react context初始加载读取不到上下文的内容

import React, { Component } from 'react';
import PropTypes from 'prop-types';
import Counter from './Counter';

class SummaryContainer extends Component {
  constructor (props) {
    super(props);
    this.onChange = this.onChange.bind(this);
    this.computSum = this.computSum.bind(this);
    this.state = {
      sum: this.computSum()
    }
  }
  componentDidMount () {
    this.context.store.subscribe(this.onChange);
  }
  onChange () {
    this.setState({
      sum: this.computSum()
    });
  }
  computSum () {
    console.log(this.context.store); // TypeError: Cannot read property 'store' of undefined
    // const data = this.context.store.getState();
    // let sum = 0;
    // Object.keys(data).forEach((v) => {
    //   sum += data[v];
    // });
    return 30;
  }

  render() {
    return <Summary sum={this.state.sum}/>;
  }
}

class Summary extends Component {
  render() {
    return (
      <div>
        <Counter caption="First" />
        <Counter caption="Second" />
        <Counter caption="Third" />
        <h3 style={{ marginTop: '30px' }}>sum: {this.props.sum}</h3>
      </div>
    );
  }
}

SummaryContainer.contextTypes = {
  store: PropTypes.object
}

export default SummaryContainer;

SummaryContainer 和 Summary分别为容器组件和傻瓜组件，SummaryContainer中的computSum方法为页面初始时调用,这里运行会报“TypeError: Cannot read property ‘store’ of undefined”，这里产生原因是在constructor中调用时，上下文未获取到值，解决修改如下

  constructor (props, context) { // 加上context参数
    super(props, context); // 加上context
    this.onChange = this.onChange.bind(this);
    this.computSum = this.computSum.bind(this);
    this.state = {
      sum: this.computSum()
    }
  }

在constructor和super分别加上context参数，再运行下，问题解决。