index.android-BUTTON.js

import React, { Component } from 'react';
import { AppRegistry, View ,Button,StyleSheet,Alert } from 'react-native';

export default class ButtonBasics extends Component {
   _onPressButton() {
       Alert.alert('You tapped the button!')
   }
    render() {
        return (
        //    <Button onPress={() => {Alert.alert('You tapped the button!')}} title="Press Me"/>
           <View style={styles.container}>
               <View style={this.buttonContainer}>
                   <Button onPress={this._onPressButton}
                   title="Press Me"/>
               </View>

               <View style={styles.buttonContainer}>
                   <Button onPress={this._onPressButton}
                   title="Press Me"
                   color="#841584"/>
               </View>
               <View style={styles.alternativeLayoutButtonContainer}>
                   <Button onPress={this._onPressButton}
                   title="This looks great!"/>
                   <Button onPress={this._onPressButton}
                   title="OK!" color="#841584"/>
               </View>
           </View> 
        );
    }
}
const styles = StyleSheet.create({
    container: {
        flex: 1,
        justifyContent: 'center'
    },
    buttonContainer: {
        margin:20
    },
    alternativeLayoutButtonContainer: {
        margin: 20,
        flexDirection: 'row',
        justifyContent: 'space-between'
    }
})

AppRegistry.registerComponent('AwesomeProject',()=>ButtonBasics);