floating-action-button.js

import React from 'react';
import { StyleSheet, View } from 'react-native';
import { MaterialIcons } from '@expo/vector-icons';

const FAB_SIZE = 56;
const ICON_SIZE = 24;

const Fab = () => (
  <View style={styles.container}>
    <MaterialIcons name="share" size={ICON_SIZE} color="#fff" />
  </View>
);

const styles = StyleSheet.create({
  container: {
    width: FAB_SIZE,
    height: FAB_SIZE,
    borderRadius: FAB_SIZE / 2,
    backgroundColor: '#2196F3',
    elevation: 6,
  },
});