onChange事件后组件未重新呈现

Question

I've checked the previous answer on different topics about re-rendering but couldn't find a proper way to fix this. I am trying to re-render a component after the onChange event is fired on the select box.

I have a userlist with 5 users and I separated them into genders array. So I have a males array and a females array. I am trying to show the selected gender user list after the onChange event.

I tried to use the 'key' attr in component but it didn't work.

here is my UserList container:

import React, { Component } from "react";
import ReactDOM from "react-dom";

import { connect } from 'react-redux';
import PropTypes from 'prop-types';

import List from '../components/List.jsx'

class UserList extends Component {

    constructor(props) {
         super(props);
     }

    renderUser = (toMap) => {
        console.log(toMap);
    return (
            <List key = {toMap.length} users = {toMap}/>
        );
  }

    renderSelect = () => {
    return (
            <select id="gender" onChange={this.handleChange}>
        <option value="select">Select a gender</option>
        <option value="males">Males</option>
        <option value="females">Females</option>
      </select>
        );
  }

    handleChange = (e) => {
        const {userList,males,females} = this.props;
        const gender = e.target.value;

        if(gender == 'males'){
            return this.renderUser(males);
        }else if(gender == 'females'){
            return this.renderUser(females);
        }
        return this.renderUser(userList);
    }

  render() {
        const {isLoading,error,userList} = this.props;

         return (
             <div>
                 {this.renderSelect()}
           <ul>
                    {this.renderUser(userList)}
           </ul>
                </div>
     );
    }
}

export default connect(mapStateToProps)(UserList);

here is my List Component:

import React from 'react';

const List = ({users}) => (
    <ul>
        {users.map( (user,index) => (
          <li key = {index} >{user.email}</li>
        ))}
    </ul>
);

export default List;

the props (users) are passing correctly when I check the console in handleChange but can't quite a visual result.

Answer 1

State and Lifecycle

You should use state / setState as a way to trigger a rendering on react .

setState() enqueues changes to the component state and tells React that this component and its children need to be re-rendered with the updated state.

Main idea here is to store gender value in state, as we want to render when this value is changed

1. Add a initial state on your contructor

    constructor(props) { super(props); this.state = { gender: [] } } 

2. Update onChange event , using setState() with a new value. This function will notify asynchroniously react about the need to rerender. You need to refactor a bit your logic to clean you change handler with just updating state. Keep in mind setState is async, might be source of misunderstanding.

     handleChange = (e) => { setState({gender: e.target.value}); } 

3. Final version should look something like this :

    import React, { Component } from "react"; import ReactDOM from "react-dom"; import { connect } from 'react-redux'; import PropTypes from 'prop-types'; import List from '../components/List.jsx' class UserList extends Component { constructor(props) { super(props); this.state = { gender: null }; } renderUser = (toMap) => { return ( <List key = {toMap.length} users = {toMap.filter(user => this.state.gender ? user.gender === this.state.gender : true)}/> ); } renderSelect = () => { return ( <select id="gender" onChange={this.handleChange}> <option value="select">Select a gender</option> <option value="males">Males</option> <option value="females">Females</option> </select> ); } handleChange = (e) => { this.setState({ gender: e.target.value}) } render() { const {isLoading,error,userList} = this.props; return ( <div> { this.renderSelect() } <ul> { this.renderUser(userList) } </ul> </div> ); } } 

   renderUser() will perform a filter based on selected gender. No need to store males and females as performance should be fine for most case.

Answer 2

Here is a working example of your code : https://codesandbox.io/s/new-wkgu6

you are not setting state anywhere because of which component is not rendering again

setState() will always lead to a re-render unless shouldComponentUpdate() returns false.

More information on setState api can be found here

https://reactjs.org/docs/react-component.html#setstate