无法获取谷歌地图实例

Question

我想通过componentDidMount()方法获取地图实例，但未成功。

请参见以下代码，我想通过

获取地图实例

const map = this.context[MAP];

但是if情况始终为假。顺便说一句，ref中的el始终是未定义的。

import React, { Component } from 'react';
import {compose, withProps, onlyUpdateForPropTypes} from "recompose";
import {withScriptjs, withGoogleMap, GoogleMap, Marker} from 'react-google-maps';
import {MAP} from 'react-google-maps/lib/constants';
import PropTypes from "prop-types";

const MyMapComponent = compose(
  withProps({
    googleMapURL: "https://maps.googleapis.com/maps/api/js?key=v2&libraries=places,geometry,drawing",
    loadingElement: <div style={{height: '100%'}} />,
    containerElement: <div style={{height: '800px'}} />,
    mapElement: <div style={{height: '100%'}} />,
  }),
  withScriptjs,
  withGoogleMap
)(props =>
  <GoogleMap
    defaultZoom={15}
    defaultCenter={{lat: 47.6129432, lng: -122.9716967}} >

    </GoogleMap>
);

export default class MapContainer extends Component {
  static contextTypes = { [MAP]: PropTypes.object };

  render() {
    return (
      <MyMapComponent 
        ref={(el) => {this.mapNode = el}} />
    )
  }

  componentDidMount() {
    console.log("Entry into componentDidMount : " + this.mapNode);

    if (this.context[MAP]) {
      const map = this.context[MAP];
      console.log("The map instance is: " + map);
    }
  }
}

有人知道如何通过componentDidMount()方法获取地图实例吗？