概述
本文以openlayers为例，结合turf.js讲一下webgis中绘制和编辑时如何实现捕捉功能。
效果
为了验证捕捉的结果是否正确，就将绘制的几个面做了一个合并的测试，效果如下：
实现思路
- 输入
- 要捕捉的图层，此处为vectorSource
- 捕捉的范围，单位是像素
- 鼠标移动时的坐标
- 输出
- 满足条件的点
捕捉工具的舒适化参数如下：
实现的大致流程如下：
实现代码
代码语言：javascript复制 /**
* 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 ;
......
}
