我正在尝试学习钩子和useState方法使我困惑。我以数组的形式将初始值赋给一个状态。useState中的set方法不适合我,无论是否使用扩展语法。

我已经在另一台PC上做了一个API,我正在调用并获取我想设置为状态的数据。

这是我的代码:

<div id="root"></div> <script type="text/babel" defer> // import React, { useState, useEffect } from "react"; // import ReactDOM from "react-dom"; const { useState, useEffect } = React; // web-browser variant const StateSelector = () => { const initialValue = [ { category: "", photo: "", description: "", id: 0, name: "", rating: 0 } ]; const [movies, setMovies] = useState(initialValue); useEffect(() => { (async function() { try { // const response = await fetch("http://192.168.1.164:5000/movies/display"); // const json = await response.json(); // const result = json.data.result; const result = [ { category: "cat1", description: "desc1", id: "1546514491119", name: "randomname2", photo: null, rating: "3" }, { category: "cat2", description: "desc1", id: "1546837819818", name: "randomname1", rating: "5" } ]; console.log("result =", result); setMovies(result); console.log("movies =", movies); } catch (e) { console.error(e); } })(); }, []); return <p>hello</p>; }; const rootElement = document.getElementById("root"); ReactDOM.render(<StateSelector />, rootElement); </script> <script src="https://unpkg.com/@babel/standalone@7/babel.min.js"></script> <script src="https://unpkg.com/react@17/umd/react.production.min.js"></script> <script src="https://unpkg.com/react-dom@17/umd/react-dom.production.min.js"></script>

setMovies(result)和setMovies(…result)都不起作用。

我希望结果变量被推入movies数组。


当前回答

我刚刚用useReducer完成了重写,遵循@kentcdobs文章(参考下面),这真的给了我一个坚实的结果,没有遭受这些闭包问题。

参见:https://kentcdodds.com/blog/how-to-use-react-context-effectively

我将他可读的样板文件压缩到我喜欢的dry级别——阅读他的沙盒实现将向你展示它是如何实际工作的。

import React from 'react'

// ref: https://kentcdodds.com/blog/how-to-use-react-context-effectively

const ApplicationDispatch = React.createContext()
const ApplicationContext = React.createContext()

function stateReducer(state, action) {
  if (state.hasOwnProperty(action.type)) {
    return { ...state, [action.type]: state[action.type] = action.newValue };
  }
  throw new Error(`Unhandled action type: ${action.type}`);
}

const initialState = {
  keyCode: '',
  testCode: '',
  testMode: false,
  phoneNumber: '',
  resultCode: null,
  mobileInfo: '',
  configName: '',
  appConfig: {},
};

function DispatchProvider({ children }) {
  const [state, dispatch] = React.useReducer(stateReducer, initialState);
  return (
    <ApplicationDispatch.Provider value={dispatch}>
      <ApplicationContext.Provider value={state}>
        {children}
      </ApplicationContext.Provider>
    </ApplicationDispatch.Provider>
  )
}

function useDispatchable(stateName) {
  const context = React.useContext(ApplicationContext);
  const dispatch = React.useContext(ApplicationDispatch);
  return [context[stateName], newValue => dispatch({ type: stateName, newValue })];
}

function useKeyCode() { return useDispatchable('keyCode'); }
function useTestCode() { return useDispatchable('testCode'); }
function useTestMode() { return useDispatchable('testMode'); }
function usePhoneNumber() { return useDispatchable('phoneNumber'); }
function useResultCode() { return useDispatchable('resultCode'); }
function useMobileInfo() { return useDispatchable('mobileInfo'); }
function useConfigName() { return useDispatchable('configName'); }
function useAppConfig() { return useDispatchable('appConfig'); }

export {
  DispatchProvider,
  useKeyCode,
  useTestCode,
  useTestMode,
  usePhoneNumber,
  useResultCode,
  useMobileInfo,
  useConfigName,
  useAppConfig,
}

用类似这样的用法:

import { useHistory } from "react-router-dom";

// https://react-bootstrap.github.io/components/alerts
import { Container, Row } from 'react-bootstrap';

import { useAppConfig, useKeyCode, usePhoneNumber } from '../../ApplicationDispatchProvider';

import { ControlSet } from '../../components/control-set';
import { keypadClass } from '../../utils/style-utils';
import { MaskedEntry } from '../../components/masked-entry';
import { Messaging } from '../../components/messaging';
import { SimpleKeypad, HandleKeyPress, ALT_ID } from '../../components/simple-keypad';

export const AltIdPage = () => {
  const history = useHistory();
  const [keyCode, setKeyCode] = useKeyCode();
  const [phoneNumber, setPhoneNumber] = usePhoneNumber();
  const [appConfig, setAppConfig] = useAppConfig();

  const keyPressed = btn => {
    const maxLen = appConfig.phoneNumberEntry.entryLen;
    const newValue = HandleKeyPress(btn, phoneNumber).slice(0, maxLen);
    setPhoneNumber(newValue);
  }

  const doSubmit = () => {
    history.push('s');
  }

  const disableBtns = phoneNumber.length < appConfig.phoneNumberEntry.entryLen;

  return (
    <Container fluid className="text-center">
      <Row>
        <Messaging {...{ msgColors: appConfig.pageColors, msgLines: appConfig.entryMsgs.altIdMsgs }} />
      </Row>
      <Row>
        <MaskedEntry {...{ ...appConfig.phoneNumberEntry, entryColors: appConfig.pageColors, entryLine: phoneNumber }} />
      </Row>
      <Row>
        <SimpleKeypad {...{ keyboardName: ALT_ID, themeName: appConfig.keyTheme, keyPressed, styleClass: keypadClass }} />
      </Row>
      <Row>
        <ControlSet {...{ btnColors: appConfig.buttonColors, disabled: disableBtns, btns: [{ text: 'Submit', click: doSubmit }] }} />
      </Row>
    </Container>
  );
};

AltIdPage.propTypes = {};

现在,在我的所有页面上,一切都很顺利

其他回答

很像扩展React创建的类组件中的. setstate()。组件或React。使用useState钩子提供的更新器的PureComponent状态更新也是异步的,并且不会立即反映出来。

此外,这里的主要问题不仅是异步性质,而且是函数基于当前闭包使用状态值,状态更新将在下一次重新呈现中反映出来,这样现有的闭包不会受到影响,但会创建新的闭包。现在,在当前状态下,钩子中的值是由现有的闭包获得的,当重新呈现发生时,闭包将根据是否再次重新创建函数而更新。

即使您添加了一个setTimeout函数,尽管超时将在重新呈现发生的一段时间后运行,但setTimeout仍将使用前一个闭包的值,而不是更新的值。

setMovies(result);
console.log(movies) // movies here will not be updated

如果你想在状态更新上执行一个动作,你需要使用useEffect钩子,就像在类组件中使用componentDidUpdate一样,因为useState返回的setter没有回调模式

useEffect(() => {
    // action on update of movies
}, [movies]);

就更新状态的语法而言,setMovies(result)将用异步请求中可用的movies值替换状态中先前的movies值。

但是,如果希望将响应与先前存在的值合并,则必须使用状态更新的回调语法以及正确使用扩展语法,如

setMovies(prevMovies => ([...prevMovies, ...result]));

使用我库中的自定义钩子,你可以等待状态值更新:

useAsyncWatcher(…values):watcherFn(peekPrevValue: boolean)=>Promise -是一个围绕useEffect的承诺包装器,它可以等待更新并返回一个新值,如果可选的peekPrevValue参数设置为true,则可能返回一个以前的值。

(现场演示)

    import React, { useState, useEffect, useCallback } from "react";
    import { useAsyncWatcher } from "use-async-effect2";
    
    function TestComponent(props) {
      const [counter, setCounter] = useState(0);
      const [text, setText] = useState("");
    
      const textWatcher = useAsyncWatcher(text);
    
      useEffect(() => {
        setText(`Counter: ${counter}`);
      }, [counter]);
    
      const inc = useCallback(() => {
        (async () => {
          await new Promise((resolve) => setTimeout(resolve, 1000));
          setCounter((counter) => counter + 1);
          const updatedText = await textWatcher();
          console.log(updatedText);
        })();
      }, []);
    
      return (
        <div className="component">
          <div className="caption">useAsyncEffect demo</div>
          <div>{counter}</div>
          <button onClick={inc}>Inc counter</button>
        </div>
      );
    }
    
    export default TestComponent;

useAsyncDeepState是一个深度状态实现(类似于此。setState (patchObject)),它的setter可以返回一个与内部效果同步的承诺。如果调用setter时不带参数,则它不会更改状态值,而只是订阅状态更新。在这种情况下,您可以从组件内部的任何地方获取状态值,因为函数闭包不再是障碍。

(现场演示)

import React, { useCallback, useEffect } from "react";
import { useAsyncDeepState } from "use-async-effect2";

function TestComponent(props) {
  const [state, setState] = useAsyncDeepState({
    counter: 0,
    computedCounter: 0
  });

  useEffect(() => {
    setState(({ counter }) => ({
      computedCounter: counter * 2
    }));
  }, [state.counter]);

  const inc = useCallback(() => {
    (async () => {
      await new Promise((resolve) => setTimeout(resolve, 1000));
      await setState(({ counter }) => ({ counter: counter + 1 }));
      console.log("computedCounter=", state.computedCounter);
    })();
  });

  return (
    <div className="component">
      <div className="caption">useAsyncDeepState demo</div>
      <div>state.counter : {state.counter}</div>
      <div>state.computedCounter : {state.computedCounter}</div>
      <button onClick={() => inc()}>Inc counter</button>
    </div>
  );
}
var [state,setState]=useState(defaultValue)

useEffect(()=>{
   var updatedState
   setState(currentState=>{    // Do not change the state by get the updated state
      updateState=currentState
      return currentState
   })
   alert(updateState) // the current state.
})

使用后台定时器库。它解决了我的问题。

const timeoutId = BackgroundTimer.setTimeout(() => {
    // This will be executed once after 1 seconds
    // even when the application is the background
    console.log('tac');
}, 1000);

之前答案的附加细节:

虽然React的setState是异步的(类和钩子都是异步的),而且很容易用这个事实来解释观察到的行为,但这并不是它发生的原因。

TLDR:原因是围绕不可变const值的闭包作用域。


解决方案:

read the value in render function (not inside nested functions): useEffect(() => { setMovies(result) }, []) console.log(movies) add the variable into dependencies (and use the react-hooks/exhaustive-deps eslint rule): useEffect(() => { setMovies(result) }, []) useEffect(() => { console.log(movies) }, [movies]) use a temporary variable: useEffect(() => { const newMovies = result console.log(newMovies) setMovies(newMovies) }, []) use a mutable reference (if we don't need a state and only want to remember the value - updating a ref doesn't trigger re-render): const moviesRef = useRef(initialValue) useEffect(() => { moviesRef.current = result console.log(moviesRef.current) }, [])


解释为什么会这样:

如果async是唯一的原因,那么可以等待setState()。

然而,道具和状态都假定在1渲染期间不变。

对待这个问题。状态,仿佛它是不可变的。

对于钩子,通过使用常量值和const关键字来加强这个假设:

const [state, setState] = useState('initial')

该值在2个渲染之间可能不同,但在渲染本身和任何闭包(即使在渲染完成后仍然存在更长的函数,例如useEffect,事件处理程序,在任何Promise或setTimeout内)中保持不变。

考虑以下伪的,但是同步的,类似react的实现:

// sync implementation: let internalState let renderAgain const setState = (updateFn) => { internalState = updateFn(internalState) renderAgain() } const useState = (defaultState) => { if (!internalState) { internalState = defaultState } return [internalState, setState] } const render = (component, node) => { const {html, handleClick} = component() node.innerHTML = html renderAgain = () => render(component, node) return handleClick } // test: const MyComponent = () => { const [x, setX] = useState(1) console.log('in render:', x) // ✅ const handleClick = () => { setX(current => current + 1) console.log('in handler/effect/Promise/setTimeout:', x) // ❌ NOT updated } return { html: `<button>${x}</button>`, handleClick } } const triggerClick = render(MyComponent, document.getElementById('root')) triggerClick() triggerClick() triggerClick() <div id="root"></div>