React Hooks useState+useEffect+event 给出过时的状态

Question

我正在尝试将事件发射器与 React useEffect 和 useState 一起使用，但它总是获取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，它会使其工作，但这会导致每次值更改时重新订阅事件发射器（由击键触发）。

我做错了什么？我尝试了useState、useRef、useReducer 和useCallback，但都无法正常工作。

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。

编辑

为了阐明我的目标，我只希望 handleEvent 中的值在所有情况下都与 Codemirror 值匹配。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它的值。除非我们每次value 更改时都重新订阅一个新的事件处理程序，否则它不会获得新的值。

解决方案 1：将第二个参数设置为发布效果 [value]。这使得事件处理程序获得正确的值，但也使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，制作一个除了在每次value 状态更改时更新此变量之外什么都不做的效果。在事件处理程序中，使用ref，而不是value。

const [value, setValue] = useState(initialValue);
const 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一些其他解决方案也可能有效。非常感谢@Dinesh 的帮助。

Answer 2

更新答案。

问题不在于钩子。初始状态值被关闭并传递给 EventEmitter 并被反复使用。

直接在handleEvent 中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

Answer 3

useCallback 应该在这里工作。

import React, { useState, useEffect, useCallback } from "react";
import PubSub from "pubsub-js";
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
  const handler = (msg, data) => {
    console.info("Value in event handler: ", value);
    document.getElementById("result").innerHTML = value;
  };

  const handleEvent = useCallback(handler, [value]);

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

  // Subscribe to events on component creation
  useEffect(() => {
    PubSub.subscribe("some_event", handleEvent);
    return () => {
      PubSub.unsubscribe(handleEvent);
    };
  }, [handleEvent]);
  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);
        }}
      />
      <button
        onClick={() => {
          ee.emit("some_event");
        }}
      >
        EventEmitter (works)
      </button>
      <button
        onClick={() => {
          PubSub.publish("some_event");
        }}
      >
        PubSub (doesnt work)
      </button>
      <button
        onClick={() => {
          setTimeout(() => handleEvent(), 100);
        }}
      >
        setTimeout (works!)
      </button>
      <div id="result" />
    </React.Fragment>
  );
}

export default App;

在此处检查代码框https://codesandbox.io/s/react-base-forked-i9ro7