解决OpenLayers postcompose事件event.vectorContext获取失败的问题？

openlayers 使用地图postcompose事件event.vectorContext 为什么获取不到呢？通过openlayers添加点然后实现点位闪烁效果

flash(feature) {
      var _this = this;
    var start = new Date().getTime();
    var listenerKey;

    function animate(event) {
        debugger;
        var duration = 1000;
        var vectorContext = event.vectorContext;
        var frameState = event.frameState;
        var flashGeom = feature.getGeometry().clone();
        var elapsed = frameState.time - start;
        var elapsedRatio = elapsed / duration;
        // var radius = ol.easing.easeOut(elapsedRatio) * 6 + 5;
        // var opacity = ol.easing.easeOut(1 - elapsedRatio);
        var radius = 10;
        var opacity = 1;
        console.log("radius：：：" + radius + "：：：opacity：：：" + opacity)
        var style = new Style({
            image: new Circle({
                radius: radius,
                snapToPixel: false,
                stroke: new Stroke({
                    color: 'rgba(255, 0, 0, ' + opacity + ')',
                    width: 0.25 + opacity
                })
            })
        });
        vectorContext.setStyle(style);
        vectorContext.drawGeometry(flashGeom);
        if (elapsed > duration) {
            ol.Observable.unByKey(listenerKey);
            return;
        }
        _this.map.render();
    }

    listenerKey = _this.map.on('postcompose', animate);
}