react 源码解析 14. 手写 hooks

import React from "react";import ReactDOM from "react-dom";
let workInProgressHook;//当前工作中的hooklet isMount = true;//是否时mount时
const fiber = {//fiber节点  memoizedState: null,//hook链表  stateNode: App//dom};
const Dispatcher = (() => {//Dispatcher对象  function mountWorkInProgressHook() {//mount时调用    const hook = {//构建hook      queue: {//更新队列        pending: null//未执行的update队列      },      memoizedState: null,//当前state      next: null//下一个hook    };    if (!fiber.memoizedState) {      fiber.memoizedState = hook;//第一个hook的话直接赋值给fiber.memoizedState    } else {      workInProgressHook.next = hook;//不是第一个的话就加在上一个hook的后面，形成链表    }    workInProgressHook = hook;//记录当前工作的hook    return workInProgressHook;  }  function updateWorkInProgressHook() {//update时调用    let curHook = workInProgressHook;    workInProgressHook = workInProgressHook.next;//下一个hook    return curHook;  }  function useState(initialState) {    let hook;    if (isMount) {      hook = mountWorkInProgressHook();      hook.memoizedState = initialState;//初始状态    } else {      hook = updateWorkInProgressHook();    }
    let baseState = hook.memoizedState;//初始状态    if (hook.queue.pending) {      let firstUpdate = hook.queue.pending.next;//第一个update
      do {        const action = firstUpdate.action;        baseState = action(baseState);        firstUpdate = firstUpdate.next;//循环update链表      } while (firstUpdate !== hook.queue.pending);//通过update的action计算state
      hook.queue.pending = null;//重置update链表    }    hook.memoizedState = baseState;//赋值新的state
    return [baseState, dispatchAction.bind(null, hook.queue)];//useState的返回  }
  return {    useState  };})();
function dispatchAction(queue, action) {//触发更新  const update = {//构建update    action,    next: null  };  if (queue.pending === null) {    update.next = update;//update的环状链表  } else {    update.next = queue.pending.next;//新的update的next指向前一个update    queue.pending.next = update;//前一个update的next指向新的update  }  queue.pending = update;//更新queue.pending
  isMount = false;//标志mount结束  workInProgressHook = fiber.memoizedState;//更新workInProgressHook  schedule();//调度更新}
function App() {  let [count, setCount] = Dispatcher.useState(1);  let [age, setAge] = Dispatcher.useState(10);  return (    <>      <p>Clicked {count} times</p>      <button onClick={() => setCount(() => count + 1)}> Add count</button>      <p>Age is {age}</p>      <button onClick={() => setAge(() => age + 1)}> Add age</button>    </>  );}
function schedule() {  ReactDOM.render(<App />, document.querySelector("#root"));}
schedule();