Type alias TransformStyleFunction

Part of Map#setStyle options, transformStyle is a convenience function that allows to modify a style after it is fetched but before it is committed to the map state this function exposes previous and next styles, it can be commonly used to support a range of functionalities like: when previous style carries certain 'state' that needs to be carried over to a new style gracefully when a desired style is a certain combination of previous and incoming style when an incoming style requires modification based on external state

Returns

resulting style that will to be applied to the map

Example

map.setStyle('https://demotiles.maplibre.org/style.json', {
  transformStyle: (previousStyle, nextStyle) => ({
      ...nextStyle,
      sources: {
          ...nextStyle.sources,
          // copy a source from previous style
          'osm': previousStyle.sources.osm
      },
      layers: [
          // background layer
          nextStyle.layers[0],
          // copy a layer from previous style
          previousStyle.layers[0],
          // other layers from the next style
          ...nextStyle.layers.slice(1).map(layer => {
              // hide the layers we don't need from demotiles style
              if (layer.id.startsWith('geolines')) {
                  layer.layout = {...layer.layout || {}, visibility: 'none'};
              // filter out US polygons
              } else if (layer.id.startsWith('coastline') || layer.id.startsWith('countries')) {
                  layer.filter = ['!=', ['get', 'ADM0_A3'], 'USA'];
              }
              return layer;
          })
      ]
  })
});
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