概述：
本文讲述如何在openlayers中实现画圆。
效果：
实现思路：
1、画中心点
通过OpenLayers.Control.DrawFeature和OpenLayers.Handler.Point实现在地图上画圆心。
2、移动鼠标设置半径
画点结束后，激活地图的鼠标移动事件，获取圆心到鼠标点的距离，并画圆，将结果展示到地图上。
3、点击地图结束绘制
点击地图，结束绘制，将最终结果展示到地图上，并停止地图的鼠标移动事件。
实现代码：
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>china EPSG:4326 image/png</title>
<link rel="stylesheet" type="text/css" href="http://localhost/olapi/theme/default/style.css"/>
<style type="text/css">
body { font-family: sans-serif; font-weight: bold; font-size: .8em; }
body { border: 0px; margin: 0px; padding: 0px; }
#map { 100%; height: 100%; border: 0px; padding: 0px; }
.query-box{
position: absolute;
top: 15pt;
right: 15pt;
z-index:1001;
border: 1px solid #ff0000;
border-radius: 3px;
background: #f2f2f2;
padding: 5px 8px;
font-family: "Trebuchet MS", Helvetica, Arial, sans-serif;
}
</style>
<script type="text/javascript" src="http://localhost/olapi/OpenLayers.js"></script>
<script type="text/javascript" src="http://localhost/olapi/lib/OpenLayers/Lang/zh-CN.js"></script>
<script src="http://localhost/jquery/jquery-1.8.3.js"></script>
<script src="circle.js"></script>
<script type="text/javascript">
var map, wfs;
function init(){
var bounds = new OpenLayers.Bounds(
87.57582859314434, 19.969920291221204,
126.56713756740385, 45.693810203800794
);
var options = {
controls: [],
maxExtent: bounds,
maxResolution: 0.1523098006807012,
projection: "EPSG:4326",
units: 'degrees'
};
map = new OpenLayers.Map('map', options);
var tiled = new OpenLayers.Layer.WMS(
"province", "http://localhost:8081/geoserver/lzugis/wms",
{
"LAYERS": 'province',
"STYLES": '',
format: 'image/png'
},
{
buffer: 0,
displayOutsideMaxExtent: true,
isBaseLayer: true,
yx : {'EPSG:4326' : true}
}
);
map.addLayer(tiled);
map.addControl(new OpenLayers.Control.PanZoomBar({
position: new OpenLayers.Pixel(2, 15)
}));
map.addControl(new OpenLayers.Control.Navigation());
map.zoomToExtent(bounds);
var drawLayer = new OpenLayers.Layer.Vector("drawLayer",{
styleMap: new OpenLayers.StyleMap()
});
var showLayer = new OpenLayers.Layer.Vector("showLayer",{
styleMap: new OpenLayers.StyleMap({'default':{
strokeColor: "#ff0000",
strokeOpacity: 1,
strokeWidth: 1,
fillColor: "#000000",
fillOpacity: 0.1
}})
});
map.addLayers([drawLayer,showLayer]);
var drawCtrl = new OpenLayers.Control.DrawFeature(drawLayer, OpenLayers.Handler.Point);
map.addControl(drawCtrl);
drawCtrl.featureAdded = onEndDraw;
function onEndDraw(feature){
drawCtrl.deactivate();
drawLayer.setVisibility(true);
var start = feature.geometry;
drawLayer.addFeatures(start);
var circle,moveEvt;
map.events.register("mousemove",map,function(e){
moveEvt = e;
drawLayer.removeAllFeatures();
var pt = map.getLonLatFromPixel(new OpenLayers.Pixel(e.x, e.y));
var end = new OpenLayers.Geometry.Point(pt.lon, pt.lat);
var line = new OpenLayers.Geometry.LineString([start, end]);
var radius = line.getLength();
var geometry = createCircle(start,radius,100,0);
var feature = new OpenLayers.Feature.Vector(geometry);
circle = feature;
drawLayer.addFeatures(feature);
});
map.events.register("click",map,function(e){
showLayer.removeAllFeatures();
showLayer.addFeatures(circle);
drawLayer.setVisibility(false);
});
}
$("#draw").on("click",function(){
drawLayer.removeAllFeatures();
showLayer.removeAllFeatures();
drawCtrl.activate();
});
}
</script>
</head>
<body onLoad="init()">
<div class="query-box">
<button id="draw">画圆</button>
</div>
<div id="map"></div>
</body>
</html>在上文中用到了createCircle，该函数如下：
/**
* Method: createCircle
* Create a regular polygon around a radius. Useful for creating circles
* and the like.
*
* Parameters:
* origin - {<OpenLayers.Geometry.Point>} center of polygon.
* radius - {Float} distance to vertex, in map units.
* sides - {Integer} Number of sides. 20 approximates a circle.
* rotation - {Float} original angle of rotation, in degrees.
*/
function createCircle(origin, radius, sides, rotation) {
var angle = Math.PI * ((1/sides) - (1/2));
if(rotation) {
angle += (rotation / 180) * Math.PI;
}
var rotatedAngle, x, y;
var points = [];
for(var i=0; i<sides; ++i) {
rotatedAngle = angle + (i * 2 * Math.PI / sides);
x = origin.x + (radius * Math.cos(rotatedAngle));
y = origin.y + (radius * Math.sin(rotatedAngle));
points.push(new OpenLayers.Geometry.Point(x, y));
}
var ring = new OpenLayers.Geometry.LinearRing(points);
return new OpenLayers.Geometry.Polygon([ring]);
};