Awesome

1. Simple usage

class MessageList extends React.Component {
//(2)Add getChildContext
 getChildContext() {
    return {color: "purple"};
  }
  render() {
    const color = "purple";
    const children = this.props.messages.map((message) =>
      <Message text={message.text} color={color} \/>
    );
    return <div>{children}<\/div>;
  }
}
//(1)Add MessageList.childContextTypes
MessageList.childContextTypes = {
  color: PropTypes.string
};

We need to focus on childContextTypes and getChildContext.

2.Referencing Context in Lifecycle Methods

If contextTypes is defined within a component, the following lifecycle methods will receive an additional parameter, the context object:

3.Referencing Context in Stateless Functional Components

Stateless functional components are also able to reference context if contextTypes is defined as a property of the function. The following code shows a Button component written as a stateless functional component.

const PropTypes = require('prop-types');
//context is second parameter of stateless function constructor
const Button = ({children}, context) =>
  <button style={{background: context.color}}>
    {children}
  <\/button>;
Button.contextTypes = {color: PropTypes.string};

4.Updating Context

Don't do it.

React has an API to update context, but it is fundamentally broken and you should not use it.

The getChildContext function will be called when the state or props changes. In order to update data in the context, trigger a local state update with this.setState. This will trigger a new context and changes will be received by the children.

const PropTypes = require('prop-types');
class MediaQuery extends React.Component {
  constructor(props) {
    super(props);
    this.state = {type:'desktop'};
  }
  //When state or props change, it will invoke
  getChildContext() {
    return {type: this.state.type};
  }
  componentDidMount() {
    const checkMediaQuery = () => {
      const type = window.matchMedia("(min-width: 1025px)").matches ? 'desktop' : 'mobile';
      //if type detected is not equal to this.state.type, we will invoke
      //this.setState
      if (type !== this.state.type) {
        this.setState({type});
        //Means setState({type:type}) invoked, then getChildContext will be invoked again~ 
      }
    };
    window.addEventListener('resize', checkMediaQuery);
    checkMediaQuery();
  }
  render() {
    return this.props.children;
  }
}
MediaQuery.childContextTypes = {
  type: PropTypes.string
};

The problem is, if a context value provided by component changes, descendants that use that value won't update if an intermediate parent returns false from shouldComponentUpdate. This is totally out of control of the components using context, so there's basically no way to reliably update the context. This blog post has a good explanation of why this is a problem and how you might get around it.

5. Why is Context + ShouldComponentUpdate problematic?

Classic use cases for context are theming, localization and routing. See example of this

Reference:

react context Api

How to safely use React context