Lazy Source

<!--
  示例：延迟设置数据源 — OpenLayers lazy-source
  核心逻辑：图层先建好但 source 为空，点击按钮后才 setSource 并渲染数据
  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>延迟设置数据源</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;flex-wrap:wrap}
  .bar button{font:inherit;padding:3px 9px;border:1px solid #ccd2da;background:#fff;
       border-radius:3px;cursor:pointer;color:#2b3140}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <button id="load">加载数据</button>
  <button id="unload">移除数据源</button>
  <span id="info">图层已创建，尚无数据源</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 infoEl = document.getElementById('info');

  /* 图层先挂上去，此时 source 为 null，不会绘制任何内容 */
  var layer = new ol.layer.Vector({
    style: new ol.style.Style({
      fill: new ol.style.Fill({ color: 'rgba(31,95,208,0.16)' }),
      stroke: new ol.style.Stroke({ color: '#1f5fd0', width: 2 }),
      image: new ol.style.Circle({
        radius: 5,
        fill: new ol.style.Fill({ color: '#1f5fd0' }),
        stroke: new ol.style.Stroke({ color: '#fff', 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'}) }),
      layer
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.391, 39.907]),
      zoom: 12,
      minZoom: 2
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  var payload = {
    type: 'FeatureCollection',
    features: [
      {
        type: 'Feature',
        properties: { name: '东城片区' },
        geometry: {
          type: 'Polygon',
          coordinates: [[
            [116.395, 39.900], [116.432, 39.903], [116.436, 39.921],
            [116.400, 39.918], [116.395, 39.900]
          ]]
        }
      },
      {
        type: 'Feature',
        properties: { name: '核心点' },
        geometry: { type: 'Point', coordinates: [116.391263, 39.907359] }
      }
    ]
  };

  function buildSource() {
    return new ol.source.Vector({
      features: new ol.format.GeoJSON().readFeatures(payload, {
        dataProjection: 'EPSG:4326',
        featureProjection: 'EPSG:3857'
      })
    });
  }

  document.getElementById('load').addEventListener('click', function () {
    infoEl.textContent = '正在读取内联 GeoJSON…';
    setTimeout(function () {
      layer.setSource(buildSource());
      infoEl.textContent = '已挂载数据源，要素 ' +
        layer.getSource().getFeatures().length + ' 个';
    }, 300);
  });

  document.getElementById('unload').addEventListener('click', function () {
    layer.setSource(null);
    infoEl.textContent = '数据源已移除，图层仍在图层列表中';
  });
})();
</script>
</body>
</html>