App-3.js

import React, {Component} from 'react';
import {Platform, StyleSheet, Text, View, Image, Button} from 'react-native';
import ToastModule from 'react-native-toast-library';
import RNAdCall from 'react-native-ad-call';

type Props = {};
var pic = {uri: 'https://developer.android.com/images/brand/Android_Robot_200.png'};
var osName = 'Android';
class OS extends Component {
    render() {
        return (
            <Text style={{fontSize: 30}}>{this.props.name}</Text>
        );
    }
}

export default class App extends Component<Props> {
    constructor(props) {
      super(props);
      //setup state
      this.state = {
        displayOS: 'android'
      };
    }
    renderPage = () => {
        if(osName === "Android") {
            pic = {uri: 'https://www.apple.com/ios/images/og.png?201805211344'}
            osName = 'iOS';
            console.log('changing to ios');
            ToastModule.show('Android');
            return this.setState({displayOS: 'ios'});
        }
        else{
            pic = {uri: 'https://developer.android.com/images/brand/Android_Robot_200.png'};
            osName = 'Android';
            console.log('changing to android');
            ToastModule.show('iOS');
            return this.setState({displayOS: 'android'});
        }
    }
    render() {
    return (
      <View style={styles.container}>
          <Image source={pic} style={{width:200, height:237}}/>
          <OS name={osName}/>
          <Button
              onPress={() => RNAdCall.callBannerActivity()}
              title="Ad Request"
              color="#000000"
              accessibilityLabel="Ad Request"
          />
      </View>
    );
    }
}

const styles = StyleSheet.create({
  container: {
    flex: 1,
    justifyContent: 'center',
    alignItems: 'center',
    backgroundColor: '#FFFFFF',
  },
});