openlayers 使用 地图postcompose事件时的问题

map.on('postcompose', moveFeature);
var moveFeature = function (event) {
var vectorContext = event.vectorContext;
var frameState = event.frameState;

        if (animating) {
            var elapsedTime = frameState.time - now;
            //通过增加速度，来获得lineString坐标
            var index = Math.round(speed * elapsedTime / 1000);

            if (index >= routeLength) {
                stopAnimation(true);
                return;
            }

            var currentPoint = new ol.geom.Point(routeCoords[index]);
            var feature = new ol.Feature(currentPoint);
            vectorContext.drawFeature(feature, styles.geoMarker);
        }
        //继续动画效果
        map.render();
    };

event.vectorContext 为何会获取不到