在react + typescript项目中正确使用useReducer

我们在Home页面增加一个组件Counter，用来保存一个数值count，来记录点击按钮的次数，如下：

// Counter.tsx
import React, { useReducer } from 'react'

type StateType = {
    count: number;
}

type ActionType = {
    type: string;
    payload: number;
}

const initialState = {count: 0}

function countReducer(state: StateType, action: ActionType) {
    switch (action.type) {
        case 'increment':
            return {count: state.count + action.payload}
            break;
        case 'decrement':
            return {count: state.count - action.payload}
            break;
        default:
            return state
            break;
    }
}

export default function Counter() {
    const [state, dispatch] = useReducer(countReducer, initialState)

    const _increment = () => {
        dispatch({
            type: 'increment',
            payload: 1
        })
    }

    const _decrement = () => {
        dispatch({
            type: 'decrement',
            payload: 1
        })
    }

    return (
        <div>
            <p>Your count is {state.count}</p>
            <button onClick={_increment}>increment</button>&nbsp;&nbsp;
            <button onClick={_decrement}>decrement</button>
        </div>
    )
}

在Home页面使用，如下：

// Home.tsx
import React from "react";
import Counter from "../components/Counter";

const Home = () => {
    return (
        <div>
            <p>这是Home页面</p>
            <Counter />
        </div>
    )
}

export default Home;

如果我们现在新增一种action的type，为重置按钮reset，即<button onClick={_reset}>reset</button>；
reducer函数也要做出相应的改变，新增一个type时的处理：

case 'reset':
	return initialState
	break;

相应的reset函数为：

const _reset = () => {
	dispatch({
		type: 'reset'
	})
}

这样写之后会有个问题，就是我们在调用_reset函数时会报错，因为我们只传入了一个type参数，没有传payload，ts编译器会告诉我们这里类型不匹配，但是我们这个函数的功能是重置，也就是根本就不需要使用到参数payload，那么怎么办呢？我们这样可以来写ActionType：

type UpdateActionType = {
    type: 'increment' | 'decrement';
    payload: number;
}

type ResetActionType = {
    type: 'reset';
}

type ActionType = UpdateActionType | ResetActionType;

这样一来，我们在增加、减少和重置的时候都会去匹配对应的类型，ts编译器也可以愉快的玩耍了。
完整代码如下：

// Counter.tsx
import React, { useReducer } from 'react'

type StateType = {
    count: number;
}

type UpdateActionType = {
    type: 'increment' | 'decrement';
    payload: number;
}

type ResetActionType = {
    type: 'reset';
}

type ActionType = UpdateActionType | ResetActionType;

const initialState = {count: 0}

function countReducer(state: StateType, action: ActionType) {
    switch (action.type) {
        case 'increment':
            return {count: state.count + action.payload}
            break;
        case 'decrement':
            return {count: state.count - action.payload}
            break;
        case 'reset':
            return initialState
            break;
    }
}

export default function Counter() {
    const [state, dispatch] = useReducer(countReducer, initialState)

    const _increment = () => {
        dispatch({
            type: 'increment',
            payload: 1
        })
    }

    const _decrement = () => {
        dispatch({
            type: 'decrement',
            payload: 1
        })
    }

    const _reset = () => {
        dispatch({
            type: 'reset'
        })
    }

    return (
        <div>
            <p>Your count is {state.count}</p>
            <button onClick={_increment}>increment</button>&nbsp;&nbsp;
            <button onClick={_decrement}>decrement</button>&nbsp;&nbsp;
            <button onClick={_reset}>reset</button>
        </div>
    )
}

代码地址及文件路径

注：
本文代码github仓库地址：ts中正确使用useReducer
文件路径：src/components/Counter.tsx。