React Native Responsive Font Size

Question

You can use PixelRatio

for example:

var React = require('react-native');

var {StyleSheet, PixelRatio} = React;

var FONT_BACK_LABEL   = 18;

if (PixelRatio.get() <= 2) {
  FONT_BACK_LABEL = 14;
}

var styles = StyleSheet.create({
  label: {
    fontSize: FONT_BACK_LABEL
  }
});

Edit:

Another example:

import { Dimensions, Platform, PixelRatio } from 'react-native';

const {
  width: SCREEN_WIDTH,
  height: SCREEN_HEIGHT,
} = Dimensions.get('window');

// based on iphone 5s's scale
const scale = SCREEN_WIDTH / 320;

export function normalize(size) {
  const newSize = size * scale 
  if (Platform.OS === 'ios') {
    return Math.round(PixelRatio.roundToNearestPixel(newSize))
  } else {
    return Math.round(PixelRatio.roundToNearestPixel(newSize)) - 2
  }
}

Usage:

fontSize: normalize(24)

you can go one step further by allowing sizes to be used on every <Text /> components by pre-defined sized.

Example:

const styles = {
  mini: {
    fontSize: normalize(12),
  },
  small: {
    fontSize: normalize(15),
  },
  medium: {
    fontSize: normalize(17),
  },
  large: {
    fontSize: normalize(20),
  },
  xlarge: {
    fontSize: normalize(24),
  },
};

We use a simple, straight-forward, scaling utils functions we wrote:

import { Dimensions } from 'react-native';
const { width, height } = Dimensions.get('window');

//Guideline sizes are based on standard ~5" screen mobile device
const guidelineBaseWidth = 350;
const guidelineBaseHeight = 680;

const scale = size => width / guidelineBaseWidth * size;
const verticalScale = size => height / guidelineBaseHeight * size;
const moderateScale = (size, factor = 0.5) => size + ( scale(size) - size ) * factor;

export {scale, verticalScale, moderateScale};

Saves you some time doing many ifs. You can read more about it on my blog post.

---

Edit: I thought it might be helpful to extract these functions to their own npm package, I also included ScaledSheet in the package, which is an automatically scaled version of StyleSheet. You can find it here: react-native-size-matters.

adjustsFontSizeToFit and numberOfLines works for me. They adjust long email into 1 line.

<View>
  <Text
    numberOfLines={1}
    adjustsFontSizeToFit
    style={{textAlign:'center',fontSize:30}}
  >
    {this.props.email}
  </Text>
</View>

Because responsive units aren't available in react-native at the moment, I would say your best bet would be to detect the screen size and then use that to infer the device type and set the fontSize conditionally.

You could write a module like:

function fontSizer (screenWidth) {
  if(screenWidth > 400){
    return 18;
  }else if(screenWidth > 250){
    return 14;
  }else { 
    return 12;
  }
}

You'll just need to look up what the default width and height are for each device. If width and height are flipped when the device changes orientation you might be able to use aspect ratio instead or just figure out the lesser of the two dimensions to figure out width.

This module or this one can help you find device dimensions or device type.