百度地图js lite api 支持点聚合

百度地图lite api 是专门为h5 绘制海量点设计的，但是偏偏忽略掉了点聚合的需求，所以需要自己动手，做一次二次改造。

我们知道点聚合需要引入开源库:

MarkerClusterer:  http://api.map.baidu.com/library/MarkerClusterer/1.2/src/MarkerClusterer.js

TextIconOverlay: http://api.map.baidu.com/library/TextIconOverlay/1.2/src/TextIconOverlay.js

由于是开源库，我们可以直接把代码撸下来保存直接放到自己的业务里使用

我们在  MarkerClusterer  addMarkers的时候浏览器会报错，

是因为lite api中的map对象及bounds对象缺乏两个方法：

map.getDistance  及   bounds.containsPoint

getDistance方法是计算两个点之间的直线距离，返回数值为米，这个方法网上就有

 const EARTH_RADIUS = 6378137.0;
 const {PI} = Math;

 function getRad(d){
     return d*PI/180.0;
 }

 function getDistance(start,end){
     let lat1 = +start.lat;
     let lat2 = +end.lat;
     let lng1 = +start.lng;
     let lng2 = +end.lng;
     if(lat1==lat2 && lng1 == lng2){
         return 0;
     }
     let f = getRad((lat1 + lat2)/2);
     let g = getRad((lat1 - lat2)/2);
     let l = getRad((lng1 - lng2)/2);
     let sg = Math.sin(g);
     let sl = Math.sin(l);
     let sf = Math.sin(f);
     let s,c,w,r,d,h1,h2;
     let a = EARTH_RADIUS;
     let fl = 1/298.257;
     sg = sg*sg;
     sl = sl*sl;
     sf = sf*sf;
     s = sg*(1-sl) + (1-sf)*sl;
     c = (1-sg)*(1-sl) + sf*sl;
     w = Math.atan(Math.sqrt(s/c));
     r = Math.sqrt(s*c)/w;
     d = 2*w*a;
     h1 = (3*r -1)/2/c;
     h2 = (3*r +1)/2/s;
     return d*(1 + fl*(h1*sf*(1-sg) - h2*(1-sf)*sg));
 }

我们把这个方法赋到map对象上

 var MarkerClusterer = function (map, options) {
     if (!map) {
         return;
     }
     map.getDistance = getDistance;  // 这里
     this._map = map;
     this._markers = [];
     this._clusters = [];

     var opts = options || {};
     this._gridSize = opts["gridSize"] || 60;
     this._maxZoom = opts["maxZoom"] || 18;
     this._minClusterSize = opts["minClusterSize"] || 2;
     this._isAverageCenter = false;
     if (opts['isAverageCenter'] != undefined) {
         this._isAverageCenter = opts['isAverageCenter'];
     }
     this._styles = opts["styles"] || [];

     var that = this;
     this._map.addEventListener("zoomend", function () {
         that._redraw();
     });

     this._map.addEventListener("moveend", function () {
         that._redraw();
     });

     var mkrs = opts["markers"];
     isArray(mkrs) && this.addMarkers(mkrs);
 };

map对象就补齐了

containsPoint 是计算点是否在bounds给定的区域中，这个更简单

 var getExtendedBounds = function (map, bounds, gridSize) {
     bounds = cutBoundsInRange(bounds);
     var pixelNE = map.pointToPixel(bounds.getNorthEast());
     var pixelSW = map.pointToPixel(bounds.getSouthWest());
     pixelNE.x += gridSize;
     pixelNE.y -= gridSize;
     pixelSW.x -= gridSize;
     pixelSW.y += gridSize;
     var newNE = map.pixelToPoint(pixelNE);
     var newSW = map.pixelToPoint(pixelSW);
     let nb =  new BMap.Bounds(newSW, newNE);
     // 写在这里
     nb.containsPoint = function(point){
         var lat = point.lat;
         var lng = point.lng;
         if(!point || !point.lat || !point.lng){
             return false;
         } else {
             return lat>this._swLat && lat < this._neLat && lng<this._neLng && lng>this._swLng;
         }
     }
     return nb;
 };

至此MarkerClusterer 改造完毕

然后是改造TextIconOverlay

由于lite api 返回的MarkerPane

 TextIconOverlay.prototype.initialize = function (map) {
             this._map = map;

             this._domElement = document.createElement('div');
             this._updateCss();
             this._updateText();
             this._updatePosition();

             this._bind();
  // 将markerMouseTarget 替换成markerPane
 this._map.getPanes().markerPane.appendChild(this._domElement);
             return this._domElement;
         };

这样就可以了