OpenLayers Vector Layer callback does not execute with GeoJSON

Question

Why does this callback get ignored:

new OpenLayers.Layer.Vector("My layer",
      {
        projection : "EPSG:4326",
        strategies : [ new OpenLayers.Strategy.Fixed(), refreshStrategy ],

        protocol : new OpenLayers.Protocol.HTTP(
        {            
          url    : "myphpurl.php",
          params : { rand : Math.random() * 1000 },
          format : new OpenLayers.Format.GeoJSON(),
          callback: function(){alert('test');}
        }),

I'm trying to get access to how many features were returned...

Answer 1

Callback get ignored because it's possible is overridden by another callback function internally (e.g. OpenLayers.Strategy.BBOX.merge). Use events mechanism for get feature count:

layer = new OpenLayers.Layer.Vector("My layer", ...)
layer.events.on({"loadend": function(e){console.log(e.object.features.length)}})

According to documentation callback property of OpenLayers.Protocol.HTTP is not marked as APIProperty, so it is for internal use only.

Answer 2

I think you're just missing the part where you ask the layer to read the data (see ** line below):

    var request = OpenLayers.Request.GET({
        url: serviceUrl.url,
        callback: function(request) {
            var geojson_format = new OpenLayers.Format.GeoJSON({
                'internalProjection': toProj,
                'externalProjection': fromProj
            });
            **gfs = geojson_format.read(request.responseText)**
            lyr.addFeatures(gfs);
            setExtentToData();

        }
    });