cesium-CallbackProperty是干啥的？

CallbackProperty就是属性回调函数，对于实体来说，比如一条线，如果我想改变其线宽，或者改变其颜色，但又不想重新绘制，那CallbackProperty就派上了用场。
请看如下示例：

    let viewer = new Cesium.Viewer('cesiumContainer');
    let entity = viewer.entities.add({
        polyline: {
            positions: new Cesium.CallbackProperty(changePosition, false),
            width: new Cesium.CallbackProperty(changeWidth, false),
            material: new Cesium.ColorMaterialProperty(new Cesium.CallbackProperty(changeColor, false)),
        }
    });

    function changePosition(time, isConstant) {
        let lat = 20;
        let lon = 100;
        return Cesium.Cartesian3.fromDegreesArray(
            [lon, lat, lon - Math.floor(Math.random() * 1000), lat - Math.floor(Math.random() * 100)],
            Cesium.Ellipsoid.WGS84,
            isConstant
        );

    }
    function changeWidth(time, isConstant) {
        return Math.floor(Math.random() * 10);
    }
    function changeColor(time, isConstant) {
        return Cesium.Color.fromRandom({
            minimumRed: 0.1,
            minimumGreen: 0.1,
            minimumBlue: 0.1,
            alpha: 1.0
        });
    }