UI = f(data)

class SomeCompoent extends Component {

  componetDidMount() {
    const node = this.refs['myRef'];
    node.addEventListener('mouseDown', handlerMouseDown);
    node.addEventListener('mouseUp', handlerMouseUp)
  }

  ...

  componetWillunmount() {
    const node = this.refs['myRef'];
    node.removeEventListener('mouseDown', handlerMouseDown)
    node.removeEventListener('mouseUp', handlerMouseUp)
  }
}

import { useState } from 'react';

function Example() {
  // Declare a new state variable, which we'll call "count"
  const [count, setCount] = useState(0);
  return (
    <div>
      <p>You clicked {count} times</p>
      <button onClick={() => setCount(count + 1)}>        Click me      </button>
    </div>
  );
}

function FunctionalComponent () {
  const [state1, setState1] = useState(1)
  const [state2, setState2] = useState(2)
  const [state3, setState3] = useState(3)

  return <div>{state1}{...}</div>
}

import React, { useState, useEffect } from 'react';

function useMousemove() {
    const [client, setClient] = useState({x: 0, y: 0});

  useEffect(() => {

    const handlerMouseCallback = (e) => {
        setClient({
          x: e.clientX,
        y: e.clientY
      })
    };
    // 在組件首次 render 之后， 既在 didMount 時調用
      document.addEventListener('mousemove', handlerMouseCallback, false);

    return () => {
      // 在組件卸載之后執(zhí)行
        document.removeEventListener('mousemove', handlerMouseCallback, false);
    }
  })

  return client;
}

type Hooks = {
    memoizedState: any, // 指向當前渲染節(jié)點 Fiber
  baseState: any, // 初始化 initialState， 已經每次 dispatch 之后 newState
  baseUpdate: Update<any> | null,// 當前需要更新的 Update ，每次更新完之后，會賦值上一個 update，方便 react 在渲染錯誤的邊緣，數據回溯
  queue: UpdateQueue<any> | null,// UpdateQueue 通過
  next: Hook | null, // link 到下一個 hooks，通過 next 串聯(lián)每一 hooks
}

type Effect = {
  tag: HookEffectTag, // effectTag 標記當前 hook 作用在 life-cycles 的哪一個階段
  create: () => mixed, // 初始化 callback
  destroy: (() => mixed) | null, // 卸載 callback
  deps: Array<mixed> | null,
  next: Effect, // 同上 
};

function createWorkInProgressHook() {
  if (workInProgressHook === null) {
    // This is the first hook in the list
    if (firstWorkInProgressHook === null) {
      currentHook = firstCurrentHook;
      if (currentHook === null) {
        // This is a newly mounted hook
        workInProgressHook = createHook();
      } else {
        // Clone the current hook.        workInProgressHook = cloneHook(currentHook);
      }
      firstWorkInProgressHook = workInProgressHook;
    } else {
      // There's already a work-in-progress. Reuse it.      currentHook = firstCurrentHook;
      workInProgressHook = firstWorkInProgressHook;
    }
  } else {
    if (workInProgressHook.next === null) {
      let hook;
      if (currentHook === null) {
        // This is a newly mounted hook
        hook = createHook();
      } else {
        currentHook = currentHook.next;
        if (currentHook === null) {
          // This is a newly mounted hook
          hook = createHook();
        } else {
          // Clone the current hook.          hook = cloneHook(currentHook);
        }
      }
      // Append to the end of the list
      workInProgressHook = workInProgressHook.next = hook;
    } else {
      // There's already a work-in-progress. Reuse it.      workInProgressHook = workInProgressHook.next;
      currentHook = currentHook !== null ? currentHook.next : null;
    }
  }
  return workInProgressHook;
}

function FunctionComponet() {

  const [ state0, setState0 ] = useState(0);
  const [ state1, setState1 ] = useState(1);
  useEffect(() => {
      document.addEventListener('mousemove', handlerMouseMove, false);
    ...
    ...
    ...
    return () => {
      ...
      ...
      ...
        document.removeEventListener('mousemove', handlerMouseMove, false);
    }
  })

  const [ satte3, setState3 ] = useState(3);
  return [state0, state1, state3];
}

import React, { useState, useEffect } from 'react';
import ReactDOM from 'react-dom';

const useWindowSize = () => {
    let [size, setSize] = useState([window.innerWidth, window.innerHeight])
    useEffect(() => {
        let handleWindowResize = event => {
            setSize([window.innerWidth, window.innerHeight])
        }
        window.addEventListener('resize', handleWindowResize)
        return () => window.removeEventListener('resize', handleWindowResize)
    }, [])
    return size
}


const App = () => {
    const [ innerWidth, innerHeight ] = useWindowSize();
  return (
    <ul>
          <li>innerWidth: {innerWidth}</li>
            <li>innerHeight: {innerHeight}</li>
    </ul>
  )
}

ReactDOM.render(<App/>, document.getElementById('root'))

function useState(initialState）{  return useReducer(
    basicStateReducer,    // useReducer has a special case to support lazy useState initializers    (initialState: any),  );}function useReducer(reducer, initialState, initialAction) {  // 解析當前正在 rendering 的 Fiber
    let fiber = (currentlyRenderingFiber = resolveCurrentlyRenderingFiber());  workInProgressHook = createWorkInProgressHook();  // 此處省略部分源碼  ...  ...  ...  // dispathAction 會綁定當前真在渲染的 Fiber， 重點在 dispatchAction 中  const dispatch = dispatchAction.bind(null, currentlyRenderingFiber,queue,)  return [workInProgressHook.memoizedState, dispatch];}function dispatchAction(fiber, queue, action) {    const alternate = fiber.alternate;  const update: Update<S, A> = {    expirationTime,    action,    eagerReducer: null,    eagerState: null,    next: null,  };  ......  ......  ......  scheduleWork(fiber, expirationTime);}