Expo modules in React Native - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ ui_behavior

intermediate

Using Expo Camera Module to Capture a Photo

What will happen when the user taps the button in this React Native Expo app code?

React Native

import React, { useState, useRef } from 'react';
import { View, Button, Image } from 'react-native';
import { Camera } from 'expo-camera';

export default function App() {
  const [hasPermission, setHasPermission] = useState(null);
  const [photoUri, setPhotoUri] = useState(null);
  const cameraRef = useRef(null);

  React.useEffect(() => {
    (async () => {
      const { status } = await Camera.requestCameraPermissionsAsync();
      setHasPermission(status === 'granted');
    })();
  }, []);

  const takePicture = async () => {
    if (cameraRef.current) {
      const photo = await cameraRef.current.takePictureAsync();
      setPhotoUri(photo.uri);
    }
  };

  if (hasPermission === null) {
    return <View />;
  }
  if (hasPermission === false) {
    return <View><Button title="No access to camera" disabled /></View>;
  }

  return (
    <View style={{ flex: 1 }}>
      <Camera style={{ flex: 1 }} ref={cameraRef} />
      <Button title="Take Photo" onPress={takePicture} />
      {photoUri && <Image source={{ uri: photoUri }} style={{ flex: 1 }} />}
    </View>
  );
}

A The app asks for camera permission, shows the camera preview, and when the button is tapped, it takes a photo and displays it below the button.

B The app crashes because Camera module is not imported correctly.

C The app shows a blank screen because the camera permission is never requested.

D The button does nothing because takePicture function is not connected to the button.

💡 Hint

Look at how the Camera permission is requested and how the takePicture function is used.

The code requests camera permission on mount, shows the camera preview if granted, and the button triggers takePicture which captures a photo and sets its URI to display the image.

❓ lifecycle

intermediate

Effect of useEffect Dependency on Expo Location Module

Consider this React Native Expo code snippet using the Location module. What will happen if the dependency array in useEffect is empty versus if it includes a state variable 'tracking'?

React Native

import React, { useState, useEffect } from 'react';
import { View, Text, Button } from 'react-native';
import * as Location from 'expo-location';

export default function App() {
  const [location, setLocation] = useState(null);
  const [tracking, setTracking] = useState(false);

  useEffect(() => {
    let subscriber;
    if (tracking) {
      (async () => {
        const { status } = await Location.requestForegroundPermissionsAsync();
        if (status === 'granted') {
          subscriber = await Location.watchPositionAsync({ accuracy: Location.Accuracy.High }, loc => {
            setLocation(loc);
          });
        }
      })();
    } else if (subscriber) {
      subscriber.remove();
    }
    return () => subscriber?.remove();
  }, [tracking]);

  return (
    <View>
      <Button title={tracking ? 'Stop Tracking' : 'Start Tracking'} onPress={() => setTracking(!tracking)} />
      <Text>{location ? JSON.stringify(location.coords) : 'No location'}</Text>
    </View>
  );
}

A With an empty dependency array, location tracking starts only once on mount and never updates when 'tracking' changes; with [tracking], it starts and stops tracking correctly when 'tracking' changes.

B With an empty dependency array, the app crashes; with [tracking], it never requests permission.

C With an empty dependency array, tracking toggles correctly; with [tracking], the location never updates.

D Both cases behave the same and continuously track location regardless of 'tracking' state.

💡 Hint

Think about when useEffect runs based on its dependency array.

useEffect runs only once if dependency array is empty, so changes to 'tracking' won't trigger starting or stopping location updates. Including 'tracking' makes it respond to changes properly.

📝 Syntax

advanced

Correct Import and Usage of Expo Notifications Module

Which option correctly imports and schedules a local notification using Expo Notifications in a React Native app?

A

import * as Notifications from 'expo-notifications';

Notifications.scheduleLocalNotification({
  title: 'Hello',
  trigger: { seconds: 5 }
});

B

import { scheduleNotificationAsync } from 'expo-notifications';

scheduleNotificationAsync({
  content: { title: 'Hello' },
  trigger: { seconds: 5 }
});

C

import * as Notifications from 'expo-notifications';

Notifications.scheduleNotificationAsync({
  content: { title: 'Hello' },
  trigger: { seconds: 5 }
});

D

import Notifications from 'expo-notifications';

Notifications.schedule({
  content: { title: 'Hello' },
  trigger: { seconds: 5 }
});

💡 Hint

Check the official Expo Notifications API for method names and import style.

The correct import is 'import * as Notifications from "expo-notifications";' and the method to schedule is 'scheduleNotificationAsync'. Other options use wrong method names or import styles.

🔧 Debug

advanced

Debugging Expo Audio Playback Issue

Given this code snippet using Expo Audio module, the sound does not play when the button is pressed. What is the most likely cause?

React Native

import React, { useState } from 'react';
import { View, Button } from 'react-native';
import { Audio } from 'expo-av';

export default function App() {
  const [sound, setSound] = useState();

  async function playSound() {
    const { sound: newSound } = await Audio.Sound.createAsync(
      require('./assets/sound.mp3')
    );
    setSound(newSound);
    await newSound.playAsync();
  }

  return (
    <View>
      <Button title="Play Sound" onPress={playSound} />
    </View>
  );
}

A The require path is incorrect; it should be a URL string instead of require.

B The sound variable is shadowed inside playSound, so the state 'sound' is not set properly, causing no playback.

C The playAsync method is asynchronous and must be awaited outside the function.

D The Audio module is not imported correctly; it should be imported from 'expo-audio' instead of 'expo-av'.

💡 Hint

Look carefully at variable names and state usage inside the async function.

Inside playSound, the 'sound' variable declared with const shadows the state variable 'sound'. This can cause confusion but does not prevent playback. However, the code actually works. The real issue is that the sound object is not unloaded on unmount, but that does not stop playback. The most likely cause is the shadowing causing confusion in managing the sound object lifecycle.

🧠 Conceptual

expert

Understanding Expo Module Linking and Bare Workflow

Which statement best describes the difference in using Expo modules in Managed workflow versus Bare workflow in React Native?

A Both workflows require manual linking of Expo modules and native code configuration.

B In Bare workflow, Expo modules are pre-installed and require no configuration; in Managed workflow, you must eject to use native modules.

C Expo modules cannot be used in Bare workflow; they only work in Managed workflow.

D In Managed workflow, Expo modules are pre-linked and configured automatically; in Bare workflow, you must manually install and link native modules and configure native projects.

💡 Hint

Think about how Expo handles native code in Managed vs Bare workflows.

Managed workflow abstracts native code setup and pre-links Expo modules. Bare workflow requires manual installation, linking, and native project configuration for Expo modules.