openlayers 使用 地图postcompose事件时的问题

event.vectorContext 为何会获取不到
event对象并没有vectorContext ，context也为null

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    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);
    }
    map.render();

var elapsedTime = getVectorContext();报错

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