React Context源码实现原理详解

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);
},
}