使用 useEffect 钩子在反应功能组件中不显示 with.map() 的元素

Question

我在 useEffect 回调中创建了一个类别数组，当我 console.log 时它工作正常。

但是当 I.map() 它时，结果数组是空的。

import React, { useEffect } from 'react';

export const Categories = (props) => {
    let categories = [];

    useEffect(() => {
        props.films.forEach((film) => {
            if (categories.findIndex(item => item === film.category) === -1)
            {
                categories.push(film.category);
                console.log(categories);
            }

        })
    }, [props.films, categories])

    return (
        <div>
            {categories.map((category) => {
                return (
                    <div>
                        {category}
                    </div>
                )
            })}
        </div>
    );
}

有人有想法吗？

Answer 1

您应该对类别使用 state 值：

const [categories, setCategories] = React.useState([])

useEffect(() => {
    let categories = []
    props.films.forEach((film) => {
        if (categories.findIndex(item => item === film.category) === -1)
        {
            categories.push(film.category);
            console.log(categories);
        }

    })
    setCategories(categories)
}, [props.films])

Answer 2

当categories中的数据发生更改时，组件不会重新呈现。 为了呈现您需要使用useState存储数据的categories 。

import React, { useEffect, useState } from 'react';

export const Categories = (props) => {
    const [categories, setCategories] = useState([]);

    useEffect(() => {
        let filmCategories = []
        props.films.forEach((film) => {
            if (categories.findIndex(item => item === film.category) === -1)
            {
                filmCategories.push(film.category);
                console.log(filmCategories);
            }
        })
        setCategories(filmCategories)
    }, [props.films])

    return (
        <div>
            {categories.map((category) => {
                return (
                    <div>
                        {category}
                    </div>
                )
            })}
        </div>
    );
}

希望这可以帮助。

Answer 3

我想你可能是新来的反应。 我建议你看看React State 和 Lifecycle

您正在使用 react 钩子useEfect将在您的组件在 DOM 中呈现后调用

我可以想到两种可能的解决方案来解决这个问题

1) 使用反应钩子useState

import React, { useEffect } from 'react';

export const Categories = (props) => {
    //Make the local variable into a variable representing state
    let [categories, setCategories] = useState([]);

    useEffect(() => {
        const result = [...categories];
        props.films.forEach((film) => {
            if (result.findIndex(item => item === film.category) === -1)
            {
                result.push(film.category);
                console.log(result);
            }

        })
        //Set the state value to trigger a re-render of your component
        if(result.length !== categories.length)
            setCategories(result);
    }, [props.films, categories])

    return (
        <div>
            {categories.map((category) => {
                return (
                    <div>
                        {category}
                    </div>
                )
            })}
        </div>
    );
}

2）如果不需要重新渲染，去掉useEffect hook

import React, { useEffect } from 'react';

export const Categories = (props) => {
    let categories = props.films.map(film => {
            if (categories.findIndex(item => item === film.category) === -1)
            {
                categories.push(film.category);
                console.log(categories);
            }
     }


    return (
        <div>
            {categories.map((category) => {
                return (
                    <div>
                        {category}
                    </div>
                )
            })}
        </div>
    );
}

如果需要 useEffect 反应钩子，那么解决方案 1 更好

如果不需要重新渲染react组件，那么方案2更好