OpenLayers点聚合

1. 引言

当页面加载的数据量过大时，拖拽、缩放时往往会产生卡顿

然而，页面实现的内容是有限的，人眼可见范围也是有限的，过于微小的部分是可以不予显示的

聚合是解决这种问题的一个办法，当数据比较多，单个又太小时，将多个数据合并为一个显示

OpenLayers提供了聚合的API，参考：

OpenLayers v6.15.1 API - Class: Cluster

OpenLayers提供了聚合的示例，参考：

Dynamic clusters (openlayers.org)

本文基于OpenLayers的Cluster API，实现点聚合效果

2. 环境准备

OpenLayers版本为最新版6.15.1，CDN引入：

<link rel="stylesheet"

      href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/css/ol.css" type="text/css">

<script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/build/ol.js"></script>

数据来源自OpenLayers的聚合示例种的数据，地址为：

https://openlayers.org/en/latest/examples/data/geojson/photovoltaic.json

3. 点聚合

构建一个基础页面，加载底图：

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta http-equiv="X-UA-Compatible" content="IE=edge">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <title>Document</title>

    <link rel="stylesheet"

        href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/css/ol.css" type="text/css">

    <script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/build/ol.js"></script>

    <style>

        html,

        body {

            height: 100%;

        }

        body {

            margin: 0;

            padding: 0;

        }

        #map {

            height: 100%;

        }

    </style>

</head>

<body>

    <div id="map"></div>

    <script>

        const map = new ol.Map({

            target: 'map',

            layers: [],

            view: new ol.View({

                center: [16.35, 48.20],

                zoom: 12,

                projection: 'EPSG:4326'

            })

        })

        map.addLayer(new ol.layer.WebGLTile({

            source: new ol.source.OSM()

        }));

    </script>

</body>

</html>

加载JSON数据，创建聚合数据源：

const source = new ol.source.Vector({

    url: 'https://openlayers.org/en/latest/examples/data/geojson/photovoltaic.json',

    format: new ol.format.GeoJSON()

})

const clusterSource = new ol.source.Cluster({

    source: source,

})

const vectorLayer = new ol.layer.Vector({

    source: clusterSource,

})

map.addLayer(vectorLayer);

参考官方API文档：

OpenLayers v6.15.1 API - Class: Cluster

默认的聚合范围为20

实现的效果：

添加聚合样式：

const vectorLayer = new ol.layer.Vector({

    source: clusterSource,

    // 聚合样式

    style: function (feature) {

        // 点的个数

        const size = feature.get('features').length

        return new ol.style.Style({

            image: new ol.style.Circle({ // 圆形

                radius: 15, // 半径

                stroke: new ol.style.Stroke({ // 边框

                    color: '#fff'

                }),

                fill: new ol.style.Fill({ // 填充

                    color: '#3399CC'

                })

            }),

            text: new ol.style.Text({ // 文字样式

                font: '15px sans-serif',

                text: size.toString(),

                fill: new ol.style.Fill({

                    color: '#fff'

                })

            })

        })

    }

})

实现效果：

4. 完整代码

完整代码如下：

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta http-equiv="X-UA-Compatible" content="IE=edge">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <title>Document</title>

    <link rel="stylesheet"

        href="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/css/ol.css" type="text/css">

    <script src="https://cdn.jsdelivr.net/gh/openlayers/openlayers.github.io@master/en/v6.15.1/build/ol.js"></script>

    <style>

        html,

        body {

            height: 100%;

        }

        body {

            margin: 0;

            padding: 0;

        }

        #map {

            height: 100%;

        }

    </style>

</head>

<body>

    <div id="map"></div>

    <script>

        const map = new ol.Map({

            target: 'map',

            layers: [],

            view: new ol.View({

                center: [16.35, 48.20],

                zoom: 12,

                projection: 'EPSG:4326'

            })

        })

        // map.addLayer(new ol.layer.WebGLTile({

        //     source: new ol.source.OSM()

        // }));

        const source = new ol.source.Vector({

            url: 'https://openlayers.org/en/latest/examples/data/geojson/photovoltaic.json',

            format: new ol.format.GeoJSON()

        })

        const clusterSource = new ol.source.Cluster({

            source: source,

            distance: 40,

        })

        const vectorLayer = new ol.layer.Vector({

            source: clusterSource,

            // 聚合样式

            style: function (feature) {

                // 点的个数

                const size = feature.get('features').length

                return new ol.style.Style({

                    image: new ol.style.Circle({ // 圆形

                        radius: 15, // 半径

                        stroke: new ol.style.Stroke({ // 边框

                            color: '#fff'

                        }),

                        fill: new ol.style.Fill({ // 填充

                            color: '#3399CC'

                        })

                    }),

                    text: new ol.style.Text({ // 文字样式

                        font: '15px sans-serif',

                        text: size.toString(),

                        fill: new ol.style.Fill({

                            color: '#fff'

                        })

                    })

                })

            }

        })

        map.addLayer(vectorLayer);

    </script>

</body>

</html>

5. 参考资料

[1]Dynamic clusters (openlayers.org)

[2]OpenLayers v6.15.1 API - Class: Cluster

[3]openlayers学习——9、openlayers聚合效果Cluster_WangConvey的博客-CSDN博客_openlayers 聚合

[4]vue+OpenLayers项目实践（三）：Cluster设置集群 - 掘金 (juejin.cn)

巴特西