GroupTileLayer is used to display a group of tile layers. Its performance is better than add TileLayers seperately and it can prevent the warning:

"WARNING: Too many active WebGL contexts. Oldest context will be lost" on chrome

index.html
<!DOCTYPE html>
<html>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1">
  <title>TileLayer and Geo-Projections - Add GroupTileLayer</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>
    <div id="map" class="container"></div>

    <script>
      var map = new maptalks.Map('map', {
        center:     [-0.113049,51.498568],
        zoom:  6,
        pitch : 40,
        attribution: {
          content: '&copy; <a href="http://osm.org">OpenStreetMap</a> contributors, &copy; <a href="https://carto.com/">CARTO</a>, &copy; BoudlessGeo'
        },
        // add 2 TileLayers with a GroupTileLayer
        baseLayer : new maptalks.GroupTileLayer('base', [
          new maptalks.TileLayer('tile2', {
            urlTemplate: 'http://{s}.basemaps.cartocdn.com/dark_nolabels/{z}/{x}/{y}.png',
            subdomains  : ['a','b','c','d']
          }),

          new maptalks.WMSTileLayer('wms', {
            'urlTemplate' : 'https://demo.boundlessgeo.com/geoserver/ows',
            'crs' : 'EPSG:3857',
            'layers' : 'ne:ne',
            'styles' : '',
            'version' : '1.3.0',
            'format': 'image/png',
            'transparent' : true,
            'uppercase' : true
          })
        ])
      });

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