App.js

import React from 'react';
import {
  StyleSheet,
  Text,
  View,
  AppState
} from 'react-native';

import { useEffect, useRef, useState } from 'react';

const App = () => {

  const appState = useRef(AppState.currentState);
  const [appStateVisible, setAppStateVisible] = useState(appState.current);

  useEffect(() => {
    const subscription = AppState.addEventListener("change", nextAppState => {
      appState.current = nextAppState;
      setAppStateVisible(appState.current);
    });

    return () => {
      subscription.remove();
    };
  }, []);

  if (appStateVisible == 'active') {
    // The actual screen when you are on app again (main view logic)
    return (
      <View>
        <Text>Hello world</Text>
      </View>
    )
  } else {
    return (
    // Blank screen (security screen)
    <View>

    </View>
    )
  }
};

export default App;