0

更新：问题直接在reducer中，与onClick函数和deleteLastItem函数无关。我用console.logs对此进行了测试，显然唯一重复两次的是直接在reducer中的那个。

我同时测试了钩子 useReducer 和 useContext ，因为我认为它们可以很好地协同工作，但显然，当我将一个函数设置为 onClick 函数时，它会被触发两次，而且它不是因为我'正在删除数组中的最后一项，因此它删除了两项而不是一项。

这是存储上下文和 reducer 挂钩的文件：

import React, {createContext, useReducer} from "react";

const initialState = {
  books: [
    {name: "Book 8", autor: "Author 8", id: 8},
    {name: "Book 7", autor: "Author 7", id: 7},
    {name: "Book 6", autor: "Author 6", id: 6},
    {name: "Book 5", autor: "Author 5", id: 5},
    {name: "Book 4", autor: "Author 4", id: 4},
    {name: "Book 3", autor: "Author 3", id: 3}
  ]
}

const reducer = (state, action) => {
  switch (action.type) {
    case "DELETE":
      state.books.pop();
      console.log("hey");
      return {...state}
    default:
      return {...state}
  }
}

export const BookContext = createContext();

export const BookProvider = props => {
  const [state, dispatch] = useReducer(reducer, initialState);

  // Actions
  const deleteLastBook = () => dispatch({type: "DELETE"});

  return (
    <BookContext.Provider value={{books: state.books, deleteLastBook}}>
      {props.children}
    </BookContext.Provider>
  )
}

如您所见，console.log("hey"), 告诉我它被执行了两次（除了看到屏幕中的项目一次被清除两次）

这是引入上下文以及出现问题的组件：

import React, {useContext} from 'react';
import {BookContext} from "../context/BookContext";

const Books = () => {

  const information = useContext(BookContext);

  const {books, deleteLastBook} = information;

  const deleteBook = e => {
    e.preventDefault();
    deleteLastBook();
  };

  return (
    <div>
      {console.log(books)}
      {books.map(book => (<h1 key={book.id}>{book.name}</h1>))}
      <button onClick={deleteBook}>Delete book</button>
    </div>
  )
}

export default Books;

最后，这里是 app.js，我认为它没有用，但以防万一：

import React from 'react';
import './App.css';
import {BookProvider} from "./context/BookContext";
import Books from "./components/Books";

const App = () => {
  return (
    <BookProvider>
      <div className="App">
        <Books />
      </div>
    </BookProvider>
  );
}

export default App;
4

1 回答 1

0

对此的解决方案如下，我使用过滤器方法更改了reducer return 语句并且它起作用了：

const reducer = (state, action) => {
  switch (action.type) {
    case "DELETE":
      const lastItem = state.books[state.books.length - 1];
      return {
        ...state,
        books: state.books.filter(book => book !== lastItem)
      }
    default:
      return {...state}
  }
}
于 2020-09-06T23:09:51.690 回答