Leaflet: Layer check box state is reset every time the map moves or zooms - leaflet

Question

I have the following code which fetches some remote GeoJSON from an API and displays the results on a Leaflet map:
<script>
// Center the map
var map = L.map('map').setView([54.233669, -4.406027], 6);
// Attribution
L.tileLayer('https://api.tiles.mapbox.com/v4/{id}/{z}/{x}/{y}.png?access_token=REMOVED', {
attribution: 'Map © OpenStreetMap',
id: 'mapbox.streets'
}).addTo(map);
// Create an empty layergroup for the data
var LayerUTMGroundHazards = L.layerGroup();
var LayerUTMAirspace = L.layerGroup();
// Style the features
function setStyle(feature) {
return {
fillColor: feature.properties.fillColor,
color: feature.properties.strokeColor,
fillOpacity: feature.properties.fillOpacity,
opacity: feature.properties.strokeOpacity
};
}
// Build Guardian UTM
function getGuardianUTMdata() {
// Clear the current Layer content
LayerUTMGroundHazards.clearLayers();
LayerUTMAirspace.clearLayers();
// Define what we want to include
function FuncGroundHazards(feature) {
if (feature.properties.category === "groundHazard") return true
}
function FuncAirspace(feature) {
if (
(feature.properties.category === "airspace" || feature.properties.category === "airport")
&& feature.properties.detailedCategory !== "uk:frz"
) return true
}
// Build the layers
fetch("https://example.com/json?n=" + map.getBounds().getNorth() + "&e=" + map.getBounds().getEast() + "&s=" + map.getBounds().getSouth() + "&w=" + map.getBounds().getWest(), { headers: { 'Authorization': 'REMOVED', 'X-AA-DeviceId': 'mySite' } })
.then(function (responseGuardianUTM) { return responseGuardianUTM.json() })
.then(function (dataGuardianUTM) {
// Create Layer: Ground Hazards
var featuresUTMGroundHazards = L.geoJson(dataGuardianUTM, {
filter: FuncGroundHazards,
style: setStyle,
pointToLayer: function (feature, latlng) { return L.marker(latlng, { icon: L.icon({ iconUrl: feature.properties.iconUrl, iconSize: [25, 25], }), }) },
onEachFeature: function (feature, layer) { layer.bindPopup(feature.properties.name); },
});
// Add the L.GeoJSON instance to the empty layergroup
LayerUTMGroundHazards.addLayer(featuresUTMGroundHazards).addTo(map);
});
// other layers are here (removed from this example)
}
// Update the Guardian UTM layer if the map moves
map.on('dragend', function () { getGuardianUTMdata(); });
map.on('zoomend', function () { getGuardianUTMdata(); });
// Layer controls
var layerControl = new L.Control.Layers(null, {
'Airspace Restrictions': LayerUTMAirspace,
'Ground Hazards': LayerUTMGroundHazards
// other layers are here (removed from this example)
}).addTo(map);
</script>
The problem is that every time the map is moved or zoomed, all of the Layer checkboxes are reset to Checked again, regardless of how many were checked before the map moved. They do not honour / remember their state when the map moves.
Given my code above, how can I store or preserve the checkbox state for each of the multiple Layers that I have so they are not reset every time the map is moved?
EDIT:
Here is a working fiddle. Remove the checkbox from the 'Ground Hazards', then move or zoom the map, you will see how it puts a tick back in the box again
https://jsfiddle.net/hdwz1b6t/1/

Answer 1

You're (re-)adding LayerUTMGroundHazards every time. This line here...
// Add the L.GeoJSON instance to the empty layergroup
LayerUTMGroundHazards.addLayer(featuresUTMGroundHazards).addTo(map);
...is not only adding featureUTMGroundHazards to layerUTMGroundHazards, it's also (re-)adding layerUTMGroundHazards to the map.
And quoting from https://leafletjs.com/examples/layers-control/ :
The layers control is smart enough to detect what layers we’ve already added and have corresponding checkboxes and radioboxes set.
So when you do LayerUTMGroundHazards.addTo(map);, the checkboxes reset.