React 深度学习：ReactHooks

path：packages/react/src/ReactHooks.js

目前 React 内置了 10 种 Hook：

• useState
• useReducer
• useContext
• useRef
• useEffect
• useLayoutEffect
• useCallback
• useMemo
• useImperativeHandle
• useDebugValue

useContext

export function useContext<T>(
  Context: ReactContext<T>,
  unstable_observedBits: number | boolean | void,
) {
  const dispatcher = resolveDispatcher();
  if (__DEV__) {
   // do something
  }
  return dispatcher.useContext(Context, unstable_observedBits);
}
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useState

export function useState<S>(initialState: (() => S) | S) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useState(initialState);
}
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useReducer

export function useReducer<S, I, A>(
  reducer: (S, A) => S,
  initialArg: I,
  init?: I => S,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useReducer(reducer, initialArg, init);
}
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useRef

export function useRef<T>(initialValue: T): {current: T} {
  const dispatcher = resolveDispatcher();
  return dispatcher.useRef(initialValue);
}
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useEffect

export function useEffect(
  create: () => (() => void) | void,
  inputs: Array<mixed> | void | null,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useEffect(create, inputs);
}
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useLayoutEffect

export function useLayoutEffect(
  create: () => (() => void) | void,
  inputs: Array<mixed> | void | null,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useLayoutEffect(create, inputs);
}
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useCallback

export function useCallback(
  callback: () => mixed,
  inputs: Array<mixed> | void | null,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useCallback(callback, inputs);
}
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useMemo

export function useMemo(
  create: () => mixed,
  inputs: Array<mixed> | void | null,
) {
  const dispatcher = resolveDispatcher();
  return dispatcher.useMemo(create, inputs);
}
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useImperativeHandle

export function useImperativeHandle<T>(
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
  create: () => T,
  inputs: Array<mixed> | void | null,
): void {
  const dispatcher = resolveDispatcher();
  return dispatcher.useImperativeHandle(ref, create, inputs);
}
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useDebugValue

export function useDebugValue(value: any, formatterFn: ?(value: any) => any) {
  if (__DEV__) {
    const dispatcher = resolveDispatcher();
    return dispatcher.useDebugValue(value, formatterFn);
  }
}
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不管哪一个 Hook，第一步都需要通过 resolveDispatcher 方法返回一个 dispatcher 对象。

resolveDispatcher

function resolveDispatcher() {
  const dispatcher = ReactCurrentDispatcher.current;
  invariant(
    dispatcher !== null,
    'Invalid hook call. Hooks can only be called inside of the body of a function component. This could happen for' +
      ' one of the following reasons:\n' +
      '1. You might have mismatching versions of React and the renderer (such as React DOM)\n' +
      '2. You might be breaking the Rules of Hooks\n' +
      '3. You might have more than one copy of React in the same app\n' +
      'See https://fb.me/react-invalid-hook-call for tips about how to debug and fix this problem.',
  );
  return dispatcher;
}
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resolveDispatcher 方法只是简单地获取了 ReactCurrentDispatcher 的 current 属性。当这个属性值为空时报错，否则返回这个对象。

ReactCurrentDispatcher

import ReactCurrentDispatcher from './ReactCurrentDispatcher';
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ReactCurrentDispatcher 文件

path：packages/react/src/ReactCurrentDispatcher.js

import type {Dispatcher} from 'react-reconciler/src/ReactFiberHooks';

/**
 * 跟踪当前 dispatcher。
 */
const ReactCurrentDispatcher = {
  /**
   * @internal
   * @type {ReactComponent}
   */
  current: (null: null | Dispatcher),
};

export default ReactCurrentDispatcher;
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ReactCurrentDispatcher 对象的 current 属性是一个 Dispatcher 类型的对象。

Dispatcher 类型定义从 ReactFiberHooks.js 中导出，下一节，我们看看 ReactFiberHooks.js 是什么样子。

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下一篇：ReactFiberHooks

转载于:https://juejin.im/post/5d097aba6fb9a07f065570ff