Question

每次我从另一个组件发出消息时，我都无法获得消息的完整列表。这是钩子和视图组件：

export function useChat() {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const socket = openSocket("http://localhost:3003");
    socket.on("chat message", msg => {
      const newState = update(messages, { $push: [msg] });
      setMessages(newState);
    });
  }, []);

  return { messages };
}

不幸的是，状态不会持续，并且总是显示最后一条消息：

export const HookSockets = () => {
  const { messages } = useChat();
  return (
    <div>
      {messages.map((message, index) => (
        <div key={index}>{message}</div>
      ))}
    </div>
  );
};

如果我按常规方式进行操作，那么一切都会按预期进行：

export class ClassSockets extends Component {
  state = {
    socket: openSocket("http://localhost:3003"),
    messages: [],
    message: ""
  };

  componentDidMount() {
    this.state.socket.on("chat message", msg => {
      const newState = update(this.state, {
        messages: { $push: [msg] }
      });
      this.setState(newState);
    });
  }

  handleClick = () => {
    this.state.socket.emit("chat message", this.state.message);
    this.setState({ message: "" });
  };

  handleChange = event => {
    this.setState({ message: event.target.value });
  };

  render() {
    return (
      <div>
        <div>Sockets</div>
        <div>{this.state.messages}</div>
        <input
          type="text"
          value={this.state.message}
          onChange={this.handleChange}
        />
        <button onClick={this.handleClick}>Send Message</button>
      </div>
    );
  }
}

Answer 1

由于您已经编写了useEffect以便在组件的初始安装时执行，因此它将创建一个闭包，该闭包引用messages的初始值，即使消息更新，它在后续调用中仍将引用相同的值

您应该将useEffect配置为在初始安装和消息更改时运行

export function useChat() {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const socket = openSocket("http://localhost:3003");
    socket.on("chat message", msg => {
      const newState = update(messages, { $push: [msg] });
      setMessages(newState);
    });
  }, [messages]);

  return { messages };
}

否则您可以使用回调模式来更新状态

export function useChat() {
  const [messages, setMessages] = useState([]);

  useEffect(() => {
    const socket = openSocket("http://localhost:3003");
    socket.on("chat message", msg => {
      setMessages(prevMessages => update(prevMessages, { $push: [msg] }););
    });
  }, []);

  return { messages };
}

useEffect内部的状态始终与React Hooks一起引用初始状态

1 个答案: