React Context源码是怎么实现的呢_2023-02-19

import React, { Component, createContext, useConText } from 'react'
const ColorContext = createContext(null)
const { Provider, Consumer } = ColorContext

console.log('ColorContext', ColorContext)
console.log('Provider', Provider)
console.log('Consumer', Consumer)

class App extends Component {
  constructor(props) {
    super(props)
    this.state = {
      color: 'red',
      background: 'cyan',
    }
  }
  render() {
    return <Provider value={this.state}>{this.props.children}</Provider>
  }
}
function Article({ children }) {
  return (
    <App>
      <h1>Context</h1>
      <p>hello world</p>
      {children}    </App>
  )
}
function Paragraph({ color, background }) {
  return (
    <div style={{ backgroundColor: background }}>
      <span style={{ color }}>text</span>
    </div>
  )
}
function TestContext() {
  return (
    <Article>
      <Consumer>{state => <Paragraph {...state} />}</Consumer>
    </Article>
  )
}

export default TestContext

// createContext 可以让我们实现状态管理
// 还能够解决传递 Props drilling 的问题
// 假如一个子组件需要父组件的一个属性，但是中间间隔了好几层，这就会出现开发和维护的一个成本。这时候就可以通过这个 API 来解决
function createContext(defaultValue, calculateChangedBits) {
  var context = {
    ?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.
    // 以下两个属性是为了适配多平台
    _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.
    _threadCount: 0,
    // These are circular
    Provider: null,
    Consumer: null
  };

  // 以下的代码很简单，就是在 context 上挂载 Provider 和 Consumer，让外部去使用
  context.Provider = {
    ?typeof: REACT_PROVIDER_TYPE,
    _context: context
  };

  var Consumer = {
    ?typeof: REACT_CONTEXT_TYPE,
    _context: context,
    _calculateChangedBits: context._calculateChangedBits
  };

  context.Consumer = Consumer;
  context._currentRenderer = null;
  context._currentRenderer2 = null;
  return context;
}

let lastContextDependency: ContextDependency<mixed> | null = null;
let currentlyRenderingFiber: Fiber | null = null;
// 在 prepareToReadContext 函数
currentlyRenderingFiber = workInProgress;


export function readContext<T>(
  context: ReactContext<T>,
  observedBits: void | number | boolean,
): T {
    let contextItem = {
      context: ((context: any): ReactContext<mixed>),
      observedBits: resolvedObservedBits,
      next: null,
    };

    if (lastContextDependency === null) {
      // This is the first dependency for this component. Create a new list.
      lastContextDependency = contextItem;
      currentlyRenderingFiber.contextDependencies = {
        first: contextItem,
        expirationTime: NoWork,
      };
    } else {
      // Append a new context item.
      lastContextDependency = lastContextDependency.next = contextItem;
    }
  }

  // isPrimaryRenderer 为 true，定义的就是 true
  // 实际就是一直会返回  context._currentValue
  return isPrimaryRenderer ? context._currentValue : context._currentValue2;
}

  var context = {
    ?typeof: REACT_CONTEXT_TYPE,
    _currentValue: defaultValue,
    _currentValue2: defaultValue,
    Provider: null,
    Consumer: null
  };

  context.Provider = {
    ?typeof: REACT_PROVIDER_TYPE,
    _context: context
  };

  var Consumer = {
    ?typeof: REACT_CONTEXT_TYPE,
    _context: context,
    _calculateChangedBits: context._calculateChangedBits
  };

import React, { useContext, createContext } from 'react'
const NameCtx = createContext({ name: 'yuny' })
function Title() {
  const { name } = useContext(NameCtx)
  return <h1># {name}</h1>
}
function App() {
  return (
    <NameCtx.Provider value={{ name: 'lxfriday' }}>
      <Title />
    </NameCtx.Provider>
  )
}
export default App

/** * useContext * @param Context {ReactContext} createContext 返回的结果 * @param unstable_observedBits {number | boolean | void} 计算新老 context 变化相关的，useContext() second argument is reserved for future  * @returns {*} 返回的是 context 的值 */
export function useContext<T>(
  Context: ReactContext<T>,  unstable_observedBits: number | boolean | void,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useContext(Context, unstable_observedBits);
}

// Invalid hook call. Hooks can only be called inside of the body of a function component. 
function resolveDispatcher() {
  const dispatcher = ReactCurrentDispatcher.current;
  return dispatcher;
}

/** * Keeps track of the current dispatcher. */
const ReactCurrentDispatcher = {
  /**   * @internal
   * @type {ReactComponent}   */
  current: (null: null | Dispatcher),
};

const HooksDispatcherOnMount: Dispatcher = {
  readContext,

  useCallback: mountCallback,
  useContext: readContext,
  useEffect: mountEffect,
  useImperativeHandle: mountImperativeHandle,
  useLayoutEffect: mountLayoutEffect,
  useMemo: mountMemo,
  useReducer: mountReducer,
  useRef: mountRef,
  useState: mountState,
  useDebugValue: mountDebugValue,
};

HooksDispatcherOnMountInDEV = {
   // ... 
   useContext<T>(
      context: ReactContext<T>,
      observedBits: void | number | boolean,
    ): T {
      return readContext(context, observedBits);
    },
}