Load Events

<!--
  示例：加载事件与进度 — OpenLayers load-events
  核心逻辑：监听瓦片与矢量源的 loadstart/loadend 事件，维护计数并驱动加载指示器
  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>加载事件与进度</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:12px;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;position:relative}
  .spinner{position:absolute;right:16px;top:16px;width:32px;height:32px;
       border:3px solid rgba(31,95,208,0.25);border-top-color:#1f5fd0;border-radius:50%;
       animation:spin 0.9s linear infinite;display:none}
  .spinner.on{display:block}
  @keyframes spin{to{transform:rotate(360deg)}}
</style>
</head>
<body>
<div class="bar">
  <button id="reload">重新加载矢量数据</button>
  <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').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');
  var spinner = document.createElement('div');
  spinner.className = 'spinner';

  var tileSource = new ol.source.XYZ({url: 'https://server.arcgisonline.com/ArcGIS/rest/services/World_Street_Map/MapServer/tile/{z}/{y}/{x}', attributions: 'Tiles © Esri'});
  var pending = 0;
  var totalLoads = 0;

  tileSource.on('tileloadstart', function () {
    pending++;
    totalLoads++;
    spinner.classList.add('on');
    render();
  });

  function done() {
    pending = Math.max(0, pending - 1);
    if (pending === 0) spinner.classList.remove('on');
    render();
  }

  tileSource.on('tileloadend', done);
  tileSource.on('tileloaderror', done);

  /* 自定义矢量加载器：延迟 800ms 后放入要素，可重复触发 */
  var vectorSource = new ol.source.Vector({
    loader: function (extent, resolution, projection, success, failure) {
      spinner.classList.add('on');
      setTimeout(function () {
        try {
          var features = [];
          for (var i = 0; i < 40; i++) {
            var lon = 116.30 + Math.random() * 0.22;
            var lat = 39.86 + Math.random() * 0.09;
            features.push(new ol.Feature({
              geometry: new ol.geom.Point(ol.proj.fromLonLat([lon, lat]))
            }));
          }
          vectorSource.addFeatures(features);
          if (success) success(features);
        } catch (e) {
          if (failure) failure();
        }
        spinner.classList.remove('on');
        render();
      }, 800);
    },
    strategy: ol.loadingstrategy.bbox
  });

  var map = new ol.Map({
    target: 'map',
    layers: [
      new ol.layer.Tile({ source: tileSource }),
      new ol.layer.Vector({
        source: vectorSource,
        style: new ol.style.Style({
          image: new ol.style.Circle({
            radius: 4,
            fill: new ol.style.Fill({ color: '#2f9e44' }),
            stroke: new ol.style.Stroke({ color: '#fff', width: 1 })
          })
        })
      })
    ],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.391, 39.907]),
      zoom: 13,
      minZoom: 2
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  map.getViewport().appendChild(spinner);

  function render() {
    infoEl.textContent = '瓦片请求 ' + totalLoads +
      ' · 进行中 ' + pending +
      ' · 矢量要素 ' + vectorSource.getFeatures().length;
  }

  vectorSource.on('featuresloadend', render);

  document.getElementById('reload').addEventListener('click', function () {
    vectorSource.clear();
    vectorSource.refresh();
    render();
  });

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