Style

React Native for Web relies on JavaScript to define styles for your application. Along with a novel JS-to-CSS conversion strategy, this allows you to avoid issues arising from the 7 deadly sins of CSS:

Global namespace
Dependency hell
No dead code elimination
No code minification
No sharing of constants
Non-deterministic resolution
Lack of isolation

Defining styles

Styles should be defined outside of the component:

class Example extends React.Component {}

const styles = StyleSheet.create({
  heading: {
    color: 'gray',
    fontSize: '2rem'
  },
  text: {
    color: 'gray',
    fontSize: '1.25rem'
  }
})

Using StyleSheet.create is optional but provides some key advantages: styles are immutable in development, styles are converted to CSS rather than applied as inline styles, and styles are only created once for the application and not on every render.

The attribute names and values are a subset of CSS. See the style documentation of individual components.

Using styles

All the React Native components accept a style attribute.

<Text style={styles.text} />
<View style={styles.view} />

A common pattern is to conditionally add style based on a condition:

// either
<View style={[
  styles.base,
  this.state.active && styles.active
]} />

// or
<View style={{
  ...styles.base,
  ...(this.state.active && styles.active)
}} />

Composing styles

In order to let a call site customize the style of your component children, you can pass styles around. Use View.propTypes.style and Text.propTypes.style in order to make sure only valid styles are being passed.

class List extends React.Component {
  static propTypes = {
    style: View.propTypes.style,
    elementStyle: View.propTypes.style,
  }

  render() {
    return (
      <View style={this.props.style}>
        {elements.map((element) =>
          <View style={[ styles.element, this.props.elementStyle ]} />
        )}
      </View>
    );
  }
}

In another file:

<List style={styles.list} elementStyle={styles.listElement} />

You also have much greater control over how styles are composed when compared to using class names. For example, you may choose to accept a limited subset of style props in the component's API, and control when they are applied:

class List extends React.Component {
  static propTypes = {
    children: React.PropTypes.any,
    // limit which styles are accepted
    style: React.PropTypes.shape({
      borderColor: View.propTypes.borderColor,
      borderWidth: View.propTypes.borderWidth
    })
  }

  render() {
    return (
      <View
        children={children}
        style={[
          this.props.style,
          // override border-color when scrolling
          isScrolling && { borderColor: 'transparent' }
        ]}
      />
    )
  }
}

Media Queries

StyleSheet.create is a way of defining the styles your application requires; it does not concern itself with where or when those styles are applied to elements.

There are various React libraries wrapping JavaScript Media Query API's, e.g., react-media-queries, media-query-fascade, or react-responsive. This has the benefit of co-locating breakpoint-specific DOM and style changes.

Pseudo-classes and pseudo-elements

Pseudo-classes like :hover and :focus can be implemented with the events (e.g. onFocus). Pseudo-elements are not supported; elements should be used instead.

How it works

Every call to StyleSheet.create extracts the unique declarations and converts them to a unique CSS rule. This is sometimes referred to as "atomic CSS". All the core components map their style property-value pairs to the corresponding className's.

By doing this, the total size of the generated CSS is determined by the total number of unique declarations (rather than the total number of rules in the application), making it viable to inline the style sheet when pre-rendering on the server. Styles are updated if new module bundle are loaded asynchronously.

JavaScript definition:

const styles = StyleSheet.create({
  heading: {
    color: 'gray',
    fontSize: '2rem'
  },
  text: {
    color: 'gray',
    fontSize: '1.25rem'
  }
})

CSS output:

._s1 { color: gray; }
._s2 { font-size: 2rem; }
._s3 { font-size: 1.25rem; }

Rendered HTML:

<span className="_s1 _s2">Heading</span>
<span className="_s1 _s3">Text</span>

Reset

You do not need to include a CSS reset or normalize.css.

React Native for Web includes a very small CSS reset taken from normalize.css. It removes unwanted User Agent styles from (pseudo-)elements beyond the reach of React (e.g., html, body) or inline styles (e.g., ::-moz-focus-inner).

html {
  font-family: sans-serif;
  -ms-text-size-adjust: 100%;
  -webkit-text-size-adjust: 100%;
}

body {
  margin: 0;
}

button::-moz-focus-inner,
input::-moz-focus-inner {
  border: 0;
  padding: 0;
}

input[type="search"]::-webkit-search-cancel-button,
input[type="search"]::-webkit-search-decoration {
  -webkit-appearance: none;
}

ol,
ul,
li {
  list-style:none
}

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