Set a boundary mask to TileLayer
index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>TileLayer and Geo-Projections - Set a boundary mask to TileLayer</title>
  <style type="text/css">
    html,body{margin:0px;height:100%;width:100%}
    .container{width:100%;height:100%}
  </style>
  <link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/[email protected]/dist/maptalks.css">
  <script type="text/javascript" src="https://cdn.jsdelivr.net/npm/[email protected]/dist/maptalks.min.js"></script>
  <body>
    <script type="text/javascript" src="boundary.js"></script>
    <div id="map" class="container"></div>

    <script>
      var map = new maptalks.Map('map', {
        center:     [114.26529, 30.60545],
        zoom:  13,
        pitch : 60,
        baseLayer : new maptalks.TileLayer('base',{
          urlTemplate: 'http://{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>'
        })
      });

      //Jianghan district's boundary, from boundary.js
      var mask = new maptalks.Polygon(boundary, {
        'symbol' : [
          {
            'lineColor' : '#ccc',
            'lineWidth' : 8,
            'polygonFillOpacity' : 0
          },
          {
            'lineColor' : '#404040',
            'lineWidth' : 6,
            'polygonFillOpacity' : 0
          }
        ]
      });

      //Copy the mask to add as mask's outline
      var outline = mask.copy();

      var maskedLayer = new maptalks.TileLayer('masked', {
        'urlTemplate' : 'http://{s}.basemaps.cartocdn.com/dark_all/{z}/{x}/{y}.png',
        'subdomains'  : ['a','b','c','d']
      })
      .setMask(mask) // set boundary as the mask to the tilelayer
      .addTo(map);

      //District's name
      var title = new maptalks.Marker(mask.getCenter(), {
        'symbol' : {
          'textName' : 'JiangHan District',
          'textFaceName' : 'sans-serif',
          'textSize' : 32,
          'textFill' : '#1bbc9b',
          'textHaloFill' : '#fff',
          'textHaloRadius' : 5,
          'textDx' : -30
        }
      });
      new maptalks.VectorLayer('v', [outline, title]).addTo(map);

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