OpenLayers学习笔记（八）— 类似比例尺的距离环（二）

openlayers 3 地图上创建一个距离环，始终以地图中心为中心，每个环之间的距离类似比例尺，随地图缩放而变化。

添加具有覆盖整个范围的特征的虚拟层，其可以被设置为围绕地图中心的环。

这篇是上一篇距离环文章的拓展和完善

GitHub:八至

作者：狐狸家的鱼

在线预览
这是模仿openlayers插件库ol-ext新出的canvas距离环功能，简直雪中送炭。
代码：
<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

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

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

    <title>距离环</title>

    <link rel="stylesheet" href="css/ol.css" type="text/css">

    <script src="build/jquery.js"></script>

    <script src="build/ol.js"></script>

    <style>

        html,

        body {

            height: 100%;

            width: 100%;

            padding: 0px;

            margin: 0px;

        }

        .map {

            height: 100%;

            width: 100%;

        }

    </style>

</head>

<body>

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

    <script>

        var map = new ol.Map({

            layers: [

                new ol.layer.Tile({

                    source: new ol.source.OSM(),

                })

            ],

            target: 'map',

            view: new ol.View()

        });

        ol.control.CanvasScaleLine = function(options) {

            ol.control.ScaleLine.call(this, options);

            this.scaleHeight_ = 6;

            // Get style options

            if (!options) options={};

            if (!options.style) options.style = new ol.style.Style();

            this.setStyle(options.style);

        }

        ol.inherits(ol.control.CanvasScaleLine, ol.control.ScaleLine);

        ol.control.CanvasScaleLine.prototype.setMap = function (map) {

            var oldmap = this.getMap();

            if (this._listener) ol.Observable.unByKey(this._listener);

            this._listener = null;

            ol.control.ScaleLine.prototype.setMap.call(this, map);

            if (oldmap) oldmap.renderSync();

            // Add postcompose on the map

            if (map) {

                this._listener = map.on('postcompose', this.drawScale_.bind(this));

            }

            // Hide the default DOM element

            this.element.style.visibility = 'hidden';

            this.olscale = this.element.querySelector(".ol-scale-line-inner");

        }

        ol.control.CanvasScaleLine.prototype.setStyle = function (style) {

            var stroke = style.getStroke();

            this.strokeStyle_ = stroke ? ol.color.asString(stroke.getColor()) : "#000";

            this.strokeWidth_ = stroke ? stroke.getWidth() : 2;

            var fill = style.getFill();

            this.fillStyle_ = fill ? ol.color.asString(fill.getColor()) : "#fff";

            var text = style.getText();

            this.font_ = text ? text.getFont() : "10px Arial";

            stroke = text ? text.getStroke() : null;

            fill = text ? text.getFill() : null;

            this.fontStrokeStyle_ = stroke ? ol.color.asString(stroke.getColor()) : this.fillStyle_;

            this.fontStrokeWidth_ = stroke ? stroke.getWidth() : 3;

            this.fontFillStyle_ = fill ? ol.color.asString(fill.getColor()) : this.strokeStyle_;

            // refresh

            if (this.getMap()) this.getMap().render();

        }

        ol.control.CanvasScaleLine.prototype.drawScale_ = function(e) {

            if ( this.element.style.visibility!=="hidden" ) return;

            var ctx = e.context;

            // Get size of the scale div

            var scalewidth = parseInt(this.olscale.style.width);

            if (!scalewidth) return;

            var text = this.olscale.textContent;

            var position = {left: this.element.offsetLeft, top: this.element.offsetTop};

            // Retina device

            var ratio = e.frameState.pixelRatio;

            ctx.save();

            ctx.scale(ratio,ratio);

            // Position if transform:scale()

            var container = this.getMap().getTargetElement();

            var scx = container.offsetWidth / container.getBoundingClientRect().width;

            var scy = container.offsetHeight / container.getBoundingClientRect().height;

            position.left *= scx;

            position.top *= scy;

            // On top

            position.top += this.element.clientHeight - this.scaleHeight_;

            // Draw scale text

            ctx.beginPath();

            ctx.strokeStyle = this.fontStrokeStyle_;

            ctx.fillStyle = this.fontFillStyle_;

            ctx.lineWidth = this.fontStrokeWidth_;

            ctx.textAlign = "center";

            ctx.textBaseline ="bottom";

            ctx.font = this.font_;

            ctx.strokeText(text, position.left+scalewidth/2, position.top);

            ctx.fillText(text, position.left+scalewidth/2, position.top);

            ctx.closePath();

            // Draw scale bar

            position.top += 2;

            ctx.lineWidth = this.strokeWidth_;

            ctx.strokeStyle = this.strokeStyle_;

            var max = 4;

            var n = parseInt(text);

            while (n%10 === 0) n/=10;

            if (n%5 === 0) max = 5;

            for (var i=0; i<max; i++) {

                ctx.beginPath();

                ctx.fillStyle = i%2 ? this.fillStyle_ : this.strokeStyle_;

                ctx.rect(position.left+i*scalewidth/max, position.top, scalewidth/max, this.scaleHeight_);

                ctx.stroke();

                ctx.fill();

                ctx.closePath();

            }

            // Calculate and draw rings corresponding to scale line

            var map = this.getMap();

            var proj = map.getView().getProjection();

            var center = ol.proj.transform(map.getView().getCenter(), proj, 'EPSG:4326');

            var sphere = ol.Sphere ? new ol.Sphere(6371008.8) : undefined;

            var t = text.split(' ');

            var multiplier = t[0]/max;

            switch (t[1]) {

                case 'km':

                    multiplier *= 1000;

                    break;

                case 'm':

                    multiplier *= 1;

                    break;

                case 'mm':

                    multiplier *= 0.001;

                    break;

                default:

            }

            for (var i=0; i<max; i++) {

                var geometry = sphere ? ol.geom.Polygon.circular(sphere, center, (i+1)*multiplier, 128).transform('EPSG:4326', proj)

                                    : ol.geom.Polygon.circular(center, (i+1)*multiplier, 128).transform('EPSG:4326', proj);

                var coordinates = geometry.getCoordinates()[0];

                var pixels = [];

                var top = [0, Infinity];

                coordinates.forEach( function(coordinate) {

                    var pixel = map.getPixelFromCoordinate(coordinate);

                    pixels.push(pixel);

                    if (pixel[1] < top[1]) { top = pixel; }

                });

                ctx.moveTo(pixels[0][0], pixels[0][1]);

                for (var j=1; j<pixels.length; j++) {

                    ctx.lineTo(pixels[j][0], pixels[j][1]);

                }

                ctx.lineWidth = this.strokeWidth_;

                ctx.strokeStyle = this.strokeStyle_;

                ctx.stroke();

            }

            // Draw scale text above the rings

            ctx.beginPath();

            ctx.strokeStyle = this.fontStrokeStyle_;

            ctx.fillStyle = this.fontFillStyle_;

            ctx.lineWidth = this.fontStrokeWidth_;

            ctx.textAlign = "center";

            ctx.textBaseline ="bottom";

            ctx.font = this.font_;

            ctx.strokeText(text, top[0], top[1]-this.strokeWidth_);

            ctx.fillText(text, top[0], top[1]-this.strokeWidth_);

            ctx.closePath();

            ctx.restore();

        }

        var proj = map.getView().getProjection();

        map.getView().setCenter(ol.proj.transform([-38, 75.9], 'EPSG:4326', proj));

        map.getView().setZoom(2);

        map.addControl(new ol.control.CanvasScaleLine());

    </script>

</body>

</html>
巴特西

OpenLayers学习笔记（八）— 类似比例尺的距离环（二）

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