Canvas Gradient Pattern

<!--
  示例：Canvas 渐变与图案填充
  要点：ol.style.Fill 的 color 可以直接接 CanvasGradient 与 CanvasPattern
  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>Canvas 渐变与图案填充</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:12px;align-items:center;font-size:12px;color:#58606e;flex-wrap:wrap}
  .bar label{display:flex;gap:5px;align-items:center;cursor:pointer}
  #map{flex:1 1 auto;min-height:0;background:#eef0f3}
</style>
</head>
<body>
<div class="bar">
  <label><input type="radio" name="fill" value="gradient" checked> 线性渐变</label>
  <label><input type="radio" name="fill" value="pattern"> 点阵图案</label>
  <label><input type="radio" name="fill" value="solid"> 纯色</label>
</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 fromLonLat = ol.proj.fromLonLat;

  /* 渐变：在 256×256 的画布上建一个对角渐变对象 */
  var gradCanvas = document.createElement('canvas');
  gradCanvas.width = gradCanvas.height = 256;
  var gctx = gradCanvas.getContext('2d');
  var gradient = gctx.createLinearGradient(0, 0, 256, 256);
  gradient.addColorStop(0, 'rgba(31,95,208,0.85)');
  gradient.addColorStop(0.5, 'rgba(122,208,200,0.85)');
  gradient.addColorStop(1, 'rgba(250,193,86,0.85)');

  /* 图案：8×8 单元内画一个圆点，再用 createPattern 平铺 */
  var patCanvas = document.createElement('canvas');
  patCanvas.width = patCanvas.height = 8;
  var pctx = patCanvas.getContext('2d');
  pctx.fillStyle = '#e8f0fb';
  pctx.fillRect(0, 0, 8, 8);
  pctx.fillStyle = '#1f5fd0';
  pctx.beginPath();
  pctx.arc(4, 4, 1.8, 0, Math.PI * 2);
  pctx.fill();
  var pattern = pctx.createPattern(patCanvas, 'repeat');

  var mode = 'gradient';

  function makePolygon(ring, name) {
    return new ol.Feature({
      geometry: new ol.Polygon([ring.map(function (p) { return fromLonLat(p); })]),
      name: name
    });
  }

  var features = [
    makePolygon([[116.30, 39.94], [116.38, 39.94], [116.38, 39.88], [116.30, 39.88]], '西城'),
    makePolygon([[116.40, 39.95], [116.47, 39.95], [116.47, 39.89], [116.40, 39.89]], '东城'),
    makePolygon([[116.42, 39.87], [116.50, 39.87], [116.50, 39.81], [116.42, 39.81]], '丰台')
  ];

  function fillColor() {
    if (mode === 'gradient') return gradient;
    if (mode === 'pattern') return pattern;
    return 'rgba(31,95,208,0.5)';
  }

  function styleFn() {
    return new ol.style.Style({
      fill: new ol.style.Fill({ color: fillColor() }),
      stroke: new ol.style.Stroke({ color: '#0f3f8f', width: 1.5 }),
      text: new ol.style.Text({
        font: '500 12px sans-serif',
        fill: new ol.style.Fill({ color: '#16181d' }),
        stroke: new ol.style.Stroke({ color: '#ffffff', width: 3 })
      })
    });
  }

  var layer = new ol.layer.Vector({
    source: new ol.source.Vector({ features: features }),
    style: styleFn
  });

  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'}) }), layer],
    view: new ol.View({ center: fromLonLat([116.40, 39.90]), zoom: 11 }),
    controls: ol.control.defaults.defaults({ rotate: false })
  });

  /* 标注文字跟着要素名走，需要在样式函数里拿到 feature */
  layer.setStyle(function (feature) {
    var s = styleFn();
    if (feature) s.getText().setText(feature.get('name'));
    return s;
  });

  document.querySelectorAll('input[name=fill]').forEach(function (el) {
    el.addEventListener('change', function () {
      mode = this.value;
      layer.changed();   /* 样式对象变了，通知图层重绘 */
    });
  });
})();
</script>
</body>
</html>