[英]React Mulitple rendering functional component
我開始反應,我嘗試使用我的 cookie 從我的 api 獲取用戶數據,我有數據,但是當我 console.log() 用戶數據時,我有 4 個未定義的用戶數據,然后是 2 個相同的 object 數據。 問題是我不知道為什么以及當我嘗試使用道具在其他組件中獲取 user.id 時,我的 tpyerror id 未定義。
import React, {useState, useEffect} from "react";
import NavBar from "./component/NavBar";
import Navigation from "./component/Navigation"
import Cookie from "js-cookie"
import axios from "axios"
import "./App.css";
function App() {
const [user, setUser] = useState()
const [logged, setLogged] = useState(false)
const cookie = Cookie.get("login")
useEffect(() => {
axios.post('/getdatafromcookie', cookie)
.then((res) => {
if(res.data.success === true){
setLogged(true)
setUser(res.data.user)
}
})
}, [])
console.log(user)
return (
<div className="App">
<header className="NavHeader">
<NavBar user={user} logged={logged} />
</header>
<Navigation user={user} logged={logged}/>
</div>
);
}
export default App;
控制台日志顯示:
導航欄
import React, { useState, useEffect } from "react";
import RegisterForm from "../component/RegisterForm";
import RegisterLogin from "../component/RegisterLogin";
import { Navbar, Nav, Dropdown } from "react-bootstrap";
import { Button } from "@material-ui/core"
export default function NavBar(props) {
const [modalLoginShow, setModalLoginShow] = useState();
const [modalRegisterShow, setModalRegisterShow] = useState();
const user = props.user
const islogged = props.logged
if(islogged === false)
{
return (
<Navbar collapseOnSelect expand="lg" bg="transparent" variant="light">
<Navbar.Brand href="/">Matchandate</Navbar.Brand>
<Navbar.Toggle aria-controls="responsive-navbar-nav" />
<Navbar.Collapse id="responsive-navbar-nav">
<Nav className="mr-auto"></Nav>
<Nav>
<Nav.Link onClick={() => setModalLoginShow(true)}>
<Button variant="contained" color="secondary">Login</Button>
</Nav.Link>
<Nav.Link onClick={() => setModalRegisterShow(true)}>
<Button variant="contained" color="secondary" >Register</Button>
</Nav.Link>
</Nav>
</Navbar.Collapse>
<RegisterLogin show={modalLoginShow} onHide={() => setModalLoginShow(false)} />
<RegisterForm show={modalRegisterShow} onHide={() => setModalRegisterShow(false)} />
</Navbar>
)
}
return(
<Navbar collapseOnSelect expand="lg" bg="transparent" variant="light">
<Navbar.Brand href="/">Matchandate</Navbar.Brand>
<Navbar.Toggle aria-controls="responsive-navbar-nav" />
<Navbar.Collapse id="responsive-navbar-nav">
<Nav className="mr-auto"></Nav>
<Nav>
<Nav.Link href="/profile">
<Button variant="contained" color="primary">Profile</Button>
</Nav.Link>
<Nav.Link href="/logout">
<Button variant="contained" color="primary" >Logout</Button>
</Nav.Link>
</Nav>
</Navbar.Collapse>
</Navbar>
)
}
導航
import React from "react"
import Slider from "./SliderHome";
import Activate from "./Activate";
import ForgotPwd from "./Pages/ForgotPwd"
import ChangePwd from "./Pages/ChangePwd"
import UserProfile from "./Pages/UserProfil";
import ErrorPage from "./Pages/ErrorPage"
import { BrowserRouter as Router, Route, Switch, Redirect } from "react-router-dom";
export default function Navigation(props){
const user = props.user
const islogged = props.logged
if(islogged){
return(
<Router>
<Switch>
<Route exact path="/" exact component={Slider} />
<Route exact path="/profile" component={() => <UserProfile user={user} />} />
{/* <Route path="/user/:id" component={ChangePwd}/> */}
<Route path="/" component={ErrorPage} />
</Switch>
</Router>
)
}
return (
<Router>
<Switch>
<Route exact path="/" exact component={Slider} />
<Route path="/activate" component={Activate} />
<Route path="/forgot-pwd" component={ForgotPwd}/>
<Route path="/changepwd" component={ChangePwd}/>
{/* <Route path="/user/:id" component={ChangePwd}/> */}
<Route path="/" component={ErrorPage} />
</Switch>
</Router>
)
}
發生這種情況是因為 useEffect 在第一次渲染后執行,因此,第一次用戶是 null,一旦用戶內部有數據,您將需要保護代碼以進行渲染
return user ? <div className=“app”><NavBar user={user}/></div> : <div>loading</div>)
日志被打印這么多次是因為開發中的嚴格模式
在你的第一次渲染中:你的用戶是未定義的,這是正常的,因為你沒有在你的 usestate 中定義一個默認值你可以通過這個const [user, setUser] = useState({})
或者這個來改變你的代碼
import React, {useState, useEffect} from "react";
import NavBar from "./component/NavBar";
import Navigation from "./component/Navigation"
import Cookie from "js-cookie"
import axios from "axios"
import "./App.css";
function App() {
const [user, setUser] = useState()
const [logged, setLogged] = useState(false)
const cookie = Cookie.get("login")
useEffect(() => {
axios.post('/getdatafromcookie', cookie)
.then((res) => {
if(res.data.success === true){
setLogged(true)
setUser(res.data.user)
}
})
}, [])
console.log(user)
return (
<div className="App">
<header className="NavHeader">
{user ? <NavBar user={user} logged={logged} /> : "loading"}
</header>
{user ? <Navigation user={user} logged={logged}: "loading" />
</div>
);
}
export default App;
問題是我不知道為什么以及當我嘗試使用道具在其他組件中獲取 user.id 時,我的 tpyerror id 未定義。
原因是您嘗試在加載用戶之前獲取用戶的 id,這意味着您正在執行類似null.id
類的未定義的操作(取決於在沒有任何 arguments 的情況下調用時useState
返回的內容,可能為空)
您在console.log
中獲得多個 Undefined 的原因是:
第一次渲染App
組件時, useEffect
還沒有完成調用 api ,所以user
仍然是undefined
,第二次是因為你調用setLogged(true)
所以 user 仍然是undefined
,第三次和第四次......我不確定,但您可能正在以某種方式更改 state 導致重新渲染
解決此問題的正確等待是等到defined
user
(即 api 調用完成),您可以使用簡單的if
語句來做到這一點,例如
if (user.id) {
// return components when the user is logged in
} else {
// return components where the user is not logged in, usually a "loading" screen
}
現在您說user.id
返回type error
,但我在您的代碼中找不到任何user.id
,所以我假設您沒有發布整個內容。
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