Synthetic Points

<!--
  示例：OpenLayers 合成点要素
  要点：两万个随机点 + 最近点吸附预览，postrender 事件里叠加临时几何
  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>合成点要素</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">
  <span>移动鼠标查看最近点</span>
  <span class="sep"></span>
  <span id="info">—</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 count = 20000;
  var features = new Array(count);
  for (var i = 0; i < count; ++i) {
    var lon = 116.391 + (Math.random() - 0.5) * 0.55;
    var lat = 39.907 + (Math.random() - 0.5) * 0.42;
    features[i] = new ol.Feature({
      geometry: new ol.geom.Point(ol.proj.fromLonLat([lon, lat])),
      size: i % 2 ? 10 : 20
    });
  }

  var styles = {
    '10': new ol.style.Style({
      image: new ol.style.Circle({
        radius: 3,
        fill: new ol.style.Fill({color: '#51606f'}),
        stroke: new ol.style.Stroke({color: '#9fd08a', width: 1})
      })
    }),
    '20': new ol.style.Style({
      image: new ol.style.Circle({
        radius: 6,
        fill: new ol.style.Fill({color: '#51606f'}),
        stroke: new ol.style.Stroke({color: '#9fd08a', width: 1})
      })
    })
  };

  var vectorSource = new ol.source.Vector({
    features: features,
    wrapX: false
  });

  var vector = new ol.layer.Vector({
    source: vectorSource,
    style: function (feature) {
      return styles[feature.get('size')];
    }
  });

  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: 0.85}),
      vector
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.391, 39.907]),
      zoom: 12
    })
  });

  var point = null;
  var line = null;

  function displaySnap(coordinate) {
    var closestFeature = vectorSource.getClosestFeatureToCoordinate(coordinate);
    if (!closestFeature) {
      point = null;
      line = null;
      document.getElementById('info').textContent = '—';
    } else {
      var closest = closestFeature.getGeometry().getClosestPoint(coordinate);
      if (!point) {
        point = new ol.geom.Point(closest);
      } else {
        point.setCoordinates(closest);
      }
      if (!line) {
        line = new ol.geom.LineString([coordinate, closest]);
      } else {
        line.setCoordinates([coordinate, closest]);
      }
      var lonlat = ol.proj.toLonLat(closest);
      document.getElementById('info').textContent =
        lonlat[0].toFixed(5) + ', ' + lonlat[1].toFixed(5);
    }
    map.render();
  }

  map.on('pointermove', function (evt) {
    if (evt.dragging) {
      return;
    }
    displaySnap(map.getEventCoordinate(evt.originalEvent));
    map.getTargetElement().style.cursor =
      map.hasFeatureAtPixel(evt.pixel) ? 'pointer' : '';
  });

  map.on('click', function (evt) {
    displaySnap(evt.coordinate);
  });

  var snapStyle = new ol.style.Style({
    stroke: new ol.style.Stroke({color: 'rgba(255, 196, 0, 0.95)', width: 2}),
    image: new ol.style.Circle({
      radius: 9,
      stroke: new ol.style.Stroke({color: 'rgba(255, 196, 0, 0.95)', width: 2})
    })
  });

  vector.on('postrender', function (evt) {
    var vectorContext = ol.render.getVectorContext(evt);
    vectorContext.setStyle(snapStyle);
    if (point) {
      vectorContext.drawGeometry(point);
    }
    if (line) {
      vectorContext.drawGeometry(line);
    }
  });

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