundary {
        fill: none;
        stroke: #777;
      }
    </style>
  </head>
  <body>
    <div id="map" class="map"></div>
    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->
    <script src="https://cdn.jsdelivr.net/npm/elm-pep@1.0.6/dist/elm-pep.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/d3@7.4.4/dist/d3.min.js"></script>
    <script src="https://cdn.jsdelivr.net/npm/topojson@3.0.2/dist/topojson.js"></script>
    <script type="module" src="main.js"></script>
  </body>
</html>