openlayers的loaders方式加载

openlayers loaders方式加载
let layerVector = new ol.layer.Vector({
source : new ol.source.Vector({

loader: function(extent, resolution, projection) {
console.log(1);
},
strategy: ol.loadingstrategy.bbox
})
});

var vectorSource = new ol.source.Vector({
format: new ol.format.GeoJSON(),
loader: function(extent, resolution, projection) {
var proj = projection.getCode();
var url = 'https://ahocevar.com/geoserver/wfs?service=WFS&' +
'version=1.1.0&request=GetFeature&typename=osm:water_areas&' +
'outputFormat=application/json&srsname=' + proj + '&' +
'bbox=' + extent.join(',') + ',' + proj;
var xhr = new XMLHttpRequest();
xhr.open('GET', url);
var onError = function() {
vectorSource.removeLoadedExtent(extent);
}
xhr.onerror = onError;
xhr.onload = function() {
if (xhr.status == 200) {
vectorSource.addFeatures(
vectorSource.getFormat().readFeatures(xhr.responseText));
} else {
onError();
}
}
xhr.send();
},
strategy: ol.loadingstrategy.bbox
});