I am new to reactJS and am writing code so that before the data is loaded from DB, it will show loading message, and then after it is loaded, render components with the loaded data. To do this, I am using both useState hook and useEffect hook. Here is the code:
问题是，当我检查console.log时，useEffect被触发了两次。因此，代码将两次查询相同的数据，这是应该避免的。
下面是我写的代码:
import React from 'react';
import './App.css';
import {useState,useEffect} from 'react';
import Postspreview from '../components/Postspreview'
const indexarray=[]; //The array to which the fetched data will be pushed
function Home() {
const [isLoading,setLoad]=useState(true);
useEffect(()=>{
/*
Query logic to query from DB and push to indexarray
*/
setLoad(false); // To indicate that the loading is complete
})
},[]);
if (isLoading===true){
console.log("Loading");
return <div>This is loading...</div>
}
else {
console.log("Loaded!"); //This is actually logged twice.
return (
<div>
<div className="posts_preview_columns">
{indexarray.map(indexarray=>
<Postspreview
username={indexarray.username}
idThumbnail={indexarray.profile_thumbnail}
nickname={indexarray.nickname}
postThumbnail={indexarray.photolink}
/>
)}
</div>
</div>
);
}
}
export default Home;
有人能帮助我理解为什么它被调用两次，以及如何正确地修复代码吗?
非常感谢!
正如其他人已经指出的那样，这很可能是由于React 18.0引入了严格模式功能。
我写了一篇博客文章，解释了为什么会发生这种情况，以及你可以做些什么来解决它。
但如果你只是想看代码，这里:
let initialized = false
useEffect(() => {
if (!initialized) {
initialized = true
// My actual effect logic...
...
}
}, [])
或作为可重复使用的钩子:
import type { DependencyList, EffectCallback } from "react"
import { useEffect } from "react"
export function useEffectUnsafe(effect: EffectCallback, deps: DependencyList) {
let initialized = false
useEffect(() => {
if (!initialized) {
initialized = true
effect()
}
}, deps)
}
请记住，只有在不得已的情况下才应该使用这种解决方案!
下面是为您的目的定制的钩子。这对你的情况可能有帮助。
import {
useRef,
EffectCallback,
DependencyList,
useEffect
} from 'react';
/**
*
* @param effect
* @param dependencies
* @description Hook to prevent running the useEffect on the first render
*
*/
export default function useNoInitialEffect(
effect: EffectCallback,
dependancies?: DependencyList
) {
//Preserving the true by default as initial render cycle
const initialRender = useRef(true);
useEffect(() => {
let effectReturns: void | (() => void) = () => {};
/**
* Updating the ref to false on the first render, causing
* subsequent render to execute the effect
*
*/
if (initialRender.current) {
initialRender.current = false;
} else {
effectReturns = effect();
}
/**
* Preserving and allowing the Destructor returned by the effect
* to execute on component unmount and perform cleanup if
* required.
*
*/
if (effectReturns && typeof effectReturns === 'function') {
return effectReturns;
}
return undefined;
}, dependancies);
}
我使用这个作为我的替代useFocusEffect。我使用嵌套的react导航堆栈，如选项卡和抽屉，使用useEffect重构并不像预期的那样对我有效。
import React, { useEffect, useState } from 'react'
import { useFocusEffect } from '@react-navigation/native'
const app = () = {
const [isloaded, setLoaded] = useState(false)
useFocusEffect(() => {
if (!isloaded) {
console.log('This should called once')
setLoaded(true)
}
return () => {}
}, [])
}
还有一个例子，你在屏幕上导航了两次。
