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

不需要任何额外的NPM包

//...
const BackendPageListing = () => {
    
    const [ myData, setMyData] = useState( {
        id: 1,
        content: "abc"
    })

    const myFunction = ( x ) => {
        
        setPagenateInfo({
        ...myData,
        content: x
        })

        console.log(myData) // not reflecting change immediately

        let myDataNew = {...myData, content: x };
        
        console.log(myDataNew) // Reflecting change immediately

    }

    return (
        <>
            <button onClick={()=>{ myFunction("New Content")} }>Update MyData</button>
        </>
    )

其他回答

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);

不需要任何额外的NPM包

//...
const BackendPageListing = () => {
    
    const [ myData, setMyData] = useState( {
        id: 1,
        content: "abc"
    })

    const myFunction = ( x ) => {
        
        setPagenateInfo({
        ...myData,
        content: x
        })

        console.log(myData) // not reflecting change immediately

        let myDataNew = {...myData, content: x };
        
        console.log(myDataNew) // Reflecting change immediately

    }

    return (
        <>
            <button onClick={()=>{ myFunction("New Content")} }>Update MyData</button>
        </>
    )

React的useEffect有自己的状态/生命周期。它与状态突变有关,直到效果被破坏才会更新状态。

只要在参数状态中传递一个参数或留下一个黑色数组,它就会完美地工作。

React.useEffect(() => {
    console.log("effect");
    (async () => {
        try {
            let result = await fetch("/query/countries");
            const res = await result.json();
            let result1 = await fetch("/query/projects");
            const res1 = await result1.json();
            let result11 = await fetch("/query/regions");
            const res11 = await result11.json();
            setData({
                countries: res,
                projects: res1,
                regions: res11
            });
        } catch {}
    })(data)
}, [setData])
# or use this
useEffect(() => {
    (async () => {
        try {
            await Promise.all([
                fetch("/query/countries").then((response) => response.json()),
                fetch("/query/projects").then((response) => response.json()),
                fetch("/query/regions").then((response) => response.json())
            ]).then(([country, project, region]) => {
                // console.log(country, project, region);
                setData({
                    countries: country,
                    projects: project,
                    regions: region
                });
            })
        } catch {
            console.log("data fetch error")
        }
    })()
}, [setData]);

或者,您可以尝试React. useref()在React钩子中进行即时更改。

const movies = React.useRef(null);
useEffect(() => {
movies.current='values';
console.log(movies.current)
}, [])

我知道已经有很好的答案了。但我想给另一个想法如何解决同样的问题,并访问最新的'电影'状态,使用我的模块react-useStateRef。

正如你所理解的,通过使用React状态,你可以在每次状态改变时渲染页面。但是通过使用React ref,你总是可以得到最新的值。

模块react-useStateRef让你同时使用状态和ref。它与React向后兼容。useState,所以你可以替换import语句

const { useEffect } = React
import { useState } from 'react-usestateref'

  const [movies, setMovies] = useState(initialValue);

  useEffect(() => {
    (async function() {
      try {

        const result = [
          {
            id: "1546514491119",
          },
        ];
        console.log("result =", result);
        setMovies(result);
        console.log("movies =", movies.current); // will give you the latest results
      } catch (e) {
        console.error(e);
      }
    })();
  }, []);

更多信息:

react-usestsateref

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.
})