Animate Line

<!--
  示例：线条流动动画
  核心逻辑：定时切换 line-dasharray 的取值，虚线间隔变化形成前进的动感
  数据：页面内构造的一条环形巡检路线
  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>线条流动动画</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>节拍 <b id="f">0</b></span>
  <span>序列 <b id="seq">4 4</b></span>
  <button id="pause">暂停</button>
  <span>虚线间隔轮流切换，方向沿线的走向</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;
  }

  /* 一条跨区域的巡检路线，首尾闭合 */
  var loop = [
    [100.0, 38.0], [104.0, 36.2], [108.0, 35.4],
    [112.0, 33.6], [116.0, 32.2], [118.2, 29.8],
    [115.0, 27.8], [110.0, 28.8], [105.0, 31.0],
    [100.8, 34.2], [100.0, 38.0]
  ];

  var map = new maplibregl.Map({
    container: 'map',
    style: 'https://demotiles.maplibre.org/style.json',
    center: [109, 33],
    zoom: 4.4,
    attributionControl: { compact: true, customAttribution: 'MapLibre demo tiles' }
  });

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

  /* 每一帧换一组 dasharray，视觉上虚线在向前走 */
  var STEPS = [[4, 4], [0, 4, 3], [0, 0, 3, 3], [0, 4], [4, 0], [3, 0, 3, 0], [2, 0, 2, 0, 2, 0]];

  var seqEl = document.getElementById('seq');
  var fEl = document.getElementById('f');
  var timer = null;
  var idx = 0;
  var frames = 0;
  var running = true;

  map.on('load', function () {
    map.addSource('loop', {
      type: 'geojson',
      data: {
        type: 'FeatureCollection',
        features: [{
          type: 'Feature',
          properties: {},
          geometry: { type: 'LineString', coordinates: loop }
        }]
      }
    });

    /* 底层实线做衬底，虚线叠在上面才看得清 */
    map.addLayer({
      id: 'loop-base',
      type: 'line',
      source: 'loop',
      layout: { 'line-cap': 'round', 'line-join': 'round' },
      paint: { 'line-color': '#2e86de', 'line-width': 5, 'line-opacity': 0.25 }
    });

    map.addLayer({
      id: 'loop-dash',
      type: 'line',
      source: 'loop',
      layout: { 'line-cap': 'butt', 'line-join': 'round' },
      paint: { 'line-color': '#f03e3e', 'line-width': 4, 'line-dasharray': STEPS[0] }
    });

    timer = setInterval(function () {
      if (!running) return;
      idx = (idx + 1) % STEPS.length;
      frames++;
      seqEl.textContent = STEPS[idx].join(' ');
      fEl.textContent = frames;
      map.setPaintProperty('loop-dash', 'line-dasharray', STEPS[idx]);
    }, 90);
  });

  document.getElementById('pause').addEventListener('click', function () {
    running = !running;
    this.textContent = running ? '暂停' : '继续';
  });

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