Tile Preload

<!--
  示例：瓦片预加载
  要点：layer.setPreload 控制预取层级数，tileloadend 统计实际请求量
  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>瓦片预加载</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}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  .bar button{padding:4px 10px;border:1px solid #cfd4dc;border-radius:4px;
              background:#fff;cursor:pointer;font-size:12px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label><input type="checkbox" id="preload"> 预加载 3 层</label>
  <span id="count">已加载 0 块</span>
  <button id="reset">清零</button>
  <span>缩放一次即可看出差别</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 base = 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'}),
    preload: 0
  });

  var loaded = 0;
  var countEl = document.getElementById('count');

  /* 先挂监听再建地图，首屏瓦片也能计入 */
  base.getSource().on('tileloadend', function () {
    loaded++;
    countEl.textContent = '已加载 ' + loaded + ' 块';
  });

  new ol.Map({
    target: 'map',
    layers: [base],
    view: new ol.View({
      center: ol.proj.fromLonLat([116.3974, 39.9093]),
      zoom: 12
    }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  document.getElementById('preload').addEventListener('change', function () {
    base.setPreload(this.checked ? 3 : 0);
    countEl.textContent = '已加载 ' + loaded + ' 块（preload=' + base.getPreload() + '）';
  });

  document.getElementById('reset').addEventListener('click', function () {
    loaded = 0;
    countEl.textContent = '已加载 0 块（preload=' + base.getPreload() + '）';
  });
})();
</script>
</body>
</html>