Tracing

<!--
  示例：OpenLayers 边界追踪绘制
  要点：Draw 的 trace 选项让绘制线沿现有面要素边界吸附前行
  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>边界追踪绘制</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;flex-wrap:wrap;font-size:12px;color:#58606e}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar select,.bar input[type=range]{font-size:12px}
  .bar .val{font-family:ui-monospace,Consolas,monospace;color:#1f5fd0;min-width:44px}
  .bar .sep{width:1px;height:16px;background:#d8dce2}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>绘制类型
    <select id="type">
      <option value="Polygon">Polygon</option>
      <option value="LineString">LineString</option>
      <option value="None">关闭</option>
    </select>
  </label>
  <button id="clear" type="button">清空</button>
  <span class="sep"></span>
  <span>勾边时靠近红色地块边界</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').textContent = 'OpenLayers 加载失败，请检查能否访问 cdn.jsdelivr.net';
    return;
  }


  var center = ol.proj.fromLonLat([116.391, 39.907]);

  var parcels = [];
  for (var row = 0; row < 4; ++row) {
    for (var col = 0; col < 4; ++col) {
      var lon = 116.360 + col * 0.012;
      var lat = 39.885 + row * 0.012;
      var ring = [
        ol.proj.fromLonLat([lon, lat]),
        ol.proj.fromLonLat([lon + 0.010, lat]),
        ol.proj.fromLonLat([lon + 0.010, lat + 0.010]),
        ol.proj.fromLonLat([lon, lat + 0.010]),
        ol.proj.fromLonLat([lon, lat])
      ];
      parcels.push(new ol.Feature(new ol.geom.Polygon([ring])));
    }
  }

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

  var baseLayer = new ol.layer.Vector({
    source: baseSource,
    style: new ol.style.Style({
      fill: new ol.style.Fill({color: 'rgba(220, 60, 60, 0.12)'}),
      stroke: new ol.style.Stroke({color: 'rgba(200, 40, 40, 0.85)', width: 1})
    })
  });

  var drawSource = new ol.source.Vector();
  var drawLayer = new ol.layer.Vector({
    source: drawSource,
    style: new ol.style.Style({
      fill: new ol.style.Fill({color: 'rgba(70, 200, 90, 0.22)'}),
      stroke: new ol.style.Stroke({color: 'rgba(30, 150, 60, 0.95)', width: 2})
    })
  });

  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'}), opacity: 0.75}),
      baseLayer,
      drawLayer
    ],
    view: new ol.View({center: center, zoom: 16})
  });

  var snap = new ol.interaction.Snap({source: baseSource});
  var drawInteraction = null;

  function addInteraction() {
    var value = document.getElementById('type').value;
    if (drawInteraction) {
      map.removeInteraction(drawInteraction);
      drawInteraction = null;
    }
    if (value !== 'None') {
      drawInteraction = new ol.interaction.Draw({
        type: value,
        source: drawSource,
        trace: true,
        traceSource: baseSource,
        style: new ol.style.Style({
          stroke: new ol.style.Stroke({color: 'rgba(255, 190, 0, 0.95)', width: 2}),
          fill: new ol.style.Fill({color: 'rgba(255, 190, 0, 0.18)'}),
          image: new ol.style.Circle({
            radius: 5,
            fill: new ol.style.Fill({color: 'rgba(255, 190, 0, 0.8)'})
          })
        })
      });
      map.addInteraction(drawInteraction);
    }
  }

  document.getElementById('type').addEventListener('change', function () {
    addInteraction();
  });

  document.getElementById('clear').addEventListener('click', function () {
    drawSource.clear();
  });

  map.addInteraction(snap);
  addInteraction();

})();
</script>
</body>
</html>