Feature Events

<!--
  示例：要素增删事件
  要点：监听 addfeature / removefeature / changefeature，同步计数与操作日志
  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>要素增删事件</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}
  .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">
  <button id="add">随机添加点</button>
  <button id="clear">清空要素</button>
  <span id="info">要素 0 个</span>
  <span id="log">—</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 center = ol.proj.fromLonLat([116.3974, 39.9093]);

  var source = new ol.source.Vector();

  /* 初始三个点，走 addFeature 之外的构造方式，不会触发自身监听 */
  [[116.386, 39.912], [116.402, 39.905], [116.412, 39.918]].forEach(function (p) {
    source.addFeature(new ol.Feature({
      geometry: new ol.geom.Point(ol.proj.fromLonLat(p))
    }));
  });

  var layer = new ol.layer.Vector({
    source: source,
    style: new ol.style.Style({
      image: new ol.style.Circle({
        radius: 6,
        fill: new ol.style.Fill({ color: '#1f5fd0' }),
        stroke: new ol.style.Stroke({ color: '#ffffff', width: 2 })
      })
    })
  });

  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: center, zoom: 13 }),
    controls: ol.control.defaults.defaults({ attribution: false })
  });

  var info = document.getElementById('info');
  var log = document.getElementById('log');

  function refresh() {
    info.textContent = '要素 ' + source.getFeatures().length + ' 个';
  }

  source.on('addfeature', function (e) {
    log.textContent = 'addfeature · ' + (e.feature.getId() || '未命名');
    refresh();
  });

  source.on('removefeature', function (e) {
    log.textContent = 'removefeature · ' + (e.feature.getId() || '未命名');
    refresh();
  });

  source.on('changefeature', function (e) {
    log.textContent = 'changefeature · ' + (e.feature.getId() || '未命名');
  });

  var seq = 0;

  document.getElementById('add').addEventListener('click', function () {
    var lon = 116.37 + Math.random() * 0.06;
    var lat = 39.895 + Math.random() * 0.03;
    var f = new ol.Feature({
      geometry: new ol.geom.Point(ol.proj.fromLonLat([lon, lat])),
      name: '点位 ' + (++seq)
    });
    f.setId('pt-' + seq);
    source.addFeature(f);
  });

  document.getElementById('clear').addEventListener('click', function () {
    var n = source.getFeatures().length;
    source.clear();
    log.textContent = 'clear · 移除 ' + n + ' 个';
    refresh();
  });

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