MUI Box 组件的用途是什么？

A Box is just that, a box. It's an element wrapped around its content which by itself contains no styling rules nor has any default effects on the visual output. But it's a place to put styling rules as needed. It doesn't offer any real functionality, just a placeholder for controlling the styles in the hierarchical markup structure.

Structurally it results in a <div>.

I often think of it as semantically similar to the JSX empty element:

<>
Some elements here
</>

In that it's used to group things. But it results in a <div> and can include some Material UI capabilities:

<Box className={classes.someStyling}>
Some elements here
</Box>

The Box component in Material UI it has a lot of useful stuff

The most important thing is that box element has been built in with material-ui/system functionalities by default that mean you can apply system functionalities to what you need if you use it as wrapper

Like this example:

<Box bgcolor="primary.main" color="primary.contrastText" p={2}>
primary.main
</Box>

and of course you can add css class to it as you like or not

the other good useful thing that it offer it can be warp in other components and be very helpful to apply system functionalities to it

Like this example:

<Typography component="div" variant="body1">
<Box color="primary.main">primary.main</Box>
</Typography>

Both of examples above i took them from documentation you can find them in this link :here

and you can find what i mean by material ui system functionalities:here

Note: you can add any of material ui system functionalities to any component like docs here but i recommend you to warp what u need with box component it make life a lot easier

EDIT: This was written in the MUI v4 days. In MUI v5, all MUI components allow you to define CSS styles via the sx prop, not just Box; but Box also accepts styling props at top-level, as well as within sx.

The other answers don't really explain the motivation for using Box.

Box renders a <div> you can apply CSS styles to directly via React props, for the sake of convenience, since alternatives like separate CSS files, CSS-in-JS, or inline styles can be more typing and hassle to use.

For example, consider this component that uses JSS:

import * as React from 'react'


import { makeStyles } from '@material-ui/styles'


const useStyles = makeStyles(theme => ({
root: {
display: 'flex',
flexDirection: 'column',
alignItems: 'center',
padding: theme.spacing(1),
}
}))


const Example = ({children, ...props}) => {
const classes = useStyles(props);


return (
<div className={classes.root}>
{children}
</div>
)
}

It's much shorter to do this with Box by passing the props you want to it:

import * as React from 'react'


import Box from '@material-ui/core/Box'


const Example = ({children}) => (
<Box display="flex" flexDirection="column" alignItems="stretch" padding={1}>
{children}
</Box>
)

Notice also how padding={1} is a shorthand for theme.spacing(1). Box provides various conveniences for working with Material-UI themes like this.

In larger files it can be more of a hassle to jump back and forth from the rendered elements to the styles than if the styles are right there on the element.

Cases where you wouldn't want to use Box (MUI v4):

• You want the enclosing component to be able to override styles by passing classes (makeStyles enables this. <Example classNames=\{\{ root: 'alert' }} /> would work in the makeStyles example, but not the Box example.)
• You need to use nontrivial selectors (example JSS selectors: $root > li > a, $root .third-party-class-name)

A Box is basically a div* on steroid. Box allows you to apply dynamic styles to an otherwise normal div very quickly like inline styles (but it's not inline styles). Besides that, it also has a first-class integration with MUI theme:

<Box
sx=\{\{
bgcolor: 'primary.main',
color: 'text.secondary',
border: 4,
borderRadius: 2,
px: 2,
fontWeight: 'fontWeightBold',
zIndex: 'tooltip',
boxShadow: 8,
}}
>
Box
</Box>

If you need to do the above with a normal div, you have to get the theme object using useTheme hook and create an inline styles which is not a good practice if abused everywhere:

<div
style=\{\{
backgroundColor: theme.palette.primary.main,
color: theme.palette.text.secondary,
border: '4px solid black',
borderRadius: theme.shape.borderRadius * 2,
padding: `0 ${theme.spacing(2)}`,
fontWeight: theme.typography.fontWeightBold,
zIndex: theme.zIndex.tooltip,
boxShadow: theme.shadows[8],
}}
>
div
</div>

Box among other components like Typography or Stack also supports system properties that lets you pass the style values to the top-level props, which resulted in even shorter code. Internally, the system properties are gathered and merged with the sx prop so they are the same thing (See this answer for more detail)

<Box
bgcolor="primary.main"
color="text.secondary"
border={4}
borderRadius={2}
px={2}
fontWeight="fontWeightBold"
zIndex="tooltip"
boxShadow={8}
>
Box
</Box>

Because Box leverages sx prop, you can also use sx features like adding responsive values:

<Box
display=\{\{
xs: 'none',
sm: 'block',
}}
width=\{\{
sm: 30,
md: 50,
lg: 100,
}}
>

Or creating nested styles:

<Box
display='flex'
sx=\{\{
'& > :not(:last-child)': {
mr: 2 // maginRight: theme.spacing(2)
},
':hover': {
bgcolor: 'green'
}
}}
>

When to use Box?

• When you want to create a styled div quickly when prototyping.
• When you want to create a one-off inline styles that is not really reusable anywhere else. This is convenient when you want to fix something that is a bit off in a specific part of your layout.
• When you want to add dynamic or responsive styles and make your code easier to understand at the same time because everything is defined in one place, plus the fact that sx syntax is highly compact.
• When you want to reference multiple MUI theme properties because many sx properties are theme-aware out-of-the-box.

When not to use Box?

• When you don't need to styles anything. Just use a normal div then.
• When you are using it in a highly reusable components like list item, grid item or table cell. This is because sx prop has the slowest performance (2x slower than the second slowest approach)
• When you're using other MUI components. In v5, all components from MUI has sx support so using Box as a wrapper or root component is unnecessary if you just want to style other MUI components.

^{*: By default a Box is a div, but you can override the root component of it. For example: <Box component='span'>}