cesium中entity的callback回调监听

//线标绘
entity.polyline.positions = new Cesium.CallbackProperty (function(){
          return entity.coords;
}, false);
//面标绘
entity.polygon.hierarchy = new Cesium.CallbackProperty (function(){
  return  new Cesium.PolygonHierarchy(entity.coords);
}, false);
//材料回调
let alpha = Cesium.Color.fromCssColorString ("#f00").withAlpha(0.5);
entity.polyline.material = new Cesium.ColorMaterialProperty (new Cesium.CallbackProperty (function(){
  return alpha;
}, false));