原理

import React from 'react';
import ReactDOM from 'react-dom';

interface WorkHook {
    memoriedState: any,
    next: WorkHook | undefined
}

let firstWorkHook: WorkHook = {
    memoriedState: undefined,
    next: undefined
};

let currentWorkHook = firstWorkHook;

function useReducer(reducer: Function, initArg: any, init?: Function) {
    let current: WorkHook = currentWorkHook;
    if (typeof current.memoriedState === 'undefined') {
        let initState = initArg;
        if (typeof init === 'function') {
            initState = init(initArg);
        }
        current.memoriedState = initState;
    }
    current.next = currentWorkHook = current.next ? current.next : { memoriedState: undefined, next: undefined };
    let dispatch = (action: any) => {
        current.memoriedState = reducer(current.memoriedState, action);
        render();
    }
    return [
        current.memoriedState,
        dispatch
    ]
}

function useEffect(callback: Function, dependencies?: any[]) {
    if (typeof dependencies === 'undefined') {
        return callback();
    }
    let current = currentWorkHook;
    let memoriedDependencies = current.memoriedState;
    if (typeof memoriedDependencies === 'undefined') {
        current.memoriedState = dependencies;
        callback();
    } else {
        if (!dependencies.every((item, index) => item === memoriedDependencies[index])) {
            callback();
            current.memoriedState = dependencies;
        }
    }
    current.next = currentWorkHook = current.next ? current.next : { memoriedState: undefined, next: undefined };

}

function useState(initialState: any) {
    return useReducer((state: any, action: any) => action, initialState);
}

const INCREMENT = 'INCREMENT';
const DECREMENT = 'DECREMENT';

let reducer = (number: any, action: any) => {
    switch (action.type) {
        case INCREMENT:
            return number + 1;
        case DECREMENT:
            return number - 1;
        default:
            return number;
    }
}
let init = (number: number) => {
    return number;
}
let Counter = () => {
    let [number, dispatch] = useReducer(reducer, 0, init);
    let [name, setName] = useState('计数器');
    useEffect(() => {
        alert('名字改变了');
    }, [name]);
    useEffect(() => {
        alert('数值改变了');
    }, [number]);
    useEffect(() => {
        alert('只出现一次');
    }, []);
    useEffect(() => {
        alert('每次都有我');
    });
    return <>
        <p>{name}:{number}</p>
        <button onClick={() => dispatch({ type: INCREMENT })}>+</button>
        <button onClick={() => dispatch({ type: DECREMENT })}>-</button>
        <button onClick={() => setName('计数器' + Date.now())}>改名字</button>
    </>
}

function render() {
    currentWorkHook = firstWorkHook;
    ReactDOM.render(<Counter />, document.getElementById('root'));
}
render();

结果

结果.gif