[英]Method not getting correct useState value despite updating state in React
我在一个试图显示分页学生信息的网站上工作。 例如,我有 12 个学生,我有分页项目 1-4,因为我一次显示 3 个学生。
当我调用我的更新分页 function 时,我遇到了一个问题。尽管我正确地递增了它,但此方法中的计数器始终为 0。 任何想法或指示将不胜感激。
import React, {useState, useEffect} from 'react';
function App() {
let students = [
{'name': 'Harry'},
{'name': 'Hermoine'},
{'name': 'Ron'},
{'name': 'Ginny'},
{'name': 'Snape'},
{'name': 'Bellatrix'},
{'name': 'Albus'},
{'name': 'Dudley'},
{'name': 'Petunia'},
{'name': 'Hagrid'},
{'name': 'Lee'},
{'name': 'James'}
];
let [loaded, setLoaded] = useState(false);
let [pagination, setPagination] = useState([]);
let [limit, setLimit] = useState(3); // how many students are visible at a time
let [pages, setPages] = useState(Math.round(students.length/limit)); // amount of pages based on total students
let [currentPage, setCurrentPage] = useState(1); // the current page
let [pageCount, setPageCount] = useState(0); // counter used to increment/decrement pages in view
function updatePagination() {
let tmpArray = [];
for(let i = pageCount; i < pages; i ++){
tmpArray.push(i+1);
}
// 1-3, 4-6 etc...
setPagination(tmpArray.slice(pageCount, pageCount + limit)); // this pageCount is always 0 despite me setting it on handleNext method
}
function handleNext(){
setCurrentPage(currentPage + 1);
if(pageCount + limit === currentPage){
if(currentPage <= pages){
setPageCount(pageCount + limit);
}
}
updatePagination();
}
useEffect(() => {
updatePagination();
setLoaded(true);
}, []);
return (
<main className={styles.app}>
<div className={styles.student}>
<h1>Three student cards</h1>
</div>
<ol className={styles.pagination}>
<div className={styles.paginationContainer}>
<button className={currentPage === 0 ? styles.prev + ' ' + styles.disabled : styles.prev}>prev</button>
{loaded && pagination.map((item, index) => {
return (
<li key={index} className={styles.paginationItem}>
<button className={currentPage === item ? styles.active : null}>{item}</button>
</li>
)
})}
<button onClick={() => {
handleNext()
}} className={currentPage === pages ? styles.next + ' ' + styles.disabled : styles.next}>next</button>
</div>
</ol>
{loaded &&
<div className={styles.info}>
<p>Page Count: {pageCount}</p>
<p>Current Page: {currentPage}</p>
<p>Limit: {limit}</p>
<p>Pages: {pages}</p>
<p>Total Students: {students.length}</p>
<p>Pagination: {pagination}</p>
</div>
}
</main>
);
}
export default App;
以下是您的代码中的问题:
在handleNext()
function 内部,您在调用setCurrentPage(...)
function 后立即使用currentPage
,但 state 是异步更新的。 所以currentPage
将是旧值而不是更新值。
另一个问题是,如果用户点击next
按钮三次,那么条件pageCount + limit === currentPage
将为true
, pageCount
将设置为pageCount + limit
,即 3。这意味着handleNext()
function 中的循环for (let i = pageCount; i < pages; i++) {...}
只会执行一次。 所以tmpArray
将只包含一个元素,即 4。
此外,由于调用handleNext()
function 会根据其先前的值更新currentPage
,因此您应该通过将 function 传递给setCurrentPage()
function 来更新 state
setCurrentPage((page) => page + 1);
类似于上一页的 go:
setCurrentPage((page) => page - 1);
这将确保 state 已正确更新。
之前并不清楚您为什么使用pageCount
,但您在评论中发布的演示清楚地表明了您要实现的目标。
您在问题中提到的问题是pageCount
的值始终为零。 查看您的代码,我认为您根本不需要pageCount
。
要实现所需的功能,您需要执行以下步骤:
要填充pagination
数组,您可以使用useEffect
钩子,它会在students
数组或limit
发生变化时执行。
useEffect(() => { // set pagination let arr = new Array(Math.ceil(students.length / limit)).fill().map((_, idx) => idx + 1); setPagination(arr); setLoaded(true); }, [students, limit]);
要一次显示有限数量的学生,请创建一个 function,它根据currentPage
和limit
对students
数组进行切片。
const getPaginatedStudents = () => { const startIndex = currentPage * limit - limit; const endIndex = startIndex + limit; return students.slice(startIndex, endIndex); };
要显示等于limit
的有限页数,您不需要pageCount
。 您可以创建一个 function 来根据currentPage
和limit
对pagination
数组进行切片。 这个 function 与步骤 2 中创建的类似,但区别在于startIndex
的计算方式。
const getPaginationGroup = () => { let start = Math.floor((currentPage - 1) / limit) * limit; let end = start + limit; return pagination.slice(start, end); };
创建将更改currentPage
的函数
function goToNextPage() { setCurrentPage((page) => page + 1); } function goToPreviousPage() { setCurrentPage((page) => page - 1); } function changePage(event) { const pageNumber = Number(event.target.textContent); setCurrentPage(pageNumber); }
这就是获得所需功能所需的全部。
以下代码片段显示了一个示例:
const studentsData = [ { name: "Harry" }, { name: "Hermoine" }, { name: "Ron" }, { name: "Ginny" }, { name: "Snape" }, { name: "Bellatrix" }, { name: "Albus" }, { name: "Dudley" }, { name: "Petunia" }, { name: "Hagrid" }, { name: "Lee" }, { name: "James" }, { name: "Lily" }, { name: "Remus" }, { name: "Voldemort" }, { name: "Dobby" }, { name: "Lucius" }, { name: "Sirius" } ]; function Student({ name }) { return ( <div className="student"> <h3>{name}</h3> </div> ); } function App() { let [students] = React.useState(studentsData); let [pagination, setPagination] = React.useState([]); let [loaded, setLoaded] = React.useState(false); let [limit] = React.useState(3); let [pages] = React.useState(Math.round(students.length / limit)); let [currentPage, setCurrentPage] = React.useState(1); function goToNextPage() { setCurrentPage((page) => page + 1); } function goToPreviousPage() { setCurrentPage((page) => page - 1); } function changePage(event) { const pageNumber = Number(event.target.textContent); setCurrentPage(pageNumber); } React.useEffect(() => { // set pagination let arr = new Array(Math.ceil(students.length / limit)).fill().map((_, idx) => idx + 1); setPagination(arr); setLoaded(true); }, [students, limit]); const getPaginatedStudents = () => { const startIndex = currentPage * limit - limit; const endIndex = startIndex + limit; return students.slice(startIndex, endIndex); }; const getPaginationGroup = () => { let start = Math.floor((currentPage - 1) / limit) * limit; let end = start + limit; return pagination.slice(start, end); }; return ( <main> <h1>Students</h1> {loaded && ( <div className="studentsContainer"> {getPaginatedStudents().map((s) => ( <Student key={s.name} {...s} /> ))} </div> )} <ol className="pagination"> <div className="paginationContainer"> <button onClick={goToPreviousPage} className={currentPage === 1? "prev disabled": "prev"} > prev </button> {loaded && getPaginationGroup().map((item, index) => { return ( <li key={index} className="paginationItem"> <button onClick={changePage} className={currentPage === item? "active": null} > {item} </button> </li> ); })} <button onClick={goToNextPage} className={currentPage === pages? "next disabled": "next"} > next </button> </div> </ol> </main> ); } ReactDOM.render(<App/>, document.getElementById('root'));
h1 { margin: 0; }.studentsContainer { display: flex; background: #efefef; padding: 15px 10px; margin: 5px 0; }.student { flex-grow: 1; box-shadow: 0 0 2px rgba(0, 0, 0, 0.4); text-align: center; max-width: 450px; margin: 0 5px; }.pagination { position: relative; padding: 0; }.paginationContainer { align-items: center; display: grid; grid-template-columns: repeat(auto-fit, minmax(40px, auto)); grid-column-gap: .65rem; justify-content: center; justify-items: center; max-width: 100%; width: 100%; position: relative; }.paginationItem { height: 40px; width: 40px; user-select: none; list-style: none; }.prev, .next { user-select: none; cursor: pointer; background: transparent; border: none; outline: none; }.prev.disabled, .next.disabled { pointer-events: none; opacity: .5; } button { padding: .65rem; display: block; height: 100%; width: 100%; border-radius: 50%; text-align: center; border: 1px solid #ccc; outline: none; cursor: pointer; } button.active { background: rgba(black, .25); border: none; color: #777; pointer-events: none; }
<script src="https://cdnjs.cloudflare.com/ajax/libs/react/16.13.1/umd/react.production.min.js"></script> <script src="https://cdnjs.cloudflare.com/ajax/libs/react-dom/16.13.1/umd/react-dom.production.min.js"></script> <div id="root"></div>
您也可以在 codesandbox 上查看此演示。
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