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 1050.html ，对应的 官网示例 https://openlayers.org/en/latest/examples/draw-features.html?q=Drag
https://openlayers.org/en/latest/examples/draw-freehand.html?q=interaction
freehand 属性，在徒手模式下操作线条、多边形和圆。这使得交互总是在徒手模式下进行，并优先于任何徒手条件选项。
image.png
<!DOCTYPE html>
<html>
<head>
<title>绘制点线面</title>
<link rel="stylesheet" href="../include/ol.css" type="text/css" />
<script src="../include/ol.js"></script>
</head>
<style>
</style>
<body>
<form class="form-inline">
<label>几何类型 </label>
<select id="type">
<option value="Point">Point</option>
<option value="LineString">LineString</option>
<option value="Polygon">Polygon</option>
<option value="Circle">Circle</option>
<option value="None">无</option>
</select>
</form>
<div id="map" class="map"></div>
<script>
var raster = new ol.layer.Tile({
source: new ol.source.OSM()
});
var source = new ol.source.Vector({
wrapX: false //地图缩放后，世界地图重复出现，这里控制绘制的要素是否重复重现，默认为true
});
var vector = new ol.layer.Vector({
source: source
});
var map = new ol.Map({
layers: [raster, vector],
target: 'map',
view: new ol.View({
center: [-11000000, 4600000],
zoom: 4
})
});
var typeSelect = document.getElementById('type');
var draw;
function addInteraction() {
var value = typeSelect.value;
if (value !== 'None') {
//关键点，定义交互绘制操作
draw = new ol.interaction.Draw({
source: source,
type: typeSelect.value
});
map.addInteraction(draw);
}
}
typeSelect.onchange = function () {
map.removeInteraction(draw); //将原有的交互操作去掉
addInteraction();
};
addInteraction();
</script>
</body>
</html>
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