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