webgis中捕捉功能的实现

 /**
  * 获取捕捉点
  * @param {Array} coords - 传入坐标
  * @return {Array}
  */
 getSnapPoint (coords) {
   if (!this._active) return []
   let result = []
   let features = this._vectorSource.getFeatures()
   const res = this._map.getView().getResolution()
   const dis = this.getPixelTolerance() * res
   const point = turfPoint.point(toLonLat(coords))
   for (let i = 0; i < features.length; i++) {
     const feature = features[i]
     const json = feature2Geojson(feature)
     const line = polygonToLine(json)
     const dist = pointToLineDistance(point, line)
     if (dist * 1000 < dis) {
       const pointLine = nearestPointOnLine(line, point, { units: 'kilometers' })
       result = pointLine.geometry.coordinates
       break
     }
   }
   this._overlay.setPosition(result.length === 0 ? null : result)
   return result
 }
// 处理map的事件，click，pointermove，dblclick
function clickEvent(e) {
  if(!isDraw) return
  const snapPoint = snap.getSnapPoint(e.coordinate)
  const coordinate = snapPoint.length > 0 ? ol.proj.fromLonLat(snapPoint) : e.coordinate ;
  ......
}
function pointerMoveEvent(e) {
  if(!isDraw) return
  const snapPoint = snap.getSnapPoint(e.coordinate)
  const coordinate = snapPoint.length > 0 ? ol.proj.fromLonLat(snapPoint) : e.coordinate ;
  ......
}
function dblclickEvent(e) {
  if(!isDraw) return
  let coords = coordinates.concat([]);
  const snapPoint = snap.getSnapPoint(e.coordinate)
  const coordinate = snapPoint.length > 0 ? ol.proj.fromLonLat(snapPoint) : e.coordinate ;
  ......
}