**解决祖孙之间组件传值** **例子** ``` ~~~ import React from "react"; import PropTypes from 'prop-types'; const { Provider, Consumer } = React.createContext('default'); class Parent extends React.Component{ state = { childContext:'123', newContext:'456' }; getChildContext(){ return {value:this.state.childContext,a:'aaa'} } render() { return( <React.Fragment> <div> <label>childContext:</label> <input type="text" value={this.state.childContext} onChange={e=>this.setState({childContext:e.target.value})} /> </div> <div> <label>newContext:</label> <input type="text" value={this.state.newContext} onChange={e=>this.setState({newContext:e.target.value})} /> <Provider value={this.state.newContext}> {this.props.children} </Provider> </div> </React.Fragment> ) } } class Child1 extends React.Component{ render() { return( <p> childContext:{this.context.value} {/*{this.context.a}*/} </p> ) } } function Child2(){ return<Consumer>{value => <p> newContext:{value} </p> }</Consumer> } Child1.contextTypes = { value:PropTypes.string, a:PropTypes.string }; Parent.childContextTypes={ value:PropTypes.string, a:PropTypes.string }; export default ()=>( <Parent> <Child2/> <Child1/> </Parent> ) ``` 源码： ~~~ import {REACT_PROVIDER_TYPE, REACT_CONTEXT_TYPE} from 'shared/ReactSymbols'; import type {ReactContext} from 'shared/ReactTypes'; import warningWithoutStack from 'shared/warningWithoutStack'; import warning from 'shared/warning'; export function createContext<T>( defaultValue: T, calculateChangedBits: ?(a: T, b: T) => number, ): ReactContext<T> { if (calculateChangedBits === undefined) { calculateChangedBits = null; } else { } //calculateChangedBits计算新老context的变化的 const context: ReactContext<T> = { $$typeof: REACT_CONTEXT_TYPE, _calculateChangedBits: calculateChangedBits, // As a workaround to support multiple concurrent renderers, we categorize // some renderers as primary and others as secondary. We only expect // there to be two concurrent renderers at most: React Native (primary) and // Fabric (secondary); React DOM (primary) and React ART (secondary). // Secondary renderers store their context values on separate fields. _currentValue: defaultValue, _currentValue2: defaultValue, //_currentValue用来记录provider提供的value有变化更新到_currentValue 记录context的值的 // Used to track how many concurrent renderers this context currently // supports within in a single renderer. Such as parallel server rendering. _threadCount: 0, // These are circular Provider: (null: any), Consumer: (null: any), }; context.Provider = { $$typeof: REACT_PROVIDER_TYPE, _context: context, }; let hasWarnedAboutUsingNestedContextConsumers = false; let hasWarnedAboutUsingConsumerProvider = false; context.Consumer = context; return context; } ~~~