openlayers check if vector has content, to avoid error message

Question

Have the following code run every second.

var new_source = new ol.source.Vector({
    url: 'pages/Coordinates.php',
    format: new ol.format.KML({
        extractStyles: false,
        extractAttributes: false
    })
});



var new_layer = new ol.layer.Vector({
    source: new_source,
    style: styling
});

map.addLayer(new_layer);


new_source.once('change', function() {
    if (x) {
        map.removeLayer(x);
    }
    x = new_layer;
});

Works fine but if there's no coordinates for the source to get I get this error message.

XML Parsing Error: no root element found Location: localhost/test/ Line Number 1, Column 1:

Any ideas on how to avoid this error message?

I've thought about checking if the source was set to ready, but it's also says ready when there's no coordinates.

Then I thought about checking if there is features in it, but then it didn't work even when there was.

So I decided to see if there was any differences between a "source" and/or "vector" object with and without a call that includes coordinates, but alas I haven't been able to find anything I can compare.

Answer 1

The error is presumably happening when OL tries to read the features, so you would need to use a custom loader as in http://openlayers.org/en/v4.6.5/apidoc/ol.source.Vector.html and use that catch the error, so something like:

var vectorSource = new ol.source.Vector({
  format: new ol.format.KML({
        extractStyles: false,
        extractAttributes: false
  }),
  loader: function(extent, resolution, projection) {
     var proj = projection.getCode();
     var url = 'pages/Coordinates.php';
     var xhr = new XMLHttpRequest();
     xhr.open('GET', url);
     var onError = function() {
       vectorSource.removeLoadedExtent(extent);
     }
     xhr.onerror = onError;
     xhr.onload = function() {
       if (xhr.status == 200) {
         try {
           vectorSource.addFeatures(
             vectorSource.getFormat().readFeatures(xhr.responseText));
         } catch(err) { onError(); }
       } else {
         onError();
       }
     }
     xhr.send();
   },
   strategy: ol.loadingstrategy.bbox
 });