使用 useState 和 useContext 在 ReactJS 中删除项目

Question

作为一个相当新的编程迷，我一直在使用一个简单的列出项目的应用程序。添加项目，OK。但是对于删除一项，我就是想不通如何实现这个功能。现在单击一个项目时，整个项目数组将被删除。我试过把 deleteItem 函数放在其他文件里，但我好像搞错了引用。

ItemContext.js:

import React, { useState, createContext } from 'react';
import { v1 as uuidv1 } from 'uuid';
export const ItemContext = createContext();

export const ItemProvider = (props) => {
  const [items, setItems] = useState([
    {
      name: "Red Beans",
      amount: 23,
      id: uuidv1()
    },
    {
      name: "Nuts for Bunnies",
      amount: 33,
      id: uuidv1()
    },
    {
      name: "Chopped Tomatoes",
      amount: 2,
      id: uuidv1() 
    }
]);
  return ( 
      <ItemContext.Provider value={[items, setItems]} >
        {props.children}
      </ItemContext.Provider>
   );
}

AddItem.js:

    import React, { useState, useContext } from 'react';
import { ItemContext } from './ItemContext';

const AddItem = () => {
  const [name, setName] = useState('');
  const [amount, setAmount] = useState('');
  const [items, setItems] = useContext(ItemContext);

  const updateName = (e) => {
    setName(e.target.value)
  }

  const updateAmount = (e) => {
    setAmount(e.target.value)
  }

  const addItem = (e) => {
    e.preventDefault();
    setItems(prevItems => [...prevItems, {name: name, amount: amount, key: items.id}])
  }

  return ( 
      <form className="new-item" onSubmit={addItem}>
        <input type="text" name="name" value={name} onChange={updateName} placeholder="Add a new item"/>
        <input type="text" name="amount" value={amount} onChange={updateAmount} placeholder="Amount"/>
        <button>Submit</button>
      </form>
   );
}
 
export default AddItem;

Item.js:

import React, { useState, useContext } from 'react';
import { ItemContext } from './ItemContext';

const Item = ({name, amount, /*deleteItem*/ }) => {
  const [id, setId] = useState('');
  const [items, setItems] = useContext(ItemContext);

  const deleteItem = (id) => {
    // const newItems = items.filter(item => items.id !== id);
    // setItems(newItems);
    const newItems = Object.assign([], ...items)
    items.splice(id, 1);
    setItems(newItems);
  }

  return (
    <div>
      <h3>{name}</h3>
      <p>{amount}</p>
      <button onClick={() => deleteItem(items.id)} className="delete-btn">Delete</button>
    </div>
  );
}
 
export default Item;

ItemsList.js:

import React, { useState, useContext } from 'react';
import Item from './Item';
import { ItemContext } from './ItemContext';
import { v1 as uuidv1 } from 'uuid';

const ItemsList = () => {
  const [items, setItems] = useContext(ItemContext);
  const [id, setId] = useState('');

  const deleteItem = (id) => {
    // const newItems = items.filter(item => items.id !== id);
    // setItems(newItems);
    const newItems = Object.assign([], ...items)
    items.splice(id, 1);
    setItems(newItems);
  }

  return (
    <div className="items-list">
      {items.map(item => (
        <Item name={item.name} amount={item.amount} key={uuidv1()} deleteItem={deleteItem}/>
      ))}
    </div> 
   );
}
 
export default ItemsList;

Answer 1

最初您需要为正在创建的 ItemContext 上下文提供一个有效的初始值。它的签名应该匹配上下文值。我建议使用对象而不是数组。

export const ItemContext = createContext({
  items: [],
  addItem: () => {},
  deleteItem: () => {}
});

然后定义 addItem 和 deleteItem 处理程序以包含在上下文值中。您希望避免直接公开 setItems 状态函数，以便 ItemProvider 保持对状态不变量的控制。

const initialState = [
  {
    name: "Red Beans",
    amount: 23,
    id: uuidv1()
  },
  {
    name: "Nuts for Bunnies",
    amount: 33,
    id: uuidv1()
  },
  {
    name: "Chopped Tomatoes",
    amount: 2,
    id: uuidv1()
  }
];

const ItemProvider = (props) => {
  const [items, setItems] = useState(initialState);

  const addItem = (item) => setItems((items) => [...items, item]);
  const deleteItem = (id) =>
    setItems((items) => items.filter((item) => item.id !== id));

  const value = {
    items,
    addItem,
    deleteItem
  };

  return (
    <ItemContext.Provider value={value}>
      {props.children}
    </ItemContext.Provider>
  );
};

然后在您使用它的地方解构 ItemContext 值。

AddItem.js - 确保为新项目分配新的 uuidV1 GUID。我认为这是您最大的问题，您将 undefined 分配给 key 属性 (key: items.id) 而不是为 id [= 创建新的 GUID 49=].

import React, { useState, useContext } from 'react';
import { ItemContext } from './ItemContext';

const AddItem = () => {
  const [name, setName] = useState("");
  const [amount, setAmount] = useState("");
  const { addItem } = useContext(ItemContext);

  const updateName = (e) => {
    setName(e.target.value);
  };

  const updateAmount = (e) => {
    setAmount(e.target.value);
  };

  const submitHandler = (e) => {
    e.preventDefault();
    addItem({
      name: name,
      amount: amount,
      id: uuidv1() // <-- new GUID here!!
    });
  };

  return (
    <form className="new-item" onSubmit={submitHandler}>
      <input
        type="text"
        name="name"
        value={name}
        onChange={updateName}
        placeholder="Add a new item"
      />
      <input
        type="text"
        name="amount"
        value={amount}
        onChange={updateAmount}
        placeholder="Amount"
      />
      <button>Submit</button>
    </form>
  );
};

Item.js - 从上下文中获取 deleteItem 并确保将项目 id 传递给组件。

import React, { useState, useContext } from 'react';
import { ItemContext } from './ItemContext';

const Item = ({ name, amount, id }) => {
  const { deleteItem } = useContext(ItemContext);

  return (
    <div>
      <h3>{name}</h3>
      <p>{amount}</p>
      <button onClick={() => deleteItem(id)} className="delete-btn">
        Delete
      </button>
    </div>
  );
};

ItemsList.js - 将 React 键设置为当前项目 id 并将 id 作为道具传递给 `Item.

import React, { useState, useContext } from 'react';
import Item from './Item';
import { ItemContext } from './ItemContext';

const ItemsList = () => {
  const { items } = useContext(ItemContext);

  return (
    <div className="items-list">
      {items.map((item) => (
        <Item
          key={item.id} // <-- Static React key to item
          id={item.id} // <-- Pass id as prop for delete handler to use
          name={item.name}
          amount={item.amount}
        />
      ))}
    </div>
  );
};

使用 useState 和 useContext 在 ReactJS 中删除项目

Deleting items in ReactJS, using useState and useContext

javascript

reactjs

react-context

use-context

演示