React.js useState hook 导致过多的重新渲染并且无法更新我的状态答案

【问题标题】：React.js useState hook causes too many re-renders and cant update my stateReact.js useState hook 导致过多的重新渲染并且无法更新我的状态
【发布时间】：2020-08-09 17:53:12
【问题描述】：

我是一名试图掌握 React 窍门的学生。到目前为止，我希望能够将从 API 接收的数据数组切割成块，以便以后可以使用分页效果。我无法弄清楚为什么函数 arrayChunk() 最终会导致过多的渲染。如何设置我的函数以将 chunkedArray 推入 setChunkedYearArray？这是我的完整代码：




import React, { useState, useEffect } from 'react';
import InformationBox from '../../component/InformationBox/InformationBox';
import arrowButton from '../../assets/arrow-button.svg';

const InformationBoxLayout = (props) => {
    const [activeYear, setActiveYear] = useState([]);
    const [chunkedYearArray, setChunkedYearArray] = useState([]);   
useEffect(() => {
        axios
            .get('http://www.mocky.io/v2/5ea446a43000005900ce2ca3')
            .then((response) =>
                setActiveYear(
                    response.data.timelineInfo.filter((item) => item.year === '2018')
                )
            );
    }, []);

    const arrayChunk = (array, chunkSize) => {
        const chunkedArray = [];
        let clonedArray = [...array];
        const splitPieces = Math.ceil(clonedArray.length / chunkSize);
        for (let i = 0; i < splitPieces; i++) {
            chunkedArray.push(clonedArray.splice(0, chunkSize));
        }
        setChunkedYearArray(chunkedArray);
    };

    return (
        <div className={style.infoBoxLayoutStyle}>
            <button className={style.leftArrow}>
                <img src={arrowButton} alt='previous-page-button' />
            </button>
            {activeYear.length > 6
                ? arrayChunk(activeYear, 6)
                : console.log('no split')}

            <button className={style.rightArrow}>
                <img
                    src={arrowButton}
                    alt='next-page-button'
                    className={style.rotateArrowRight}
                />
            </button>
        </div>
    );
};

export default InformationBoxLayout;

【问题讨论】：

标签： reactjs react-hooks rerender use-state

【解决方案1】：

任何时候你调用 setState 组件都会重新渲染，所以你不能在渲染函数中调用 setState。没有理由为chunkedYearArray 使用状态。只需从函数返回值，然后使用它：

const InformationBoxLayout = props => {
  const [activeYear, setActiveYear] = useState([]);

  useEffect(() => {
    axios
      .get("http://www.mocky.io/v2/5ea446a43000005900ce2ca3")
      .then(response =>
        setActiveYear(
          response.data.timelineInfo.filter(item => item.year === "2018")
        )
      );
  }, []);

  const arrayChunk = (array, chunkSize) => {
    const chunkedArray = [];
    let clonedArray = [...array];
    const splitPieces = Math.ceil(clonedArray.length / chunkSize);
    for (let i = 0; i < splitPieces; i++) {
      chunkedArray.push(clonedArray.splice(0, chunkSize));
    }
    return chunkedArray;
  };

  // Now you have calculated it, so use it for whatever you want in your JSX
  const chunkedYearArray = activeYear.length > 6
        ? arrayChunk(activeYear, 6)
        : [];

  return (
    <div className={style.infoBoxLayoutStyle}>
      <button className={style.leftArrow}>
        <img src={arrowButton} alt="previous-page-button" />
      </button>


      <button className={style.rightArrow}>
        <img
          src={arrowButton}
          alt="next-page-button"
          className={style.rotateArrowRight}
        />
      </button>
    </div>
  );
};

export default InformationBoxLayout;

或者，如果您真的希望它处于状态，您可以通过 API 调用在 then 中进行计算：

  const [chunkedYearArray, setChunkedYearArray] = useState([]);
  useEffect(() => {
    axios
      .get("http://www.mocky.io/v2/5ea446a43000005900ce2ca3")
      .then(response =>
        setActiveYear(
          response.data.timelineInfo.filter(item => item.year === "2018")
        )

        const arrayChunk = (array, chunkSize) => {
          const chunkedArray = [];
          let clonedArray = [...array];
          const splitPieces = Math.ceil(clonedArray.length / chunkSize);
          for (let i = 0; i < splitPieces; i++) {
            chunkedArray.push(clonedArray.splice(0, chunkSize));
          }
          return chunkedArray;
        };

        const newChunkedYearArray = activeYear.length > 6 ? arrayChunk(activeYear, 6)
        : [];

        // Because we are in useEffect we are only calculated once!
        setChunkedYearArray(newChunkedYearArray)
      );
  }, []);

这里肯定有清理的余地，但我认为其中任何一个都会让你走上正轨。希望对您有所帮助！

【讨论】：

【解决方案2】：

您最终会调用arrayChunk 每个渲染设置一些状态并触发重新渲染。仅当 activeYear 更新应该有帮助时，才使用效果挂钩重新计算分块数组。

useEffect(() => {
  activeYear.length > 6 && arrayChunk(activeYear, 6);
}, [activeYear]);

然后只渲染{ chunkedYearArray }

【讨论】：