Box Select

<!--
  示例：框选要素
  核心逻辑：Shift 拖拽触发 boxzoomend，用返回的 bounds 查询要素并写 feature-state
  数据：页面内生成的 120 个监测点，带编号与数值
  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>框选要素</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}
  .bar button{background:#fff;border:1px solid #cfd6df;border-radius:4px;padding:3px 10px;
       font:12px/1.4 inherit;color:#3a4250;cursor:pointer}
  .bar button:hover{border-color:#2e86de;color:#2e86de}
  #map{flex:1 1 auto;min-height:0}
</style>
</head>
<body>
<div class="bar">
  <span>总数 <b>120</b></span>
  <span>选中 <b id="n">0</b></span>
  <button id="clear">清空选择</button>
  <span>按住 Shift 在图上拖出一个框</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 features = [];
  for (var i = 0; i < 120; i++) {
    var c = cities[i % cities.length];
    features.push({
      type: 'Feature',
      properties: { id: i, code: 'S' + (1000 + i), value: 10 + Math.round(Math.random() * 90) },
      geometry: {
        type: 'Point',
        coordinates: [c[0] + (Math.random() - 0.5) * 4.5, c[1] + (Math.random() - 0.5) * 3.5]
      }
    });
  }

  var map = new maplibregl.Map({
    container: 'map',
    style: 'https://demotiles.maplibre.org/style.json',
    center: [110, 34],
    zoom: 3.4,
    attributionControl: { compact: true, customAttribution: 'MapLibre demo tiles' }
  });

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

  var nEl = document.getElementById('n');
  var selected = [];

  map.on('load', function () {
    map.addSource('sensors', {
      type: 'geojson',
      data: { type: 'FeatureCollection', features: features },
      promoteId: 'id'
    });

    map.addLayer({
      id: 'sensors',
      type: 'circle',
      source: 'sensors',
      paint: {
        'circle-radius': ['case', ['boolean', ['feature-state', 'selected'], false], 7, 4],
        'circle-color': ['case', ['boolean', ['feature-state', 'selected'], false], '#f03e3e', '#4dabf7'],
        'circle-stroke-width': 1,
        'circle-stroke-color': '#ffffff'
      }
    });
  });

  function clear() {
    selected.forEach(function (id) {
      map.setFeatureState({ source: 'sensors', id: id }, { selected: false });
    });
    selected = [];
    nEl.textContent = 0;
  }

  /* boxzoomend 只在框选结束时触发一次，比监听拖拽过程省事 */
  map.on('boxzoomend', function (e) {
    if (!map.getLayer('sensors')) return;
    clear();

    var hit = map.queryRenderedFeatures(e.boxZoomBounds, { layers: ['sensors'] });
    var seen = {};

    hit.forEach(function (f) {
      var id = f.properties.id;
      if (seen[id]) return;
      seen[id] = true;
      selected.push(id);
      map.setFeatureState({ source: 'sensors', id: id }, { selected: true });
    });

    nEl.textContent = selected.length;
  });

  document.getElementById('clear').addEventListener('click', clear);

  map.on('mousemove', 'sensors', function (e) {
    map.getCanvas().style.cursor = 'pointer';
    var p = e.features[0].properties;
    e.target.getCanvas().title = p.code + ' · ' + p.value;
  });

  map.on('mouseleave', 'sensors', function () {
    map.getCanvas().style.cursor = '';
    map.getCanvas().title = '';
  });

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