index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>Geometry Styles - Line gradient fill</title>
  <style type="text/css">
    html,body{margin:0px;height:100%;width:100%}
    .container{width:100%;height:100%}
  </style>
  <link rel="stylesheet" href="https://unpkg.com/maptalks/dist/maptalks.css">
  <script type="text/javascript" src="https://unpkg.com/maptalks/dist/maptalks.min.js"></script>
  <body>

    <div id="map" class="container"></div>
    <script>
      var map = new maptalks.Map('map', {
        center: [-0.113049, 51.49856],
        zoom: 14,
        baseLayer: new maptalks.TileLayer('base', {
          urlTemplate: 'https://{s}.basemaps.cartocdn.com/light_all/{z}/{x}/{y}.png',
          subdomains: ["a","b","c","d"],
          attribution: '&copy; <a href="http://osm.org">OpenStreetMap</a> contributors, &copy; <a href="https://carto.com/">CARTO</a>'
        })
      });

      var layer = new maptalks.VectorLayer('vector').addTo(map);

      var c = map.getCenter();

      var line = new maptalks.LineString(
        [c.sub(0.01, 0), c],
        {
          symbol:{
            // linear gradient
            'lineColor' : {
              'type' : 'linear',
              'colorStops' : [
                [0.00, 'red'],
                [1 / 4, 'orange'],
                [2 / 4, 'green'],
                [3 / 4, 'aqua'],
                [1.00, 'white']
              ]
            },
            'lineWidth' : 10
          }
        }).addTo(layer);

      var line1 = new maptalks.LineString(
        [c.sub(0.015, 0.005), c.sub(0, 0.005)],
        {
          symbol:{
            // radial gradient
            'lineColor' : {
              'type' : 'radial',
              'colorStops' : [
                [0.00, 'red'],
                [1 / 3, 'orange'],
                [2 / 3, 'green'],
                [1.00, 'white']
              ]
            },
            'lineWidth' : 10
          }
        }).addTo(layer);

    </script>
  </body>
</html>
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