实现效果

今天介绍一个有趣的gis小功能：动态轨迹播放！效果就像这样：

这效果看着还很丝滑！别急，接下来教你怎么实现。代码示例基于parcel打包工具和es6语法，本文假设你已经掌握相关知识和技巧。

gis初学者可能对openlayers（后面简称ol）不熟悉，这里暂时不介绍ol了，直接上代码，先体验下感觉。

创建一个地图容器

引入地图相关对象

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import Map from 'ol/Map';

import View from 'ol/View';

import XYZ from 'ol/source/XYZ';

import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';

创建地图对象

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const center = [-5639523.95, -3501274.52];

const map = new Map({

  target: document.getElementById('map'),

  view: new View({

    center: center,

    zoom: 10,

    minZoom: 2,

    maxZoom: 19,

  }),

  layers: [

    new TileLayer({

      source: new XYZ({

        attributions: attributions,

        url: 'https://api.maptiler.com/maps/hybrid/{z}/{x}/{y}.jpg?key=' + key,

        tileSize: 512,

      }),

    }),

  ],

});

创建一条线路

画一条线路

可以用这个geojson网站随意画一条线，然后把数据内容复制下来，保存为json文件格式，作为图层数据添加到地图容器中。

你可以用异步加载的方式，也可以用require方式，这里都介绍下吧：

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// fetch

fetch('data/route.json').then(function (response) {

  response.json().then(function (result) {

    const polyline = result.routes[0].geometry;

  }),

};

// require

var roadData = require('data/route.json')

后面基本一样了，就以fetch为准，现在把线路加载的剩余部分补充完整：

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fetch('data/route.json').then(function (response) {

  response.json().then(function (result) {

    const polyline = result.routes[0].geometry;

    // 线路数据坐标系转换

    const route = new Polyline({

      factor: 1e6,

    }).readGeometry(polyline, {

      dataProjection: 'EPSG:4326',

      featureProjection: 'EPSG:3857',

    });

    // 线路图层要素

    const routeFeature = new Feature({

      type: 'route',

      geometry: route,

    });

    // 起点要素

    const startMarker = new Feature({

      type: 'icon',

      geometry: new Point(route.getFirstCoordinate()),

    });

    // 终点要素

    const endMarker = new Feature({

      type: 'icon',

      geometry: new Point(route.getLastCoordinate()),

    });

    // 取起点值

    const position = startMarker.getGeometry().clone();

    // 游标要素

    const geoMarker = new Feature({

      type: 'geoMarker',

      geometry: position,

    });

    // 样式组合

    const styles = {

        // 路线

      'route': new Style({

        stroke: new Stroke({

          width: 6,

          color: [237, 212, 0, 0.8],

        }),

      }),

      'icon': new Style({

        image: new Icon({

          anchor: [0.5, 1],

          src: 'data/icon.png',

        }),

      }),

      'geoMarker': new Style({

        image: new CircleStyle({

          radius: 7,

          fill: new Fill({color: 'black'}),

          stroke: new Stroke({

            color: 'white',

            width: 2,

          }),

        }),

      }),

    };

    // 创建图层并添加以上要素集合

    const vectorLayer = new VectorLayer({

      source: new VectorSource({

        features: [routeFeature, geoMarker, startMarker, endMarker],

      }),

      style: function (feature) {

        return styles[feature.get('type')];

      },

    });

    // 在地图容器中添加图层

    map.addLayer(vectorLayer);

以上代码很完整，我加了注释，整体思路总结如下：

  • 先加载路线数据
  • 构造路线、起始点及游标对应图层要素对象
  • 构造图层并把要素添加进去
  • 在地图容器中添加图层

添加起、终点

这个上面的代码已经包括了，我这里列出来是为了让你更清晰，就是startMarker和endMarker对应的代码。

添加小车

同样的，这里的代码在上面也写过了，就是geoMarker所对应的代码。

准备开车

线路有了，车也有了，现在就到了激动人心的开车时刻了，接下来才是本文最核心的代码！

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const speedInput = document.getElementById('speed');

    const startButton = document.getElementById('start-animation');

    let animating = false;

    let distance = 0;

    let lastTime;

    function moveFeature(event) {

      const speed = Number(speedInput.value);

      // 获取当前渲染帧状态时刻

      const time = event.frameState.time;

      // 渲染时刻减去开始播放轨迹的时间

      const elapsedTime = time - lastTime;

      // 求得距离比

      distance = (distance + (speed * elapsedTime) / 1e6) % 2;

      // 刷新上一时刻

      lastTime = time;

      // 反减可实现反向运动，获取坐标点

      const currentCoordinate = route.getCoordinateAt(

        distance > 1 ? 2 - distance : distance

      );

      position.setCoordinates(currentCoordinate);

      // 获取渲染图层的画布

      const vectorContext = getVectorContext(event);

      vectorContext.setStyle(styles.geoMarker);

      vectorContext.drawGeometry(position);

      map.render();

    }

    function startAnimation() {

      animating = true;

      lastTime = Date.now();

      startButton.textContent = 'Stop Animation';

      vectorLayer.on('postrender', moveFeature);

      // 隐藏小车前一刻位置同时触发事件

      geoMarker.setGeometry(null);

    }

    function stopAnimation() {

      animating = false;

      startButton.textContent = '开车了';

      // 将小车固定在当前位置

      geoMarker.setGeometry(position);

      vectorLayer.un('postrender', moveFeature);

    }

    startButton.addEventListener('click', function () {

      if (animating) {

        stopAnimation();

      } else {

        startAnimation();

      }

    });

简单说下它的原理就是利用postrender事件触发一个函数，这个事件本来是地图渲染结束事件，但是它的回调函数中，小车的坐标位置一直在变，那就会不停地触发地图渲染，当然最终也会触发postrender。这样就实现的小车沿着轨迹的动画效果了。这段代码有点难理解，最好自己尝试体验下，比较难理解部分我都加上了注释。

好了，ol动态巡查已经介绍完了，动手试下吧！看你的车能否开起来？

完整代码

index.html

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<!DOCTYPE html>

<html lang="en">

  <head>

    <meta charset="UTF-8">

    <title>Marker Animation</title>

    <!-- Pointer events polyfill for old browsers, see https://caniuse.com/#feat=pointer -->

    <script src="https://unpkg.com/elm-pep"></script>

    <style>

      .map {

        width: 100%;

        height:400px;

      }

    </style>

  </head>

  <body>

    <div id="map" class="map"></div>

    <label for="speed">

      speed:

      <input id="speed" type="range" min="10" max="999" step="10" value="60">

    </label>

    <button id="start-animation">Start Animation</button>

    <script src="main.js"></script>

  </body>

</html>

main.js

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import 'ol/ol.css';

import Feature from 'ol/Feature';

import Map from 'ol/Map';

import Point from 'ol/geom/Point';

import Polyline from 'ol/format/Polyline';

import VectorSource from 'ol/source/Vector';

import View from 'ol/View';

import XYZ from 'ol/source/XYZ';

import {

  Circle as CircleStyle,

  Fill,

  Icon,

  Stroke,

  Style,

} from 'ol/style';

import {Tile as TileLayer, Vector as VectorLayer} from 'ol/layer';

import {getVectorContext} from 'ol/render';

const key = 'Get your own API key at https://www.maptiler.com/cloud/';

const attributions =

  '<a href="https://www.maptiler.com/copyright/" rel="external nofollow"  target="_blank">&copy; MapTiler</a> ' +

  '<a href="https://www.openstreetmap.org/copyright" rel="external nofollow"  target="_blank">&copy; OpenStreetMap contributors</a>';

const center = [-5639523.95, -3501274.52];

const map = new Map({

  target: document.getElementById('map'),

  view: new View({

    center: center,

    zoom: 10,

    minZoom: 2,

    maxZoom: 19,

  }),

  layers: [

    new TileLayer({

      source: new XYZ({

        attributions: attributions,

        url: 'https://api.maptiler.com/maps/hybrid/{z}/{x}/{y}.jpg?key=' + key,

        tileSize: 512,

      }),

    }),

  ],

});

// The polyline string is read from a JSON similiar to those returned

// by directions APIs such as Openrouteservice and Mapbox.

fetch('data/polyline/route.json').then(function (response) {

  response.json().then(function (result) {

    const polyline = result.routes[0].geometry;

    const route = new Polyline({

      factor: 1e6,

    }).readGeometry(polyline, {

      dataProjection: 'EPSG:4326',

      featureProjection: 'EPSG:3857',

    });

    const routeFeature = new Feature({

      type: 'route',

      geometry: route,

    });

    const startMarker = new Feature({

      type: 'icon',

      geometry: new Point(route.getFirstCoordinate()),

    });

    const endMarker = new Feature({

      type: 'icon',

      geometry: new Point(route.getLastCoordinate()),

    });

    const position = startMarker.getGeometry().clone();

    const geoMarker = new Feature({

      type: 'geoMarker',

      geometry: position,

    });

    const styles = {

      'route': new Style({

        stroke: new Stroke({

          width: 6,

          color: [237, 212, 0, 0.8],

        }),

      }),

      'icon': new Style({

        image: new Icon({

          anchor: [0.5, 1],

          src: 'data/icon.png',

        }),

      }),

      'geoMarker': new Style({

        image: new CircleStyle({

          radius: 7,

          fill: new Fill({color: 'black'}),

          stroke: new Stroke({

            color: 'white',

            width: 2,

          }),

        }),

      }),

    };

    const vectorLayer = new VectorLayer({

      source: new VectorSource({

        features: [routeFeature, geoMarker, startMarker, endMarker],

      }),

      style: function (feature) {

        return styles[feature.get('type')];

      },

    });

    map.addLayer(vectorLayer);

    const speedInput = document.getElementById('speed');

    const startButton = document.getElementById('start-animation');

    let animating = false;

    let distance = 0;

    let lastTime;

    function moveFeature(event) {

      const speed = Number(speedInput.value);

      const time = event.frameState.time;

      const elapsedTime = time - lastTime;

      distance = (distance + (speed * elapsedTime) / 1e6) % 2;

      lastTime = time;

      const currentCoordinate = route.getCoordinateAt(

        distance > 1 ? 2 - distance : distance

      );

      position.setCoordinates(currentCoordinate);

      const vectorContext = getVectorContext(event);

      vectorContext.setStyle(styles.geoMarker);

      vectorContext.drawGeometry(position);

      // tell OpenLayers to continue the postrender animation

      map.render();

    }

    function startAnimation() {

      animating = true;

      lastTime = Date.now();

      startButton.textContent = 'Stop Animation';

      vectorLayer.on('postrender', moveFeature);

      geoMarker.setGeometry(null);

    }

    function stopAnimation() {

      animating = false;

      startButton.textContent = '开车了';

      geoMarker.setGeometry(position);

      vectorLayer.un('postrender', moveFeature);

    }

    startButton.addEventListener('click', function () {

      if (animating) {

        stopAnimation();

      } else {

        startAnimation();

      }

    });

  });

});

package.json

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{

  "name": "feature-move-animation",

  "dependencies": {

    "ol": "6.9.0"

  },

  "devDependencies": {

    "parcel": "^2.0.0-beta.1"

  },

  "scripts": {

    "start": "parcel index.html",

    "build": "parcel build --public-url . index.html"

  }

}

 

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