Expo modules in React Native - Build, Publish & Deploy

Build & Publish - Expo modules

Performance Impact of Expo Modules

Using Expo modules can affect your app's performance in several ways. They provide native features with JavaScript bridges, which may add slight overhead compared to fully native code. However, Expo modules are optimized to maintain smooth UI rendering at 60fps on most devices. Memory usage is generally moderate, but including many modules can increase RAM consumption and battery usage due to background tasks or sensors.

💻

How to Optimize Expo Modules for 60fps Rendering

To keep your app smooth, only include the Expo modules you actually use. Avoid importing large modules unnecessarily. Use lazy loading or dynamic imports when possible to reduce initial load. Profile your app with React Native DevTools and Expo's performance monitors to spot bottlenecks. Also, minimize heavy computations on the JavaScript thread to prevent frame drops.

Impact on App Bundle Size and Startup Time

Each Expo module adds native code and JavaScript to your app bundle, increasing its size. This can slow down app startup and increase download size. Using only required modules and enabling Expo's tree shaking can reduce bundle size. Managed Expo projects handle this automatically, but bare workflow apps should carefully manage dependencies to keep the app lightweight.

iOS vs Android Differences for Expo Modules

Expo modules are designed to work cross-platform, but some native features behave differently on iOS and Android. For example, permissions, background tasks, and hardware access may have platform-specific APIs or limitations. Testing on both platforms is essential. Also, iOS requires code signing and provisioning profiles for native modules, while Android uses APK/AAB signing.

Relevant Store Review Guidelines and Requirements

When using Expo modules, ensure your app complies with Apple App Store and Google Play policies. For iOS, follow Apple's Human Interface Guidelines and ensure privacy permissions are clearly requested and explained. For Android, comply with Google Play's permissions and background activity policies. Both stores require proper code signing and metadata for native modules.

Self-Check: Your App Takes 5 Seconds to Load This Screen. What's Likely Wrong?

Possible issues include importing too many Expo modules at startup, causing large bundle size and slow initialization. Heavy JavaScript computations or blocking native calls on the main thread can also delay rendering. Optimizing imports, lazy loading modules, and profiling startup performance can help identify and fix the problem.

Key Result

Expo modules provide easy access to native features but can impact app size and performance if overused. Optimize by importing only needed modules and profiling startup to maintain smooth 60fps UI and fast load times.

Practice

1. What is the main purpose of Expo modules in a React Native app?

easy

A. To create custom UI components from scratch

B. To write backend server code

C. To manage app navigation between screens

D. To easily add device features like camera and location

Solution

1. Step 1: Understand Expo modules role

   Expo modules provide access to device features such as camera, location, and notifications.

2. Step 2: Compare options with this role

   Only To easily add device features like camera and location matches this purpose; others relate to UI, navigation, or backend, which Expo modules do not handle.

3. Final Answer:

   To easily add device features like camera and location -> Option D

4. Quick Check:

   Expo modules = device features access [OK]

Hint: Expo modules = device features access, not UI or backend [OK]

Common Mistakes:

• Confusing Expo modules with navigation libraries
• Thinking Expo modules create UI components
• Assuming Expo modules handle backend logic

2. Which is the correct way to import the Camera module from Expo in a React Native app?

easy

A. import { Camera } from 'expo-camera';

B. import Camera from 'react-native-camera';

C. import { Camera } from 'expo-device';

D. import Camera from 'expo-camera-module';

Solution

1. Step 1: Identify the official Expo Camera import

   The correct import is from 'expo-camera' with named import { Camera }.

2. Step 2: Check other options for correctness

   import Camera from 'react-native-camera'; uses a different library name, C imports from wrong module, D uses a non-existent module name.

3. Final Answer:

   import { Camera } from 'expo-camera'; -> Option A

4. Quick Check:

   Expo Camera import = { Camera } from 'expo-camera' [OK]

Hint: Expo modules import from 'expo-<feature>' with named imports [OK]

Common Mistakes:

• Using default import instead of named import
• Importing from wrong package name
• Confusing Expo modules with React Native community packages

3. What will the following code output if the user denies camera permission?

import { Camera } from 'expo-camera';
import React, { useState, useEffect } from 'react';

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

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

  if (hasPermission === null) {
    return Requesting permission...;
  }
  if (hasPermission === false) {
    return No access to camera;
  }
  return Camera access granted;
}

medium

A. Displays 'No access to camera'

B. Displays 'Requesting permission...' indefinitely

C. Displays 'Camera access granted'

D. Throws a runtime error

Solution

1. Step 1: Understand permission request flow

   The code requests camera permission asynchronously and sets hasPermission to true if granted, false otherwise.

2. Step 2: Analyze user denies permission case

   If user denies, status !== 'granted', so hasPermission is false, triggering the second if block to show 'No access to camera'.

3. Final Answer:

   Displays 'No access to camera' -> Option A

4. Quick Check:

   User denies permission = show 'No access to camera' [OK]

Hint: Permission denied sets hasPermission false, shows denial message [OK]

Common Mistakes:

• Assuming permission request never finishes
• Confusing null and false states
• Expecting granted message even if denied

4. Identify the error in this Expo Location usage code:

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

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

  useEffect(() => {
    (async () => {
      let { status } = await Location.requestForegroundPermissionsAsync();
      if (status !== 'granted') {
        return;
      }
      let loc = await Location.getCurrentPositionAsync({});
      setLocation(loc);
    })();
  }, []);

  return location ? {JSON.stringify(location)} : Loading...;
}

medium

A. Using getCurrentPositionAsync without options

B. Using useState incorrectly with null initial value

C. Missing parentheses when calling requestForegroundPermissionsAsync

D. Not importing React properly

Solution

1. Step 1: Check async function calls

   The code uses await Location.requestForegroundPermissionsAsync without parentheses, so it references the function instead of calling it.

2. Step 2: Verify other parts

   useState with null is valid, React import is correct, getCurrentPositionAsync can be called with empty options object.

3. Final Answer:

   Missing parentheses when calling requestForegroundPermissionsAsync -> Option C

4. Quick Check:

   Async function call needs () to execute [OK]

Hint: Async calls need () after function name to run [OK]

Common Mistakes:

• Forgetting () on async function calls
• Assuming useState(null) is wrong
• Thinking getCurrentPositionAsync requires options

5. You want to build a React Native app using Expo modules that shows the user's current location only if permission is granted. Which sequence of steps is correct?

hard

A. Request permission first, then import Location module, then get location

B. Import Location module, request permission, check status, get location if granted, display location

C. Import Location module, get location immediately, then request permission if needed

D. Display location first, then request permission, then import Location module

Solution

1. Step 1: Understand correct usage order

   You must import the Location module first to use its functions.

2. Step 2: Request permission before accessing location

   Request permission and check status before calling getCurrentPositionAsync to avoid errors.

3. Step 3: Only get and display location if permission is granted

   This ensures app behaves correctly and respects user privacy.

4. Final Answer:

   Import Location module, request permission, check status, get location if granted, display location -> Option B

5. Quick Check:

   Import -> request permission -> check -> get location -> display [OK]

Hint: Always request permission before accessing device features [OK]

Common Mistakes:

• Trying to get location before permission
• Requesting permission after accessing location
• Importing modules after usage