I am a React noob. I am using Material UI and Material UI icons and React to create a title with a forward and back button, so the user can scroll through the pictures on display. When I press the forward button, the picture in "index 2" shows nothing. When I press the back button, the picture in "index 0" is blank (weird, right?). This seems like a super weird bug in my mind. Is there something I am missing here? It is making no sense to me.

Here is my code (as you can see I'm still editing things, so I still have the default text from when I grabbed this from Material UI).

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>
  );
}

Picture of the Forward and Backward Icons

CodePudding user response：

In your handler functions, the array index may go out of bound. So you should modify your handler functions like below:

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);
  }
}