Check if a GeoJSON source is present in mapbox viewport - mapbox

Question

I have a map with several layers of GeoJSON each with their own unique layer name:
var map = new mapboxgl.Map({
container: 'map',
center: [-97.5651505, 37.89549,],
zoom: 4
});
var sources = {
'ord': 'chicago',
'pit': 'pittsburgh',
'atl': 'atlanta'
};
map.on('load', function () {
for (var s in sources) {
map.addSource(s, { type: 'geojson', data: `/geojson/${s}.json` });
map.addLayer({
'id': sources[s],
'type': 'fill',
'source': s,
'layout': {
'visibility': 'visible'
},
'paint': {
'fill-color': '#088',
'fill-opacity': 0.5
}
});
}
});
I would like to check if a user has zoomed in past zoom level 13 evaluate if any of these three layers is in the viewport. If it is I'll take action to add a button to the overlay. However, I'm having issues finding any documentation other than leaflet on how to check if a layer is inside the viewport. I've found some mention of markers that that doesn't seem to apply.

Answer 1

You can achieve this with queryRenderedFeatures which returns an array of features rendered within a given bounding box. However, if you omit the bounding box argument, queryRenderedFeatures will query within the entire viewport. You can also use the options.layers argument to limit your query to specific layers to avoid getting a bunch of features that are in the underlying style (for example, streets and lakes). You can do this query in a zoomend event listener to achieve your desired outcome. Putting it all together would look something like this:
map.on('zoomend', () => {
if (map.getZoom() > 13) {
const visibleFeatures = map.queryRenderedFeatures(null, {layers: ['ord', 'pit', 'atl']});
// if none of the layers are visible, visibleFeatures will be an empty array
if (visibleFeatures.length) {
// figure out which layers are showing and add your button
}
}
});