import React, { useReducer } from 'react'
// useReducer 它就是一個小型的redux，它幾乎和redux是一樣的，也可以管理數據
// 初始化數據
const initState = {
  count: 100
}
// reducer純函數中的state無需在定義函數時指定初始值，initState 會賦值給 reducer
// const reducer = (state,action)=>{}
// type, payload 是從 action 中結構出來的
const reducer = (state, { type, payload }) => {
  if (type === 'add') {
    return { ...state, count: state.count + payload }
  }
  return state
}
const App = () => {
  // state 它就是初始化后數據的對象，狀態
  // dispatch 它就是用來發送指令讓reducer來修改state中的數據
  // reducer它就是一個純函數，用來修改initState初始化后數據狀態函數
  // initState 初始化數據
  const [state, dispatch] = useReducer(reducer, initState)
  const addCount = () => {
    // 數據以改變就會觸發 useReducer 中的 reducer 函數
    dispatch({ type: 'add', payload: 2 })
  }
  return (
    <div>
      <h3>App組件 -- {state.count}</h3>
      <button onClick={addCount}>++++</button>
    </div>
  );
}
export default App;

import React, { useReducer } from 'react'
const initState = {
  count: 100
}
const reducer = (state, { type, payload }) => {
  if (type === 'add') {
    return { ...state, count: state.count + payload }
  }
  return state
}
const App = () => {
  // initState 初始化數據
  // 如果useReducer它有第3個參數，則第2個參數就沒有意義，它以第3個參數優先，第3個參數，惰性初始化，提升性能
  // 第3參數它是一個回調函數且一定要返回一個對象數據,當然你也可以直接返回一個值也可以，不建議
  const [state, dispatch] = useReducer(reducer, null, () => ({ count: 2000 }))
  const addCount = () => {
    dispatch({ type: 'add', payload: 2 })
  }
  return (
    <div>
      <h3>App組件 -- {state.count}</h3>
      <button onClick={addCount}>++++</button>
    </div>
  );
}
export default App;

// 導出初始化數據
export const initState = {
  // 任務列表數據源
  todos: []
}
// 導出用于操作當前todos任務列表的純函數
export const reducer = (state, { type, payload }) => {
  // [...state.todos, payload] 追加數據
  if ('add' === type) return { ...state, todos: [...state.todos, payload] }
  if ('del' === type) return { ...state, todos: state.todos.filter(item => item.id != payload) }
  return state
}

import React from 'react';
import Todo from './Todo';
const App = () => {
  return (
    <div>
      <Todo />
    </div>
  );
}
export default App;

import React, { useReducer } from 'react'
import Form from './components/Form'
import List from './components/List'
// 導入 reducer 文件中的初始化數據和操作數據函數
import { initState, reducer } from './reducer'
const Todo = () => {
  const [{ todos }, dispatch] = useReducer(reducer, initState)
  return (
    <div>
      {/* 表單項 */}
      <Form dispatch={dispatch} />
      {/* 任務項 */}
      <List dispatch={dispatch} todos={todos} />
    </div>
  )
}
export default Todo

import React from 'react'
// 表單項組件
const Form = ({ dispatch }) => {
  // 回車事件
  const onEnter = evt => {
    if (evt.keyCode === 13) {
      const title = evt.target.value.trim()
      // todo每項中的數據
      const todo = {
        id: Date.now(),
        title,
        done: false
      }
      dispatch({ type: 'add', payload: todo })
      evt.target.value = ''
    }
  }
  return (
    <div>
      <input onKeyUp={onEnter} />
    </div>
  )
}
export default Form

import React from 'react'
// 任務項組件
const List = ({ todos, dispatch }) => {
  const del = id => {
    dispatch({ type: 'del', payload: id })
  }
  return (
    <div>
      <h3>任務項</h3>
      <hr />
      <ul>
        {todos.map(item => (
          <li key={item.id}>
            <span>{item.title}</span>
            <span onClick={() => del(item.id)}>刪除</span>
          </li>
        ))}
      </ul>
    </div>
  )
}
export default List