Image Load Events

<!--
  说明：影像加载事件
  核心逻辑：监听 ImageStatic 的 imageloadstart / end / error，维护加载状态与耗时
  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>影像加载事件</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:10px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar button{font:inherit;padding:4px 10px;border:1px solid #c9d0da;border-radius:5px;
              background:#fff;color:#1f2a37;cursor:pointer}
  #dot{width:8px;height:8px;border-radius:50%;background:#8b96a3;display:inline-block}
  #dot.on{background:#1f5fd0}
  #dot.err{background:#c8281c}
  #info{margin-left:auto}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <span id="dot"></span>
  <span id="state">等待加载</span>
  <button id="reload">重新加载</button>
  <button id="fail">模拟失败</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 fromLonLat = ol.proj.fromLonLat;

  function sample() {
    var canvas = document.createElement('canvas');
    canvas.width = 420;
    canvas.height = 300;
    var ctx = canvas.getContext('2d');
    for (var y = 0; y < 300; y += 10) {
      for (var x = 0; x < 420; x += 10) {
        ctx.fillStyle = 'hsl(' + (x + y) % 360 + ', 60%, ' + (40 + (y % 30)) + '%)';
        ctx.fillRect(x, y, 10, 10);
      }
    }
    ctx.fillStyle = 'rgba(10,20,35,0.7)';
    ctx.font = '600 20px sans-serif';
    ctx.fillText('Canvas 生成的静态影像', 18, 40);
    return canvas.toDataURL();
  }

  var extent = ol.proj.transformExtent([108, 31, 119, 39], 'EPSG:4326', 'EPSG:3857');

  function makeSource(bad) {
    return new ol.source.ImageStatic({
      url: bad ? 'data:image/png;base64,AAAA' : sample(),
      imageExtent: extent,
      projection: 'EPSG:3857'
    });
  }

  var imageLayer = new ol.layer.Image({ source: makeSource(false) });

  var 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'}) }), imageLayer],
    view: new ol.View({ center: fromLonLat([113, 35]), zoom: 5, minZoom: 2, maxZoom: 19 }),
    controls: ol.control.defaults.defaults({ attribution: false, rotate: false })
  });

  var dot = document.getElementById('dot');
  var state = document.getElementById('state');
  var info = document.getElementById('info');
  var startedAt = 0;

  function bind(src) {
    src.on('imageloadstart', function () {
      startedAt = Date.now();
      dot.className = 'on';
      state.textContent = '加载中';
      info.textContent = 'imageloadstart';
    });
    src.on('imageloadend', function () {
      dot.className = '';
      state.textContent = '加载完成';
      info.textContent = 'imageloadend · 耗时 ' + (Date.now() - startedAt) + ' ms';
    });
    src.on('imageloaderror', function () {
      dot.className = 'err';
      state.textContent = '加载失败';
      info.textContent = 'imageloaderror';
    });
  }

  bind(imageLayer.getSource());

  function replace(bad) {
    var src = makeSource(bad);
    bind(src);
    imageLayer.setSource(src);
  }

  document.getElementById('reload').addEventListener('click', function () {
    replace(false);
  });

  document.getElementById('fail').addEventListener('click', function () {
    replace(true);
  });
})();
</script>
</body>
</html>