React Hooks 仅在表单提交时更新状态

Question

So I've been at this all morning and can't figure out how to update my state correctly using useState .

I have a single controlled user input with name . When a user enters text and submits I would like to take the input value and push that to another state object namesList and map over that array in a child component.

Parent Component

import React, { Fragment, useState } from 'react';

import TextField from '@material-ui/core/TextField';

import NameInputList from './NameInputList';

const NameInputContainer = () => {
  const [name, setName] = useState('');
  const [namesList, setNamesList] = useState([]);

  const handleChange = event => {
    const { value } = event.target;
    setName(value);
  };

  const handleSubmit = event => {
    event.preventDefault();
    setNamesList(prevState => [...prevState, name]);
    setName('');
  };

  return (
    <Fragment>
      <form onSubmit={handleSubmit}>
        <TextField
          id="name"
          label="Enter New Name"
          variant="outlined"
          value={name}
          onChange={handleChange}
        />
      </form>
      {namesList.length > 0 && <NameInputList names={namesList} />}
    </Fragment>
  );
};

export default NameInputContainer;

Child Component

import React from 'react';
import PropTypes from 'prop-types';

const NameInputList = ({ names }) => {
  console.log('child component names: ', names);
  const generateKey = val => {
    return `${val}_${new Date().getTime()}`;
  };

  return (
    <ul>
      {names.map((name, index) => ( // <--- Adding the index here seems to resolve the problem. I want to say the error was happening based on issues with having Unique keys. 
        <li key={generateKey(name + index)}>{name}</li>
      ))}
    </ul>
  );
};

NameInputList.propTypes = {
  names: PropTypes.arrayOf(PropTypes.string)
};

NameInputList.defaultProps = {
  names: []
};

export default NameInputList;

Seems like when I submit the first time the child component gets the correct value and renders as expected. When I go to input a new name there's a rerender on every handleChange . I'm not sure how to consistently: enter text > submit input > update namesList > render updated namesList in child component without handleChange breaking the functionality.

Answer 1

Just adding to that, as namesList prop is an array, a new copy will be sent for each parent re-render. Hence it's comparison will always be false and React will trigger a re-render of the child.

You can prevent the re-render by doing this :

export default React.memo(NameInputList, function(prevProps, nextProps) {
  return prevProps.names.join("") === nextProps.names.join("");
});

This will ensure that NameInputList only re-renders when the contents of namesList actually change.