七章-80-动态点的效果
2020-03-27 本文已影响0人
彩云飘过
本文基于腾讯课堂老胡的课《跟我学Openlayers--基础实例详解》做的学习笔记，使用的openlayers 5.3.x api。
源码 见 1080.html ，对应的 官网示例 https://openlayers.org/en/latest/examples/dynamic-data.html
image.png
核心技术点：
map.on('postcompose', function(event) {})
map在渲染之后会触发一个postcompose消息，在这个监听函数中，使用cavas绘制好此次渲染的每个点的位置以及样式，最后再调用一次render()，继续触发postcompose，如此形成不间断的cavas的重绘。
image.png
<!DOCTYPE html>
<html>
<head>
<title>动态点的效果</title>
<link rel="stylesheet" href="../include/ol.css" type="text/css" />
<script src="../include/ol.js"></script>
</head>
<style></style>
<body>
<div id="map" class="map"></div>
<div id="info"> </div>
<script>
var map = new ol.Map({
layers: [
new ol.layer.Tile({
source: new ol.source.OSM()
})
],
target: 'map',
view: new ol.View({
center: [0, 0],
zoom: 2
})
});
//用于动态的样式
var imageStyle = new ol.style.Style({
image: new ol.style.Circle({
radius: 5,
fill: new ol.style.Fill({color: 'yellow'}),
stroke: new ol.style.Stroke({color: 'red', width: 1})
})
});
var headInnerImageStyle = new ol.style.Style({
image: new ol.style.Circle({
radius: 2,
fill: new ol.style.Fill({color: 'blue'})
})
});
var headOuterImageStyle = new ol.style.Style({
image: new ol.style.Circle({
radius: 5,
fill: new ol.style.Fill({color: 'black'})
})
});
var n = 200;
var omegaTheta = 30000;
var R = 7e6;
var r = 2e6;
var p = 2e6;
//关键点：postcompose 渲染完成之后 ； precompose 渲染完成之前
map.on('postcompose', function(event) {
var vectorContext = event.vectorContext;
var frameState = event.frameState;
var theta = 2 * Math.PI * frameState.time / omegaTheta;
var coordinates = [];
var i;
for (i = 0; i < n; ++i) {
var t = theta + 2 * Math.PI * i / n;
var x = (R + r) * Math.cos(t) + p * Math.cos((R + r) * t / r);
var y = (R + r) * Math.sin(t) + p * Math.sin((R + r) * t / r);
coordinates.push([x, y]);
}
vectorContext.setStyle(imageStyle);
vectorContext.drawGeometry(new ol.geom.MultiPoint(coordinates));
//头部的效果，获取头部的坐标，设置内部样式/外部样式
var headPoint = new ol.geom.Point(coordinates[coordinates.length - 1]);
vectorContext.setStyle(headOuterImageStyle);
vectorContext.drawGeometry(headPoint);
vectorContext.setStyle(headInnerImageStyle);
vectorContext.drawGeometry(headPoint);
map.render(); //render调用时会再次触发postcompose
});
map.render();
</script>
</body>
</html>
