React Hooks useState + useEffect + event给出陈旧状态

Question

我正在尝试使用React useEffect和useState的事件发射器，但它总是获得初始状态而不是更新状态。如果我直接调用事件处理程序，即使使用setTimeout，它也可以工作。

如果我将值传递给useEffect()第二个参数，它会使它工作，但是每次值改变时（这会触发击键），这会导致重新订阅事件发射器。

我究竟做错了什么？我已经尝试过useState，useRef，useReducer和useCallback，并且无法正常工作。

这是一个复制品：

import React, { useState, useEffect } from "react";
import { Controlled as CodeMirror } from "react-codemirror2";
import "codemirror/lib/codemirror.css";
import EventEmitter from "events";

let ee = new EventEmitter();

const initialValue = "initial value";

function App(props) {
  const [value, setValue] = useState(initialValue);

  // Should get the latest value, both after the initial server load, and whenever the Codemirror input changes.
  const handleEvent = (msg, data) => {
    console.info("Value in event handler: ", value);
    // This line is only for demoing the problem. If we wanted to modify the DOM in this event, we would instead call some setState function and rerender in a React-friendly fashion.
    document.getElementById("result").innerHTML = value;
  };

  // Get value from server on component creation (mocked)
  useEffect(() => {
    setTimeout(() => {
      setValue("value from server");
    }, 1000);
  }, []);

  // Subscribe to events on component creation
  useEffect(() => {
    ee.on("some_event", handleEvent);
    return () => {
      ee.off(handleEvent);
    };
  }, []);

  return (
    <React.Fragment>
      <CodeMirror
        value={value}
        options={{ lineNumbers: true }}
        onBeforeChange={(editor, data, newValue) => {
          setValue(newValue);
        }}
      />
      {/* Everything below is only for demoing the problem. In reality the event would come from some other source external to this component. */}
      <button
        onClick={() => {
          ee.emit("some_event");
        }}
      >
        EventEmitter (doesnt work)
      </button>
      <div id="result" />
    </React.Fragment>
  );
}

export default App;

这是App2中相同的代码沙箱：

https://codesandbox.io/s/ww2v80ww4l

App组件有3种不同的实现 - EventEmitter，pubsub-js和setTimeout。只有setTimeout有效。

谢谢！

编辑：为了澄清我的目标，我只想让handleEvent中的值在所有情况下都匹配Codemirror值。单击任何按钮时，应显示当前的codemirror值。而是显示初始值。

Answer 1

value在事件处理程序中是陈旧的，因为它从定义它的闭包中获取其值。除非我们每次value更改时重新订阅新的事件处理程序，否则它将不会获得新值。

解决方案1：为发布效果[value]创建第二个参数。这使得事件处理程序获得正确的值，但也会导致效果在每次击键时再次运行。

解决方案2：使用ref将最新的value存储在组件实例变量中。然后，创建一个效果，除了每次value状态更改时都会更新此变量。在事件处理程序中，使用ref，而不是value。

const [value, setValue] = useState(initialValue);
let refValue = useRef(value);
useEffect(() => {
    refValue.current = value;
});
const handleEvent = (msg, data) => {
    console.info("Value in event handler: ", refValue.current);
};

https://reactjs.org/docs/hooks-faq.html#what-can-i-do-if-my-effect-dependencies-change-too-often

看起来该页面上还有其他一些可能也有效的解决方案。非常感谢@Dinesh提供的帮助。

Answer 2

更新的答案。

问题不在于钩子。初始状态值已关闭并传递给EventEmitter并一次又一次地使用。

在handleEvent中直接使用状态值不是一个好主意。相反，我们需要在发出事件时将它们作为参数传递。

import React, { useState, useEffect } from "react";
import { Controlled as CodeMirror } from "react-codemirror2";
import "codemirror/lib/codemirror.css";
import EventEmitter from "events";

let ee = new EventEmitter();

const initialValue = "initial value";

function App(props) {
  const [value, setValue] = useState(initialValue);
  const [isReady, setReady] = useState(false);

  // Should get the latest value
  function handleEvent(value, msg, data) {
    // Do not use state values in this handler
    // the params are closed and are executed in the context of EventEmitter
    // pass values as parameters instead
    console.info("Value in event handler: ", value);
    document.getElementById("result").innerHTML = value;
  }

  // Get value from server on component creation (mocked)
  useEffect(() => {
    setTimeout(() => {
      setValue("value from server");
      setReady(true);
    }, 1000);
  }, []);

  // Subscribe to events on component creation
  useEffect(
    () => {
      if (isReady) {
        ee.on("some_event", handleEvent);
      }
      return () => {
        if (!ee.off) return;
        ee.off(handleEvent);
      };
    },
    [isReady]
  );

  function handleClick(e) {
    ee.emit("some_event", value);
  }

  return (
    <React.Fragment>
      <CodeMirror
        value={value}
        options={{ lineNumbers: true }}
        onBeforeChange={(editor, data, newValue) => {
          setValue(newValue);
        }}
      />
      <button onClick={handleClick}>EventEmitter (works now)</button>
      <div id="result" />
    </React.Fragment>
  );
}

export default App;

这是一个有效的codesandbox