React Leaflet Esri聚集要素：实现地图点要素聚合

问题描述

1. esri-leaflet以插件(plugins)形式，直接在前端提供了要素图层的自动聚合功能,即esri.cluster.Featurelayer
2. 在react环境下，esri.cluster.Featurelayer的使用与直接引入CDN有一些使用的细节差异

环境准备

1. 安装lealet/esri-leaflet

操作步骤省略

1. 安装esri-leaflet-cluster插件

npm install esri-leaflet-cluster leaflet.markercluster

注意：因为esri.cluster.Featurelayer是基于leaflet.markercluster进行扩展开发的。故而安装时，两个包都需要安装。后者本身不是leaflet的官方组件，其实也是第三方扩展的，具体使用细节参考，https://github.com/Leaflet/Leaflet.markercluster

1. 引入esri.cluster.Featurelayer和leaflet.markercluster
   
       的API和CSS样式文件支持

import React, { useEffect } from 'react';
import L from "leaflet"
import * as esri from "esri-leaflet"
import * as markercluster from "leaflet.markercluster"
import * as cluster from  "esri-leaflet-cluster"
import "leaflet.markercluster/dist/MarkerCluster.Default.css"

注意：若未引入leaflet.markercluster则编译运行后，浏览器很有可能报如下错误，主要就是基础类找不到的原因

TypeError: Object(…) is not a function

  21 |    this._layers = {};
  22 |    this._leafletIds = {};
  23 | 
> 24 |    this.cluster = markerClusterGroup(options);
     | ^  25 |    this._key = 'c' + (Math.random() * 1e9).toString(36).replace('.', '_');

实例 代码

import React, { useEffect } from 'react';
import L from "leaflet"
import * as esri from "esri-leaflet"
import * as markercluster from "leaflet.markercluster"
import * as cluster from  "esri-leaflet-cluster"
import "leaflet.markercluster/dist/MarkerCluster.Default.css"

function ClusterFeatureLayer(props) {
    useEffect(()=>{
        
        //对于clusterFeatureLayer必须设置maxZoom属性，否则编译会出错
        let map = L.map("clusterfeaturelayer_map", { maxZoom: 14}).setView([29.776,106.308],10)
        let layer = cluster.featureLayer({ 
            url: "http://localhost:6080/arcgis/rest/services/02_Tky_GDCGK/FeatureServer/4",
            pointToLayer: function (geojson, latlng) {
                return L.circleMarker(latlng, 2, {
                  color: "#2D84C8"
                });
              }
            })
        layer.addTo(map)
    },[])

    return (
        <div id="clusterfeaturelayer_map" style={{ width: "100%", height: "800px" }}></div>
    );
}

export default ClusterFeatureLayer;

代码 说明

1. import的内容一定要完整，否则编译会出现各种奇怪的错误
2. 添加了聚集图层的map一定要设置maxZoom属性，否则会引起Uncaught Map has no maxZoom specified而导致地图不能加载 TypeError: Cannot read property '_addChild' of undefined
3. 聚集功能是完全基于前端实现展示层实现的，因此不需要对后台的FS有任何的特殊设置
4. 一定要分清esri.cluster.Featurelayer和leaflet.markercluster的职责范围：
      
5. 因为esri.cluster.Featurelayer继承扩展自leaflet.markercluster因此，前者可以使用后者所提供的各种函数接口或config参数

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