Snap Draw

<!--
  示例：绘制捕捉
  要点：Snap 与 Draw 同时加入地图，pixelTolerance 控制吸附距离，可 setActive 开关
  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>绘制捕捉</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:12px;align-items:center;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .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>单击开始绘制折线，双击结束</span>
  <label><input type="checkbox" id="snapOn" checked> 启用捕捉</label>
  <span id="info">已绘 0 条</span>
  <button id="clear">清除绘制</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 fromLonLat = ol.proj.fromLonLat;

  /* 已有要素：两条参考线，作为捕捉目标 */
  var baseLines = [
    [[116.372, 39.906], [116.396, 39.914], [116.420, 39.906]],
    [[116.378, 39.922], [116.400, 39.918], [116.424, 39.924]]
  ].map(function (coords) {
    return new ol.Feature({
      geometry: new ol.geom.LineString(coords.map(function (p) {
        return fromLonLat(p);
      }))
    });
  });

  var baseSource = new ol.source.Vector({ features: baseLines });
  var drawSource = new ol.source.Vector();

  var baseLayer = new ol.layer.Vector({
    source: baseSource,
    style: new ol.style.Style({
      stroke: new ol.style.Stroke({ color: '#8a919d', width: 4 }),
      image: new ol.style.Circle({
        radius: 5,
        fill: new ol.style.Fill({ color: '#8a919d' })
      })
    })
  });

  var drawLayer = new ol.layer.Vector({
    source: drawSource,
    style: new ol.style.Style({
      stroke: new ol.style.Stroke({ color: '#c2410c', width: 3 })
    })
  });

  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'}) }),
      baseLayer,
      drawLayer
    ],
    view: new ol.View({
      center: fromLonLat([116.398, 39.915]),
      zoom: 14
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  var draw = new ol.interaction.Draw({
    source: drawSource,
    type: 'LineString'
  });

  var snap = new ol.interaction.Snap({
    source: baseSource,
    pixelTolerance: 12
  });

  map.addInteraction(draw);
  map.addInteraction(snap);

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

  draw.on('drawend', function () {
    info.textContent = '已绘 ' + drawSource.getFeatures().length + ' 条';
  });

  document.getElementById('snapOn').addEventListener('change', function () {
    snap.setActive(this.checked);
  });

  document.getElementById('clear').addEventListener('click', function () {
    drawSource.clear();
    info.textContent = '已绘 0 条';
  });
})();
</script>
</body>
</html>