Callback Property

<!--
  示例：用 CallbackProperty 每帧算位置与半径
  说明：物体的位置和圆半径都由函数实时给出，不用预先采样
  核心逻辑：new Cesium.CallbackProperty(fn, false) 挂在 position 与 ellipse.semiMajorAxis 上
  依赖：CesiumJS 1.121 · 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>回调属性</title>
<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Widgets/widgets.css">
<style>
  html,body{height:100%;margin:0}
  body{background:#0e1116;color:#e6e8ec;overflow:hidden;
       font:13px/1.6 -apple-system,"Segoe UI","PingFang SC",sans-serif}
  #c{height:100%}
  .hud{position:absolute;left:12px;top:12px;z-index:5;background:rgba(23,28,35,.9);
       border:1px solid #2a313b;border-radius:6px;padding:9px 12px;font-size:12px;
       color:#a2a9b6;max-width:260px}
  .hud b{color:#e6e8ec;font-weight:600}
  .cesium-viewer-bottom .cesium-widget-credits,.cesium-viewer-animationContainer,
  .cesium-viewer-timelineContainer{display:none!important}
</style>
</head>
<body>
<div id="c"></div>
<div class="hud">
  <div><b>回调属性</b></div>
  <div>位置在绕圈，圆的半径按正弦变化</div>
  <div>两个属性都由 <code>isConstant = false</code> 的回调给出</div>
</div>
<script src="https://cdn.jsdelivr.net/npm/cesium@1.121.0/Build/Cesium/Cesium.js"></script>
<script>
(function () {
  'use strict';

  var viewer = new Cesium.Viewer('c', {
    baseLayer: false,
    baseLayerPicker: false, geocoder: false, homeButton: false,
    sceneModePicker: false, navigationHelpButton: false,
    animation: false, timeline: false, fullscreenButton: false,
    infoBox: false, selectionIndicator: false
  });

  var scene = viewer.scene;
  scene.globe.baseColor = Cesium.Color.fromCssColorString('#1c2733');
  scene.backgroundColor = Cesium.Color.fromCssColorString('#0e1116');
  scene.skyBox.show = false;
  scene.moon.show = false;
  if (scene.skyAtmosphere) scene.skyAtmosphere.show = false;

  var start = Cesium.JulianDate.now().clone();

  /* 半径按正弦变化，半轴在 6 km 到 16 km 之间摆动 */
  var radius = new Cesium.CallbackProperty(function (time) {
    var s = Cesium.JulianDate.secondsDifference(time, start);
    return 8000 + 6000 * Math.sin(s * 1.2);
  }, false);

  /* 位置绕中心点转圈，高度固定 */
  var position = new Cesium.CallbackProperty(function (time, result) {
    var s = Cesium.JulianDate.secondsDifference(time, start);
    var lon = 116.392 + 0.045 * Math.cos(s * 0.5);
    var lat = 39.900 + 0.028 * Math.sin(s * 0.5);
    return Cesium.Cartesian3.fromDegrees(lon, lat, 2600, Cesium.Ellipsoid.WGS84, result);
  }, false);

  viewer.entities.add({
    position: Cesium.Cartesian3.fromDegrees(116.392, 39.900, 0),
    ellipse: {
      semiMajorAxis: radius,
      semiMinorAxis: radius,
      height: 0,
      material: Cesium.Color.fromCssColorString('#4d9bff').withAlpha(0.18),
      outline: true,
      outlineColor: Cesium.Color.fromCssColorString('#8fc2ff').withAlpha(0.9)
    }
  });

  viewer.entities.add({
    position: position,
    point: {
      pixelSize: 12,
      color: Cesium.Color.fromCssColorString('#ffd166'),
      outlineColor: Cesium.Color.WHITE,
      outlineWidth: 2,
      disableDepthTestDistance: Number.POSITIVE_INFINITY
    },
    label: {
      text: '回调算出的位置',
      font: '500 12px "Microsoft YaHei", sans-serif',
      fillColor: Cesium.Color.WHITE,
      pixelOffset: new Cesium.Cartesian2(0, -22),
      disableDepthTestDistance: Number.POSITIVE_INFINITY
    }
  });

  /* 轨迹线的位置也用回调给出，实时把走过的点连成圆 */
  viewer.entities.add({
    polyline: {
      positions: new Cesium.CallbackProperty(function (time, result) {
        var s = Cesium.JulianDate.secondsDifference(time, start);
        var n = 64;
        var arr = new Array((n + 1) * 3);
        for (var i = 0; i <= n; i++) {
          var a = s * 0.5 + (i / n) * Math.PI * 2;
          arr[i * 3] = 116.392 + 0.045 * Math.cos(a);
          arr[i * 3 + 1] = 39.900 + 0.028 * Math.sin(a);
          arr[i * 3 + 2] = 2600;
        }
        return Cesium.Cartesian3.fromDegreesArrayHeights(arr);
      }, false),
      width: 1.5,
      material: Cesium.Color.fromCssColorString('#ffd166').withAlpha(0.5)
    }
  });

  viewer.camera.flyTo({
    destination: Cesium.Cartesian3.fromDegrees(116.392, 39.884, 95000),
    orientation: {
      heading: 0,
      pitch: Cesium.Math.toRadians(-52),
      roll: 0
    },
    duration: 0
  });
})();
</script>
</body>
</html>