ol.source.Vector.getFeatures() 填坑

1 var vectorChina = new ol.layer.Vector({
2     source: new ol.source.Vector({
3         projection : ‘EPSG:4326‘,
4         url: ‘geojson/china.json‘,
5         format: new ol.format.GeoJSON()
6     }),
7     zIndex: 1,
8     visible:true
9 });

var features = vectorChina.getSource().getFeatures()；

console.log(features);　　// 调试显示 undefined

 1 vectorChina.on(‘change‘, function (evt) {
 2     // 获取矢量图层数据源
 3     var source = evt.target.getSource();
 4     if (source.getState() === ‘ready‘) {
 5         // 获取数据集
 6         var features = source.getFeatures();
 7         console.log(features);
 8         console.log(features.length);
 9         // console.log(features[0].get(‘要素名称‘));
10     }
11 });