Draw Circle

<!--
  示例：绘制圆
  要点：Draw 的 type 为 Circle，getRadius 取半径并换算周长与面积
  依赖：OpenLayers 10.3.1 · jsDelivr CDN
-->
<!DOCTYPE html>
<html lang="zh-CN">
<head>
<meta name="robots" content="noindex,follow">
<meta charset="utf-8">
<meta name="viewport" content="width=device-width, initial-scale=1">
<title>绘制圆</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/ol@10.3.1/ol.css">
<style>
  html,body{height:100%;margin:0}
  body{display:flex;flex-direction:column;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:10px;align-items:center;font-size:12px;color:#58606e}
  .bar button{padding:4px 10px;border:1px solid #cfd4dc;border-radius:4px;
              background:#fff;cursor:pointer;font-size:12px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span>按住并拖动绘制服务范围</span>
  <span id="info">—</span>
  <button id="clear">清空圆</button>
</div>
<div id="map"></div>
<script src="https://cdn.jsdelivr.net/npm/ol@10.3.1/dist/ol.js"></script>
<script>
(function () {
  'use strict';

  if (typeof ol === 'undefined') {
    document.getElementById('map').innerHTML =
      '<div style="padding:20px;color:#b42318">OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  var source = new ol.source.Vector();

  var layer = new ol.layer.Vector({
    source: source,
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(31, 95, 208, 0.15)' }),
      stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2, lineDash: [6, 4] })
    })
  });

  var draw = new ol.interaction.Draw({
    source: source,
    type: 'Circle',
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(31, 95, 208, 0.12)' }),
      stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2 }),
      image: new ol.style.Circle({
        radius: 5,
        fill: new ol.style.Fill({ color: '#1f5fd0' })
      })
    })
  });

  var map = new ol.Map({
    target: 'map',
    layers: [new ol.layer.Tile({ source: new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'}) }), layer],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.3974, 39.9093]),
      zoom: 13
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  map.addInteraction(draw);

  var info = document.getElementById('info');

  draw.on('drawend', function (evt) {
    var geom = evt.feature.getGeometry();
    var r = geom.getRadius();
    var circumference = 2 * Math.PI * r / 1000;
    var area = Math.PI * r * r / 1e6;

    info.textContent = '半径 ' + r.toFixed(0) + ' m · 周长 ' +
      circumference.toFixed(2) + ' km · 面积 ' + area.toFixed(2) + ' km²';
  });

  document.getElementById('clear').addEventListener('click', function () {
    source.clear();
    info.textContent = '—';
  });
})();
</script>
</body>
</html>