OpenLayers 4:使用ol.format.WFS时未在地图上显示WFS图层

var wfsSource = new ol.source.Vector({
  format: new ol.format.WFS(),
  url: 'http://example.com/wfs',
  params: {
    'VERSION': '1.1.0',
    'TYPENAME': 'myFeatureType'
  }
});

var wfsLayer = new ol.layer.Vector({
  source: wfsSource
});

map.addLayer(wfsLayer);

var style = new ol.style.Style({
  fill: new ol.style.Fill({
    color: 'rgba(255, 255, 255, 0.6)'
  }),
  stroke: new ol.style.Stroke({
    color: '#319FD3',
    width: 1
  }),
  image: new ol.style.Circle({
    radius: 5,
    fill: new ol.style.Fill({
      color: 'rgba(255, 255, 255, 0.6)'
    }),
    stroke: new ol.style.Stroke({
      color: '#319FD3',
      width: 1
    })
  })
});

wfsLayer.setStyle(style);