Make view 80% width of parent in React Native

If you are simply looking to make the input relative to the screen width, an easy way would be to use Dimensions:

// De structure Dimensions from React
var React = require('react-native');
var {
  ...
  Dimensions
} = React; 

// Store width in variable
var width = Dimensions.get('window').width; 

// Use width variable in style declaration
<TextInput style={{ width: width * .8 }} />

I've set up a working project here. Code is also below.

https://rnplay.org/apps/rqQPCQ

'use strict';

var React = require('react-native');
var {
  AppRegistry,
  StyleSheet,
  Text,
  View,
  TextInput,
  Dimensions
} = React;

var width = Dimensions.get('window').width;

var SampleApp = React.createClass({
  render: function() {
    return (
      <View style={styles.container}>
        <Text style={{fontSize:22}}>Percentage Width In React Native</Text>
        <View style={{marginTop:100, flexDirection: 'row',justifyContent: 'center'}}>
            <TextInput style={{backgroundColor: '#dddddd', height: 60, width: width*.8 }} />
          </View>
      </View>
    );
  }
});

var styles = StyleSheet.create({
  container: {
    flex: 1,
    marginTop:100
  },

});

AppRegistry.registerComponent('SampleApp', () => SampleApp);

As of React Native 0.42 height: and width: accept percentages.

Use width: 80% in your stylesheets and it just works.

• Screenshot
  
  

• Live Example
  Child Width/Height as Proportion of Parent
  
  

• Code

  import React from 'react';
  import { Text, View, StyleSheet } from 'react-native';
  
  const width_proportion = '80%';
  const height_proportion = '40%';
  
  const styles = StyleSheet.create({
    screen: {
      flex: 1,
      alignItems: 'center',
      justifyContent: 'center',
      backgroundColor: '#5A9BD4',
    },
    box: {
      width: width_proportion,
      height: height_proportion,
      alignItems: 'center',
      justifyContent: 'center',
      backgroundColor: '#B8D2EC',
    },
    text: {
      fontSize: 18,
    },
  });
  
  export default () => (
    <View style={styles.screen}>
      <View style={styles.box}>
        <Text style={styles.text}>
          {width_proportion} of width{'\n'}
          {height_proportion} of height
        </Text>
      </View>
    </View>
  );
  

That should fit your needs:

var yourComponent = React.createClass({
    render: function () {
        return (
            <View style={{flex:1, flexDirection:'column', justifyContent:'center'}}>
                <View style={{flexDirection:'row'}}>
                    <TextInput style={{flex:0.8, borderWidth:1, height:20}}></TextInput>
                    <View style={{flex:0.2}}></View> // spacer
                </View>
            </View>
        );
    }
});

In your StyleSheet, simply put:

width: '80%';

instead of:

width: 80%;

Keep Coding........ :)