React Native Extended StyleSheet

Drop-in replacement of React Native StyleSheet with media-queries, variables, dynamic themes, relative units, percents, math operations, scaling and other styling stuff.

Installation
Usage
Features
API
- .create()
- .build()
- .value()
- .memoize()
- .child()
- .subscribe()
FAQ
Changelog
Feedback
License

Installation

npm i react-native-extended-stylesheet --save

Usage

Import EStyleSheet and call EStyleSheet.build() in entry point of your app:

// app.js
import EStyleSheet from 'react-native-extended-stylesheet';
 
EStyleSheet.build({
  $textColor: 'green' // variable 
});

Define styles using EStyleSheet.create() in components:

// component.js
import EStyleSheet from 'react-native-extended-stylesheet';
 
const styles = EStyleSheet.create({
  column: {
    width: '80%'         // 80% of screen width
  },
  text: {
    color: '$textColor', // use variable $textColor
    fontSize: '1.5rem'   // use relative unit - CSS3 rem
  }
});
 
class MyComponent extends React.Component {
  render() {
    return (
      // use styles as normal react-native StyleSheet
      <View style={styles.column}>
        <Text style={styles.text}>Hello</Text>
      </View>
    );
  }
}

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Features

Global variables

Global variables are useful for global theming or A/B testing of your app. They are passed to EStyleSheet.build() and available in any stylesheet.

// app entry: set global variables and calc styles
EStyleSheet.build({
  $textColor: '#0275d8'
});
 
// component: use global variables
const styles = EStyleSheet.create({
  text: {
    color: '$textColor'
  }
});

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Theming

There can be two types of themes:

static (app reload needed to theme change)
dynamic (theme can be changed in runtime)

Please see examples of static themes and dynamic themes.
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Local variables

Local variables can be defined directly in sylesheet and have priority over global variables. To define local variable just start it with $:

const styles = EStyleSheet.create({
  $textColor: '#0275d8',
  text: {
    color: '$textColor'
  },
  icon: {
    color: '$textColor'
  },
});

Local variables are also available in result style: styles.$textColor.
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Math operations

Any value can contain one of following math operations: *, /, +, -. Operands can be numbers, variables and percents.
For example, to render circle you may create style:

const styles = EStyleSheet.create({
  $size: 20,
  circle: {
    width: '$size',
    height: '$size',
    borderRadius: '0.5 * $size'
  }
});

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REM units

Similar to CSS3 rem unit it allows to define any integer value as relative to the root element. In our case root value is special rem global variable that can be set in EStyleSheet.build(). It makes easy to scale app depending on screen size and other conditions. Default rem is 16.

// component
const styles = EStyleSheet.create({
  text: {
    fontSize: '1.5rem',
    marginHorizontal: '2rem'
  }
});
// app entry
let {height, width} = Dimensions.get('window');
EStyleSheet.build({
  rem: width > 340 ? 18 : 16
});

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Percents

Percent values are supported natively since React Native 0.43. Style properties that contain only percent value (e.g. "80%") are passed as is to the native code. The only case when JavaScript calculation is applied by this library - operations with percent values, e.g. "100% - 20". Percents are calculated relative to screen width/height on application launch.

const styles = EStyleSheet.create({
  column: {
    width: '100% - 20'
  }
});

Percents in nested components
If you need sub-components with percent operations - you can use variables.
For example, to render 2 sub-columns with 30%/70% width of parent:

render() {
  return (
    <View style={styles.column}>
      <View style={styles.subColumnLeft}></View>
      <View style={styles.subColumnRight}></View>
    </View>
  );
}
 
...
 
const styles = EStyleSheet.create({
  $columnWidth: '80%',
  column: {
    width: '$columnWidth',
    flexDirection: 'row'
  },
  subColumnLeft: {
    width: '0.3 * $columnWidth'
  },
  subColumnRight: {
    width: '0.7 * $columnWidth'
  }
});

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Media queries

Media queries allows to have different styles for different screens, platform and orienation. They are supported as properties with @media prefix (thanks for idea to @grabbou, #5).

Media queries can operate with the following values:

media type: ios|android
width, min-width, max-width
height, min-height, max-height
orientation (landscape|portrait)
aspect-ratio

Example:

const styles = EStyleSheet.create({
  column: {
    width: '80%',
  },
  '@media (min-width: 350) and (max-width: 500)': {
    column: {
      width: '90%',
    }
  }
});

Also you can use media queries on style level:

const styles = EStyleSheet.create({
  header: {
    fontSize: 18,
    '@media ios': {
      color: 'green',
    },
    '@media android': {
      color: 'blue',
    },
  }
});

See full example here.
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Scaling

You can apply scale to components by setting special $scale variable.

const styles = EStyleSheet.create({
  $scale: 1.5,
  button: {
    width: 100,
    height: 20,
    marginLeft: 10
  }
});

This helps to create reusable components that could be scaled depending on prop:

class Button extends React.Component {
  static propTypes = {
    scale: React.PropTypes.number
  };
  render() {
    let style = getStyle(this.props.scale)
    return (
      <View style={style.button}>
      </View>
    );
  }
}
 
let getStyle = function (scale = 1) {
  return EStyleSheet.create({
    $scale: scale,
    button: {
      width: 100,
      height: 20,
      marginLeft: 10
    }
  });
}

To cache calculated styles please have a look on caching section.
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Underscored styles

Original react-native stylesheets are calculated to integer numbers and original values are unavailable. But sometimes they are needed. Let's take an example:
You want to render text and icon with the same size and color. You can take this awesome icon library and see that <Icon> component has size and color props. It would be convenient to define style for text and keep icon's size/color in sync.

const styles = EStyleSheet.create({
  text: {
    fontSize: '1rem',
    color: 'gray'
  }
});

In runtime styles created with original react's StyleSheet will look like:

styles = {
  text: 0
}

But extended stylesheet saves calculated values under _text property:

styles = {
  text: 0,
  _text: {
    fontSize: 16,
    color: 'gray'
  }
}

To render icon we just take styles from _text:

return (
  <View>
    <Icon name="rocket" size={styles._text.fontSize} color={styles._text.color} />
    <Text style={styles.text}>Hello</Text>
  </View>
);

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Pseudo classes (:nth-child)

Extended stylesheet supports 4 pseudo classes: :first-child, :nth-child-even, :nth-child-odd, :last-child. As well as in traditional CSS it allows to apply special styling for first/last items or render stripped rows.
To get style for appropriate index you should use EStyleSheet.child() method. It's signature: EStyleSheet.child(stylesObj, styleName, index, count).

const styles = EStyleSheet.create({
  row: {
    fontSize: '1.5rem',
    borderTopWidth: 1
  },
  'row:nth-child-even': {
    backgroundColor: 'gray' // make stripped
  },
  'row:last-child': {
    borderBottomWidth: 1 // render bottom edge for last row
  }
});
...
render() {
  return (
    <View>
      {items.map((item, index) => {
        return (
          <View key={index} style={EStyleSheet.child(styles, 'row', index, items.length)}></View>
        );
      })}
    </View>
  );
}

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Value as a function

For the deepest customization you can specify any value as a function that will be executed on EStyleSheet build. For example, you may darken or lighten color of variable via npm color package:

import Color from 'color';
import EStyleSheet from 'react-native-extended-stylesheet';
 
const styles = EStyleSheet.create({
  button: {
    backgroundColor: '$buttonColor',
  },
  $underlayColor: () => Color(EStyleSheet.value('$buttonColor')).darken(0.1).hexString();
});
...
render() {
  return (
    <TouchableHighlight style={styles.button} underlayColor={styles.$underlayColor}>
      ...
    </TouchableHighlight>
  );
}

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Caching

If you use dynamic styles depending on runtime prop or you are making reusable component with dynamic styling you may need stylesheet creation in every render() call. Let's take example from scaling section:

class Button extends React.Component {
  static propTypes = {
    scale: React.PropTypes.number
  };
  render() {
    let style = getStyle(this.props.scale)
    return (
      <View style={style.button}>
      </View>
    );
  }
}
 
let getStyle = function (scale = 1) {
  return EStyleSheet.create({
    $scale: scale,
    button: {
      width: 100,
      height: 20,
      marginLeft: 10
    }
  });
}

To avoid creating styles on every render you can use EStyleSheet.memoize() wrapper method that works similar to lodash.memoize: store result for particular parameters and returns it from cache when called with the same parameters. Updated example:

let getStyle = EStyleSheet.memoize(function (scale = 1) {
  return EStyleSheet.create({
    $scale: scale,
    button: {
      width: 100,
      height: 20,
      marginLeft: 10
    }
  });
});

Now if you call getStyle(1.5) 3 times actually style will be created on the first call and two other calls will get it from cache.
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Outline for debug

To outline all components for debug purpuses just set special $outline variable:

// outline all stylesheets
EStyleSheet.build({outline: 1}); 
 
// outline particular stylesheet
const styles = EStyleSheet.create({
  $outline: 1,
  column: {
    width: '80%',
    flexDirection: 'row'
  },
  ...
});

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EStyleSheet API

.create()

/**
 * Creates extended stylesheet object
 *
 * @param {Object} source style
 * @returns {Object} extended stylesheet object
 */
 create (source) {...}

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.build()

/**
 * Calculates all stylesheets
 *
 * @param {Object} [globalVars] global variables for all stylesheets
 */
 build (globalVars) {...}

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.value()

/**
 * Calculates particular value
 *
 * @param {*} value 
 * @param {String} [prop] property for which value is calculated. Needed for example for percent values.
 * @returns {*} calculated result
 */
 value (value, prop) {...}

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.memoize()

/**
 * Wraps function to cache calls with the same parameters
 *
 * @param {Function} fn 
 * @returns {Function} wrapped function
 */
 memoize (fn) {...}

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.child()

/**
 * Returns styles with pseudo classes :first-child, :nth-child-even, :last-child according to index and count
 *
 * @param {Object} stylesheet 
 * @param {String} styleName 
 * @param {Number} index index of item for style
 * @param {Number} count total count of items
 * @returns {Object|Array} styles
 */
 child (styles, styleName, index, count) {...}

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.subscribe()

/**
 * Subscribe to events. Currently only 'build' event is supported
 *
 * @param {String} event 
 * @param {Function} listener 
 */
 subscribe (event, listener) {...}

This method is useful when you want to pre-render some component on init. As extended style is calculated after call of EStyleSheet.build(), it is not available instantly after creation so you should wrap pre-render info listener to build event:

const styles = EStyleSheet.create({
  button: {
    width: '80%',
  }
});
 
// this will NOT work as styles.button is not calculated yet
let Button = <View style={styles.button}></View>;
 
// but this will work
let Button;
EStyleSheet.subscribe('build', () => {
  Button = <View style={styles.button}></View>;
});

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FAQ

What about orientation change?
Currently orientation change is not properly supported. Please see this issue for more details.

Changelog

CHANGELOG.md

Feedback

If you have any ideas or something goes wrong feel free to open new issue.

License

MIT
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