如何在react-google-map中获得圆心？

Question

将圆环拖到其他位置后，我试图获取圆心。从react-google-map文档中提供的示例代码中，他们在搜索框中使用它们来获取位置：

lifecycle({
componentWillMount() {
  const refs = {}

  this.setState({
    bounds: null,
    center: {
      lat: 41.9, lng: -87.624
    },
    markers: [],
    onMapMounted: ref => {
      refs.map = ref;
    },
    onBoundsChanged: () => {
      this.setState({
        bounds: refs.map.getBounds(),
        center: refs.map.getCenter(),
      })
    },
    onSearchBoxMounted: ref => {
      refs.searchBox = ref;
    },
    onPlacesChanged: () => {
      const places = refs.searchBox.getPlaces();
      const bounds = new google.maps.LatLngBounds();

我也用它来获得圆心：

  lifecycle({
componentWillMount () {
  const refs = {}
    this.setState({
    onCircleMounted: ref => {
      refs.circle = ref
    },
    onCenterChanged: () => {
      console.log(refs.circle.getCenter())
    }
  })
}
  }),
  withScriptjs,
  withGoogleMap
)((props) =>
  <GoogleMap
defaultZoom={props.zoomValue}
defaultCenter={props.centerFocus}
  >
<Circle
  key={marker.id}
  center={{ lat: marker.lat, lng: marker.lng }}
  radius={500}
  draggable={true}
  editable={true}
  ref={props.onCircleMounted}
  onCenterChanged={props.onCenterChanged}
/>
  )
  </GoogleMap>
)

但是我听不懂。它为refs.circle.getCenter()返回了null。

有人知道怎么了吗？