ReactJS：影像有時不會出現在按鈕點擊上

我是一個 React 菜鳥。我正在使用 Material UI 和Material UI 圖示以及 React 創建一個帶有前進和后退按鈕的標題，因此用戶可以滾動瀏覽顯示的圖片。當我按下前進按鈕時，“索引 2”中的圖片什么也沒有顯示。當我按下后退按鈕時，“索引 0”中的圖片是空白的（很奇怪，對嗎？）。在我看來，這似乎是一個非常奇怪的錯誤。我在這里缺少什么嗎？這對我來說毫無意義。

這是我的代碼（如您所見，我仍在編輯內容，因此當我從 Material UI 中獲取此代碼時，我仍然擁有默認文本）。

import React from 'react';
import { makeStyles } from '@material-ui/core/styles';
import clsx from 'clsx';
import Card from '@material-ui/core/Card';
import CardHeader from '@material-ui/core/CardHeader';
import CardMedia from '@material-ui/core/CardMedia';
import CardContent from '@material-ui/core/CardContent';
import CardActions from '@material-ui/core/CardActions';
import Collapse from '@material-ui/core/Collapse';
import Avatar from '@material-ui/core/Avatar';
import IconButton from '@material-ui/core/IconButton';
import Typography from '@material-ui/core/Typography';
import { red } from '@material-ui/core/colors';
import FavoriteIcon from '@material-ui/icons/Favorite';
import ShareIcon from '@material-ui/icons/Share';
import ExpandMoreIcon from '@material-ui/icons/ExpandMore';
import MoreVertIcon from '@material-ui/icons/MoreVert';
import ArrowForwardIosIcon from '@material-ui/icons/ArrowForwardIos';
import ArrowBackIosIcon from '@material-ui/icons/ArrowBackIos';

import image2019_0201 from '../images/2019_0201.jpeg';
import image2019_0202 from '../images/2019_0202.jpeg';
import image2019_0203 from '../images/2019_0203.jpeg';

const images = [
    image2019_0201,
    image2019_0202,
    image2019_0203,
];

const imageText = [
    "February 15, 2019",
    "text2",
    "text3"
]

const useStyles = makeStyles((theme) => ({
  root: {
    maxWidth: 345,
  },
  media: {
    height: 0,
    paddingTop: '56.25%', // 16:9
  },
  expand: {
    transform: 'rotate(0deg)',
    marginLeft: 'auto',
    transition: theme.transitions.create('transform', {
      duration: theme.transitions.duration.shortest,
    }),
  },
  expandOpen: {
    transform: 'rotate(180deg)',
  },
  avatar: {
    backgroundColor: red[500],
  },
}));

export default function Year2019() {
  const classes = useStyles();
  const [expanded, setExpanded] = React.useState(false);
  const [currentImageIndex, setCurrentImageIndex] = React.useState(0)

  const handleExpandClick = () => {
    setExpanded(!expanded);
  };

  const handleForwardClick = () => {
    console.log("current index", currentImageIndex);
    if (currentImageIndex < images.length) {
      setCurrentImageIndex(currentImageIndex   1);
    }
    else {
      setCurrentImageIndex(0);
    }
    console.log("new index", currentImageIndex);
  }

  const handleBackClick = () => {
    console.log("current index", currentImageIndex);
    if (currentImageIndex == 0) {
      setCurrentImageIndex(images.length);
    }
    else {
      setCurrentImageIndex(currentImageIndex - 1);
    }
    console.log("new index", currentImageIndex);

  }

  return (
    <Card className={classes.root}>
      <CardHeader
        title="Title"
        subheader="February 15, 2019"
      />
      <CardMedia
        className={classes.media}
        image={images[currentImageIndex]}
        title="image2019_0201"
      />
      <CardContent>
        <Typography variant="body2" color="textSecondary" component="p">
          Waiting for the shuttle.
        </Typography>
      </CardContent>
      <CardActions disableSpacing>
        <IconButton aria-label="back" onClick={handleBackClick}>
          <ArrowBackIosIcon/>
        </IconButton>
        <IconButton aria-label="forward" onClick={handleForwardClick}>
        <ArrowForwardIosIcon/>
        </IconButton>
        <IconButton
          className={clsx(classes.expand, {
            [classes.expandOpen]: expanded,
          })}
          onClick={handleExpandClick}
          aria-expanded={expanded}
          aria-label="show more"
        >
          <ExpandMoreIcon />
        </IconButton>
      </CardActions>
      <Collapse in={expanded} timeout="auto" unmountOnExit>
        <CardContent>
          <Typography paragraph>Method:</Typography>
          <Typography paragraph>
            Heat 1/2 cup of the broth in a pot until simmering, add saffron and set aside for 10
            minutes.
          </Typography>
          <Typography paragraph>
            Heat oil in a (14- to 16-inch) paella pan or a large, deep skillet over medium-high
            heat. Add chicken, shrimp and chorizo, and cook, stirring occasionally until lightly
            browned, 6 to 8 minutes. Transfer shrimp to a large plate and set aside, leaving chicken
            and chorizo in the pan. Add pimentón, bay leaves, garlic, tomatoes, onion, salt and
            pepper, and cook, stirring often until thickened and fragrant, about 10 minutes. Add
            saffron broth and remaining 4 1/2 cups chicken broth; bring to a boil.
          </Typography>
          <Typography paragraph>
            Add rice and stir very gently to distribute. Top with artichokes and peppers, and cook
            without stirring, until most of the liquid is absorbed, 15 to 18 minutes. Reduce heat to
            medium-low, add reserved shrimp and mussels, tucking them down into the rice, and cook
            again without stirring, until mussels have opened and rice is just tender, 5 to 7
            minutes more. (Discard any mussels that don’t open.)
          </Typography>
          <Typography>
            Set aside off of the heat to let rest for 10 minutes, and then serve.
          </Typography>
        </CardContent>
      </Collapse>
    </Card>
  );
}

前進和后退圖示的圖片

uj5u.com熱心網友回復：

在您的處理程式函式中，陣列索引可能會超出范圍。所以你應該修改你的處理函式，如下所示：

const handleForwardClick = () => {
  if (currentImageIndex < (images.length - 1)) {
    setCurrentImageIndex(currentImageIndex   1);
  }
  else {
    setCurrentImageIndex(0);
  }
}

const handleBackClick = () => {
  if (currentImageIndex == 0) {
    setCurrentImageIndex(images.length - 1);
  }
  else {
    setCurrentImageIndex(currentImageIndex - 1);
  }
}

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