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Version: 2021fa

Lecture 10

Lecture Slides

Final Project Instructions

Final Project - Milestone 2 due 12/2 by 11:59pm

Final Project - Milestone 3 due 12/9 by 11:59pm

React Native

What is React Native?

React Native allows for cross platform mobile development using a webdev framework we already know--React!

React Native allows you to build UIs independent of the platform. Usually when developing an app you have to develop an Android version (using Java/Kotlin) and iOS version (using Objective-C/Swift) separately. React Native takes care of this conversion for you.

Core React Native Components

Since React Native is really just React, many of the same concepts (useState, props, React Hooks, etc) still apply to React. However, instead of HTML we have Views. A view is the basic building block of UI in mobile development. Views can display images, hold text, handle user input, etc.

Some core React Native components are:

  • <View>: A container that supports layout with flexbox, style, some touch handling, and accessibility controls. Similar to a non-scrolling <div>.
  • <Text>: Displays, styles, and nests strings of text and even handles touch events. Similar to a <p>
  • <Image>: Displays images like <img>
  • <ScrollView>: A generic scrolling container than can hold nested components and views. Similar to a <div>.
  • <TextInput>: User text input field similar to <input type="text" />.
  • ... and you can also define your own custom components (and use those built by the community)!

How to start a React Native Project?

A popular way to use React Native is through the Expo framework, which allows developing, building, and iterating on iOS, Android and webapps. Expo provides a UI for you to view your changes and if you download the Expo app (Android, iOS) you can see those changes on your phone as well! After all, we're doing mobile development. Expo also has a lot of packages specific to accessing features on mobile devices like the status bar, battery level, I/O devices, etc.

To start a React Native project run the following:

yarn global add expo-cli
expo init <project name>
yarn start

expo init is similar to create-react-app in that it generates boilerplate code for you.


As part of the demo we built the simple TODO list app from assignment 4 and battery tracker in React Native!

Before starting though, we have to install the expo-battery package by running expo install expo-battery. We use expo install to install expo packages instead of yarn add or npm install because it prevents you from installing incompatible versions with your particular Expo SDK version.

import { addBatteryLevelListener, getBatteryLevelAsync } from 'expo-battery';
import { StatusBar } from 'expo-status-bar';
import React, { useEffect, useState } from 'react';
import { Button, StyleSheet, Text, View } from 'react-native';
import TaskList from './TaskList';

export default function App() {
const [battery, setBattery] = useState(0);
const [count, setCount] = useState(0);

// Example of using a battery package from expo to access mobile features
useEffect(() => {
// initially gets battery level
// subscribe to battery level to keep it in sync
const listener = addBatteryLevelListener((event) =>
// remove the subscription after unmounting
return listener.remove;
}, []);

return (
<View style={styles.container}>
<Text>Battery: {battery * 100}%</Text>
<Text>Count: {count}</Text>
<Button title="Increment" onPress={() => setCount((x) => x + 1)} />
<TaskList />
<StatusBar style="auto" />

// how to style elements
const styles = StyleSheet.create({
container: {
flex: 1,
backgroundColor: '#fff',
alignItems: 'center',
justifyContent: 'center',
gap: '2rem',
import {
} from 'react-native';
import React, { useState } from 'react';

const styles = StyleSheet.create({
// space out the container a bit
container: { display: 'flex', gap: '8px' },
// give our textbox a border
input: {
borderWidth: 1,

export default () => {
const [tasks, setTasks] = useState<string[]>([]);
const [input, setInput] = useState('');
return (
<View style={styles.container}>
<Text>Task List</Text>
{/* flatlist is a canonical way to render a list of items instead of using
map like you would in regular react */}
<FlatList data={tasks} renderItem={(info) => <Text>{info.item}</Text>} />
placeholder="Item Name"
onPress={() => {
setTasks((tasks) => [...tasks, input]);

Once you yarn start, you should be taken to Expo where you can view your changes on your browser. On the bottom left, there should also be a QR code. If you download the Expo app and scan the QR code with your phone camera (Android, iOS), it should take you to the Expo app where you can see your application in mobile form!

Learn more

This was a very cursory introduction to React Native. You can learn more by referring to React docs.

React Docs:

Expo Docs:

Extras - Prettifying your UI

Material UI is a library that ships a bunch of pre-style and customizable components for you to use in your own React projects. Material UI comes with a variety of components ranging from buttons to icons to drawers, all of which are customizable and come with their own props that can do common tweaks to their existing components to fit your individual need.

Semantic UI and Bootstrap are two other popular libraries used for styling and do not require projects written in React and provide pre-written classes for styling elements and organizing layout. Nevertheless - they do come with "React component" versions (Semantic UI React and React Bootstrap) similar to Material UI!