为什么在 state 更改后 React 不重新渲染页面？

Question

so I was working on a basic Todo app using React.js and I was wondering why the todo component does not automatically re-render once the state changed (the state contains the list of todos- so adding a new todo would update this array)? It is supposed to re-render the Header and the Todo component of the page with the updated array of todos passed in as props. Here is my code:

import React from 'react';
import './App.css';

class Header extends React.Component {
  render() {
    let numTodos = this.props.todos.length;
    return <h1>{`You have ${numTodos} todos`}</h1>
  }
}

class Todos extends React.Component {
  render() {
    return (
    <ul>
      {
    this.props.todos.map((todo, index) => {
      return (<Todo index={index} todo={todo} />)
    })
    }
    </ul>
    )
  }
}

class Todo extends React.Component {
  render() {
    return <li key={this.props.index}>{this.props.todo}</li>
  }
}

class Form extends React.Component {
  constructor(props) {
    super(props);
    this.addnewTodo = this.addnewTodo.bind(this);
  }

  addnewTodo = () => {
    let inputBox = document.getElementById("input-box");
    if (inputBox.value === '') {
      return;
    }
    this.props.handleAdd(inputBox.value);
  }

  render() {
    return (
      <div>
        <input id="input-box" type="text"></input>
        <button type="submit" onClick={this.addnewTodo}>Add</button>
      </div>
    )
  }
}

class App extends React.Component {
  constructor(props) {
    super(props);
    this.state = { todos: ['task 1', 'task 2', 'task 3']}
    this.handleNewTodo = this.handleNewTodo.bind(this);
  }

  handleNewTodo(todo) {
    let tempList = this.state.todos;
    tempList.push(todo);
    this.setState = { todos: tempList };
  }

  render() {
    return (
      <div>
      <Header todos={this.state.todos} />
      <Todos todos={this.state.todos} /> 
      <Form todos={this.state.todos} handleAdd={this.handleNewTodo} />
      </div>
      )
  }
}

Answer 1

You are not updating the state correctly.

You need to make a copy of the this.state.todos , add the new todo in the copied array and then call this.setState function

handleNewTodo(todo) {
    let tempList = [...this.state.todos];
    tempList.push(todo);
    this.setState({ todos: tempList });
}

Notice that this.setState is a function

Answer 2

You're updating state incorrectly,

handleNewTodo(todo) {
    let tempList = [...this.state.todos];
    tempList.push(todo);
    this.setState({ todos: tempList });
  }

This is the correct syntax.