Draw Modify Trace Snap

<!--
  说明：绘制、追踪与吸附
  核心逻辑：Draw + Modify(trace) + Snap 共用同一矢量源，捕捉开关控制 snap 交互
  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、追踪与吸附</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;flex-wrap:wrap}
  .bar button{font:inherit;padding:4px 10px;border:1px solid #c9d0da;border-radius:5px;
              background:#fff;color:#1f2a37;cursor:pointer}
  #info{margin-left:auto}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label><input type="checkbox" id="snap" checked> 吸附</label>
  <label><input type="checkbox" id="trace" checked> 追踪</label>
  <button id="clear">清空绘制</button>
  <span id="info">沿灰色边界画线试试</span>
</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;

  var referenceRing = [
    [116.32, 39.88], [116.46, 39.88], [116.46, 39.98], [116.32, 39.98], [116.32, 39.88]
  ].map(function (p) { return fromLonLat(p); });

  var refFeature = new ol.Feature({ geometry: new ol.geom.Polygon([referenceRing]) });
  var refSource = new ol.source.Vector({ features: [refFeature] });

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

  var refLayer = new ol.layer.Vector({
    source: refSource,
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(123,135,148,0.10)' }),
      stroke: new ol.style.Stroke({ color: '#8b96a3', width: 2, lineDash: [8, 6] })
    })
  });

  var drawLayer = new ol.layer.Vector({
    source: drawSource,
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(31,95,208,0.14)' }),
      stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2.5 })
    })
  });

  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'}) }),
      refLayer,
      drawLayer
    ],
    view: new ol.View({ center: fromLonLat([116.39, 39.93]), zoom: 12, minZoom: 2, maxZoom: 19 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var draw = new ol.interaction.Draw({ source: drawSource, type: 'LineString' });
  var modify = new ol.interaction.Modify({ source: drawSource, trace: true });
  var snap = new ol.interaction.Snap({
    source: refSource,
    pixelTolerance: 12
  });

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

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

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

  document.getElementById('snap').addEventListener('change', function () {
    snap.setActive(this.checked);
    info.textContent = this.checked ? '吸附已开启（12 px）' : '吸附已关闭';
  });

  document.getElementById('trace').addEventListener('change', function () {
    map.removeInteraction(modify);
    modify = new ol.interaction.Modify({ source: drawSource, trace: this.checked });
    map.addInteraction(modify);
    info.textContent = this.checked ? '追踪已开启' : '追踪已关闭';
  });

  document.getElementById('clear').addEventListener('click', function () {
    drawSource.clear();
    info.textContent = '绘制已清空';
  });
})();
</script>
</body>
</html>