Preload

<!--
  示例：瓦片预加载
  要点：layer 的 preload 控制视野外缓冲圈层，Infinity 表示提前备齐
  依赖：OpenLayers 10.3.1 · jsDelivr CDN（底图 OSM，无 Key）
-->
<!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;color:#16181d}
  .bar{flex:0 0 auto;padding:8px 12px;background:#f6f7f9;border-bottom:1px solid #e3e6ea;
       display:flex;gap:14px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar b{color:#16181d;font-weight:600}
  .bar select{font-size:12px;padding:2px 4px}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label>preload
    <select id="pl">
      <option value="0">0（默认）</option>
      <option value="1">1</option>
      <option value="2">2</option>
      <option value="inf">Infinity</option>
    </select>
  </label>
  <span>本层请求瓦片 <b id="cnt">0</b></span>
  <span>切换后缩放，观察请求数变化</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 (!window.ol) {
    document.getElementById('map').textContent = 'OpenLayers 加载失败，请检查 cdn.jsdelivr.net 是否可访问';
    return;
  }

  var count = 0;
  var map = null;
  var overlayLayer = null;

  function build(preload) {
    var layer = 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: preload,
      opacity: 0.65
    });
    layer.getSource().on('tileloadstart', function () {
      count++;
      document.getElementById('cnt').textContent = count;
    });
    return layer;
  }

  function swap(value) {
    var next = build(value);
    if (overlayLayer) map.removeLayer(overlayLayer);
    map.addLayer(next);
    overlayLayer = next;
  }

  map = new ol.Map({
    target: 'map',
    layers: [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'}) })],
    view: new ol.View({ center: ol.proj.fromLonLat([116.39, 39.91]), zoom: 8 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  swap(0);

  document.getElementById('pl').addEventListener('change', function () {
    swap(this.value === 'inf' ? Infinity : Number(this.value));
  });
})();
</script>
</body>
</html>