前言：OL可以让我们实现自定义的标注样式，用过CAD的同学应该都知道CAD中有上下文字中间用横线隔开的样式，这样标注让人一目了然的了解信息。给人以直观的的信息表达，本文实现就是分式的标注。
先来张图：
一、关于ol.style.Icon类
该类中有一个img参数，该参数非常好用，该参数的输入类型为Image | HTMLCanvasElement，第二个类型就是canvas标签，作为一个GIS开发人员，相信大家对canvas标签不在陌生，支持我们画一些图形等，但是该参数必须和imgSize参数一块使用。
二、实现原理
上下的文字通过设置ol.style.Text样式，底部的文字进行在Y方向上偏移一部分，中间的运用ol.style.Icon，用canvas画横线，将横线通过ol.style.Icon封成一个ol.style.Style，最终通过一个函数将三者的结合一起来返回一个styles数组，赋值vector图层的styleFunc函数
三、主要代码
//构造顶部文字样式的样式
var createTextStyleA = function (fieldValue) {
var style = new ol.style.Style({
text: new ol.style.Text({
textAlign: “center”,
textBaseline: “middle”,
font: ” 20px serif”,
text: “北京”,
fill: new ol.style.Fill({ color: ‘green’ }),
stroke: new ol.style.Stroke({ color: ‘rgba(255,255,255,0)’ }),
offsetX: 10,
offsetY: 0,
rotation: 0
})
});
return style;
}
//构造底部文字样式
var createTextStyleB = function (fieldValue) {
var style = new ol.style.Style({
text: new ol.style.Text({
textAlign: “center”,
textBaseLine: “middle”,
font: “20px serif”,
text: “30万”,
fill: new ol.style.Fill({ color: ‘green’ }),
stroke: new ol.style.Stroke({ color: ‘rgba(255,255,255,0)’ }),
offsetX: 10,
offsetY: 20,
rotation: 0
})
});
return style;
}
//构造线样式
var createLineStyle = function (textA) {
var canvas = document.createElement(‘canvas’);
canvas.width = 80;
canvas.height = 20;
var ctx = canvas.getContext(“2d”);
ctx.lineWidth = 2;
ctx.strokeStyle = “red”;
ctx.beginPath(); //新建一条path
ctx.moveTo(0, 0);
ctx.lineTo(textA.length*30,0);
//画线
ctx.stroke();
//封闭路径
ctx.closePath();
var lineStyle = new ol.style.Style({
image: new ol.style.Icon({
img: canvas,
imgSize: [canvas.width, canvas.height],
rotation: 180 * Math.PI/180
})
});
return lineStyle;
}
//三种样式结合
var combineStyle = function (textA,textB) {
var styles = [];
var textStyleA = createTextStyleA(textA);
var line = createLineStyle(textA);
var textStyleB = createTextStyleB(textB);
styles.push(textStyleA);
styles.push(line);
styles.push(textStyleB);
return styles;
}
该部分的代码我只是进行简单的应用，有兴趣的同学可以进步进行封装，横线的图标需要进行适当的旋转，还有进行横线的长度，应该选用最长的文字那部分，作为横线的长度。
四、完整demo
<!DOCTYPE html>
<html>
<head>
<meta charset=”utf-8″ />
<title>自定义标注</title>
<link rel=”stylesheet” href=”https://openlayers.org/en/v4.5.0/css/ol.css” type=”text/css”>
<script src=”https://openlayers.org/en/v4.5.0/build/ol.js”></script>
<script src=”../../Scripts/jquery/jquery-3.1.1.min.js”></script>
<style type=”text/css”>
body, #map {
border: 0px;
margin: 0px;
padding: 0px;
width: 100%;
height: 100%;
font-size: 13px;
overflow: hidden;
}
</style>
</head>
<body>
<div id=”map”></div>
<script>
var projection = new ol.proj.Projection({
code: ‘EPSG:4326’,
units: ‘degrees’
});
function getNavmapLayer() {
return new ol.layer.Tile({
source: new ol.source.XYZ({
url: ‘http://webrd01.is.autonavi.com/appmaptile?x={x}&y={y}&z={z}&lang=zh_cn&size=1&scale=1&style=8’//7,8
}),
projection: projection
});
}
var navlayer = getNavmapLayer();
map = new ol.Map({
controls: ol.control.defaults({
attribution: false
}),
target: ‘map’,
layers: [navlayer],
view: new ol.View({
projection: projection,
center: [116.456, 40.251],
zoom: 4
})
});
var coordinate = [116.456, 40.251];
//构造顶部文字样式的样式
var createTextStyleA = function (fieldValue) {
var style = new ol.style.Style({
text: new ol.style.Text({
textAlign: “center”,
textBaseline: “middle”,
font: ” 20px serif”,
text: “北京”,
fill: new ol.style.Fill({ color: ‘green’ }),
stroke: new ol.style.Stroke({ color: ‘rgba(255,255,255,0)’ }),
offsetX: 10,
offsetY: 0,
rotation: 0
})
});
return style;
}
//构造底部文字样式
var createTextStyleB = function (fieldValue) {
var style = new ol.style.Style({
text: new ol.style.Text({
textAlign: “center”,
textBaseLine: “middle”,
font: “20px serif”,
text: “30万”,
fill: new ol.style.Fill({ color: ‘green’ }),
stroke: new ol.style.Stroke({ color: ‘rgba(255,255,255,0)’ }),
offsetX: 10,
offsetY: 20,
rotation: 0
})
});
return style;
}
//构造线样式
var createLineStyle = function (textA) {
var canvas = document.createElement(‘canvas’);
canvas.width = 80;
canvas.height = 20;
var ctx = canvas.getContext(“2d”);
ctx.lineWidth = 2;
ctx.strokeStyle = “red”;
ctx.beginPath(); //新建一条path
ctx.moveTo(0, 0);
ctx.lineTo(textA.length*30,0);
//画线
ctx.stroke();
//封闭路径
ctx.closePath();
var lineStyle = new ol.style.Style({
image: new ol.style.Icon({
img: canvas,
imgSize: [canvas.width, canvas.height],
rotation: 180 * Math.PI/180
})
});
return lineStyle;
}
//三种样式结合
var combineStyle = function (textA,textB) {
var styles = [];
var textStyleA = createTextStyleA(textA);
var line = createLineStyle(textA);
var textStyleB = createTextStyleB(textB);
styles.push(textStyleA);
styles.push(line);
styles.push(textStyleB);
return styles;
}
var source = new ol.source.Vector();
var vector = new ol.layer.Vector({
source: source,
style: combineStyle(“北京”)
});
map.addLayer(vector);
var point = new ol.geom.Point(coordinate);
var feature = new ol.Feature({
geometry: point,
});
source.addFeature(feature);
</script>
</body>
</html>