Openlayers2 :我想根据用户输入的长度在openlayer2上绘制一个缓冲区

var map = new OpenLayers.Map("map");

var osmLayer = new OpenLayers.Layer.OSM();
map.addLayer(osmLayer);

var vectorLayer = new OpenLayers.Layer.Vector("Vector Layer");
map.addLayer(vectorLayer);

var drawControl = new OpenLayers.Control.DrawFeature(vectorLayer, OpenLayers.Handler.Path);
map.addControl(drawControl);

drawControl.events.register("featureadded", drawControl, function(event) {
  var feature = event.feature;
  var geometry = feature.geometry;

  // 获取用户输入的长度
  var length = parseFloat(prompt("请输入缓冲区长度："));

  // 创建缓冲区
  var bufferedGeometry = geometry.buffer(length);

  // 创建缓冲区的样式
  var style = {
    strokeColor: "#ff0000",
    strokeWidth: 2,
    fillColor: "#ff0000",
    fillOpacity: 0.3
  };

  // 创建缓冲区的要素
  var bufferedFeature = new OpenLayers.Feature.Vector(bufferedGeometry, null, style);

  // 添加缓冲区要素到矢量图层
  vectorLayer.addFeatures([bufferedFeature]);

  // 取消绘制控件的激活状态
  drawControl.deactivate();
});