_onpressbutton = async () => {

    axios.post('apivalidacion', {
      username: this.state.user,
      password: this.state.pass
    }).then(function (response){
      this.props.navigation.navigate('Main');
    }).catch(function(error){
      console.log(error);
    });
};

import React from 'react';
import axios from 'axios';
import {
  Alert,
  StyleSheet,
  View,
  AppRegistry,
  ImageBackground,

} from 'react-native';
import { TextField } from 'react-native-material-textfield';
import { Button , Text} from 'native-base'