React源码解析之React.createContext()

前言：
由于childContext在React17中会被废弃，所以不去分析它了，主要是新 API— —createContext()的讲解

一、React.createContext()

作用：
方便祖先组件与后代组件（中间隔了好多层组件）传值

使用：
context.js：

import React from 'react';

const contextTestOne={
  name:'chen',
  length:22,
}

export const wrapContext=React.createContext(contextTestOne.name)

祖先组件：

import { wrapContext } from '@/utils/context';

const Father=props=>{
  return (<wrapContext.Provider value={'this is provider'}>
     <Child />
</wrapContext.Provider>)
}

子孙组件：

import { wrapContext } from '@/utils/context';

const getProviderValue=()=>{
  return <wrapContext.Consumer>{value=><span>{value}</span>}</wrapContext.Consumer>
}

const Child=props=>{
return (
    getProviderValue()
  );
}

结果：

注意：
将undefined传递给<Provider>的value时，createContext中的defaultValue不会生效，Consumer的value显示空值

React 官方文档：
https://zh-hans.reactjs.org/docs/context.html#contextprovider

源码：

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */

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,
  //使用Object.is()计算新老context的差异
  calculateChangedBits: ?(a: T, b: T) => number,
): ReactContext<T> {
  if (calculateChangedBits === undefined) {
    calculateChangedBits = null;
  } else {
    //不看
    if (__DEV__) {
      warningWithoutStack(
        calculateChangedBits === null ||
          typeof calculateChangedBits === 'function',
        'createContext: Expected the optional second argument to be a ' +
          'function. Instead received: %s',
        calculateChangedBits,
      );
    }
  }

  const context: ReactContext<T> = {
    //还是那句话，ReactContext中的$$typeof是
    // 作为createElement中的属性type中的对象进行存储的，并不是ReactElement的$$typeof
    $$typeof: REACT_CONTEXT_TYPE,
    _calculateChangedBits: calculateChangedBits,
    //作为支持多个并发渲染器的解决方法，我们将一些渲染器分类为主要渲染器，将其他渲染器分类为辅助渲染器。
    // As a workaround to support multiple concurrent renderers, we categorize
    // some renderers as primary and others as secondary.

    //我们只希望最多有两个并发渲染器：React Native（主要）和Fabric（次要）;
    // React DOM（主要）和React ART（次要）。
    // 辅助渲染器将自己的context的value存储在单独的字段中。
    // 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.

    //<Provider value={xxx}>中的value就是赋值给_currentValue的

    //也就是说_currentValue和_currentValue2作用是一样的，只是分别给主渲染器和辅助渲染器使用
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    // Used to track how many concurrent renderers this context currently
    // supports within in a single renderer. Such as parallel server rendering.

    //用来追踪该context的并发渲染器的数量
    _threadCount: 0,
    // These are circular
    Provider: (null: any),
    Consumer: (null: any),
  };
  //const obj={}
  //obj.provider._obj = obj
  context.Provider = {
    $$typeof: REACT_PROVIDER_TYPE,
    _context: context,
  };

  let hasWarnedAboutUsingNestedContextConsumers = false;
  let hasWarnedAboutUsingConsumerProvider = false;
  //不看
  if (__DEV__) {
    // A separate object, but proxies back to the original context object for
    // backwards compatibility. It has a different $$typeof, so we can properly
    // warn for the incorrect usage of Context as a Consumer.
    const Consumer = {
      $$typeof: REACT_CONTEXT_TYPE,
      _context: context,
      _calculateChangedBits: context._calculateChangedBits,
    };
    // $FlowFixMe: Flow complains about not setting a value, which is intentional here
    Object.defineProperties(Consumer, {
      Provider: {
        get() {
          if (!hasWarnedAboutUsingConsumerProvider) {
            hasWarnedAboutUsingConsumerProvider = true;
            warning(
              false,
              'Rendering <Context.Consumer.Provider> is not supported and will be removed in ' +
                'a future major release. Did you mean to render <Context.Provider> instead?',
            );
          }
          return context.Provider;
        },
        set(_Provider) {
          context.Provider = _Provider;
        },
      },
      _currentValue: {
        get() {
          return context._currentValue;
        },
        set(_currentValue) {
          context._currentValue = _currentValue;
        },
      },
      _currentValue2: {
        get() {
          return context._currentValue2;
        },
        set(_currentValue2) {
          context._currentValue2 = _currentValue2;
        },
      },
      _threadCount: {
        get() {
          return context._threadCount;
        },
        set(_threadCount) {
          context._threadCount = _threadCount;
        },
      },
      Consumer: {
        get() {
          if (!hasWarnedAboutUsingNestedContextConsumers) {
            hasWarnedAboutUsingNestedContextConsumers = true;
            warning(
              false,
              'Rendering <Context.Consumer.Consumer> is not supported and will be removed in ' +
                'a future major release. Did you mean to render <Context.Consumer> instead?',
            );
          }
          return context.Consumer;
        },
      },
    });
    // $FlowFixMe: Flow complains about missing properties because it doesn't understand defineProperty
    context.Consumer = Consumer;
  }

  else {
    //const obj={}
    //obj.consumer=obj
    //也就是Consumber对象指向React.Context对象

    //在<Consumer>进行渲染时，为了保证Consumer拿到最新的值，
    //直接让Consumer=React.Context，
    // React.Context中的_currentValue已经被<Provider>的value给赋值了
    //所以Consumer能立即拿到最新的值
    context.Consumer = context;
  }
  //不看
  if (__DEV__) {
    context._currentRenderer = null;
    context._currentRenderer2 = null;
  }

  return context;
}

解析：
不看__DEV__的话，还是挺简单的，需要注意的是context.Consumer = context，让<Consumer>等于React.context，这样能立即拿到<Provider>提供的最新值

二、为什么要弃用childContext？
因为childContext对下层的组件影响太大了，即使子孙组件没有用到childContext，子孙组件仍然要进行Update，严重影响了性能

（完）