var vectorLoader = function(extent, resolution, projection) {
    var url = 'http://demo.boundlessgeo.com/geoserver/wfs?service=WFS&' +
    'version=1.1.0&request=GetFeature&typename=osm:builtup_area&' +
    'outputFormat=text/javascript&format_options=callback:loadFeatures' +
    '&srsname=EPSG:3857&bbox=' + extent.join(',') + ',EPSG:3857';
    $.ajax({
        url: url,
        dataType: 'jsonp'
    });
};

var geoJsonFormat = new ol.format.GeoJSON();

var loadFeatures = function (response) {
    var features = geoJsonFormat.readFeatures(response);
    vectorSource.addFeatures(features);
};

var vectorSource = new ol.source.Vector({
    format: geoJsonFormat,
    loader: vectorLoader,
    strategy: ol.loadingstrategy.tile(ol.tilegrid.createXYZ({
        maxZoom: 19
    })),
    projection: 'EPSG:3857'
});

var serverVector = new ol.layer.Vector({
    source: vectorSource,
    style: vectorStyle
});