Hover Effect

<!--
  示例：悬停高亮
  核心逻辑：mouseenter 给要素写 feature-state，circle 的绘制表达式读 hover 决定半径与颜色
  数据：页面内生成的 40 个站点，promoteId 把属性 id 提为要素 id
  依赖：MapLibre GL JS 4 · jsDelivr CDN · demotiles 样式
-->
<!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>悬停高亮</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/maplibre-gl@4/dist/maplibre-gl.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;color:#16181d}
  .bar{flex:0 0 auto;display:flex;gap:14px;align-items:center;padding:8px 12px;
       background:#f6f7f9;border-bottom:1px solid #e3e6ea;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  #map{flex:1 1 auto;min-height:0}
</style>
</head>
<body>
<div class="bar">
  <span>站点 <b>40</b></span>
  <span>悬停 <b id="cur">—</b></span>
  <span>feature-state 只在 GPU 侧变化，不重建源</span>
</div>
<div id="map"></div>
<script src="https://cdn.jsdelivr.net/npm/maplibre-gl@4/dist/maplibre-gl.js"></script>
<script>
(function () {
  'use strict';

  if (typeof maplibregl === 'undefined') {
    document.body.innerHTML =
      '<div style="padding:24px;color:#b42318">MapLibre 加载失败，请检查能否访问 cdn.jsdelivr.net</div>';
    return;
  }

  var cities = [
    [116.39, 39.90], [121.47, 31.23], [113.26, 23.12], [104.06, 30.57],
    [108.94, 34.34], [114.30, 30.59], [120.15, 30.28], [106.55, 29.56]
  ];
  var names = ['北站', '东站', '南站', '西站', '中站'];

  var features = [];
  for (var i = 0; i < 40; i++) {
    var c = cities[i % cities.length];
    features.push({
      type: 'Feature',
      properties: {
        id: i,
        name: names[i % 5] + ' ' + (i + 1),
        load: 20 + Math.round(Math.random() * 80)
      },
      geometry: {
        type: 'Point',
        coordinates: [c[0] + (Math.random() - 0.5) * 3.2, c[1] + (Math.random() - 0.5) * 2.4]
      }
    });
  }

  var map = new maplibregl.Map({
    container: 'map',
    style: 'https://demotiles.maplibre.org/style.json',
    center: [110, 33],
    zoom: 3.6,
    attributionControl: { compact: true, customAttribution: 'MapLibre demo tiles' }
  });

  map.addControl(new maplibregl.NavigationControl(), 'top-right');

  var curEl = document.getElementById('cur');

  map.on('load', function () {
    map.addSource('stations', {
      type: 'geojson',
      data: { type: 'FeatureCollection', features: features },
      /* 让 properties.id 直接充当 feature-state 的 id */
      promoteId: 'id'
    });

    map.addLayer({
      id: 'stations-circle',
      type: 'circle',
      source: 'stations',
      paint: {
        'circle-radius': [
          'case',
          ['boolean', ['feature-state', 'hover'], false],
          11,
          6
        ],
        'circle-color': [
          'case',
          ['boolean', ['feature-state', 'hover'], false],
          '#ff922b',
          '#2e86de'
        ],
        'circle-stroke-width': 1.5,
        'circle-stroke-color': '#ffffff'
      }
    });
  });

  var hoveredId = null;

  function setHover(id, on) {
    if (id === null || id === undefined) return;
    map.setFeatureState({ source: 'stations', id: id }, { hover: on });
  }

  map.on('mousemove', 'stations-circle', function (e) {
    var f = e.features[0];
    if (hoveredId === f.id) return;
    setHover(hoveredId, false);
    hoveredId = f.id;
    setHover(hoveredId, true);
    map.getCanvas().style.cursor = 'pointer';
    curEl.textContent = f.properties.name + ' · 负载 ' + f.properties.load + '%';
  });

  map.on('mouseleave', 'stations-circle', function () {
    setHover(hoveredId, false);
    hoveredId = null;
    map.getCanvas().style.cursor = '';
    curEl.textContent = '—';
  });

  map.on('error', function (e) {
    if (e.error) console.warn('地图错误：' + e.error.message);
  });
})();
</script>
</body>
</html>