Preparing for iOS Networking Interview Questions

Networking is a fundamental part of iOS development. Mastering concepts like HTTP requests, JSON parsing, authentication, caching, and more can make or break your next iOS interview This comprehensive guide covers some of the most common iOS networking interview questions you’re likely to encounter, along with detailed explanations and code snippets to help you prepare

Basics of iOS Networking

Most apps need to communicate with remote servers in some form. Here are some typical questions interviewers may ask to test your basic networking skills:

Q How does URLSession work in Swift? Can you explain URLSession and URLSessionTask?

URLSession provides an API for performing network data transfers such as HTTP requests. It is built on three main components – the session itself, session tasks, and session configuration The session represents the context for network operations and its configuration controls policies like caching and timeouts URLSessionTask is the base class for the actual tasks like data, upload or download that perform the transfers.

Q: What is the difference between GET and POST HTTP methods? When should each be used?

GET requests data from a server. The parameters are appended to the URL so it is less secure but cacheable. Use GET when retrieving data without side effects, like a search query.

POST sends data to the server in the request body. This is more secure and used when creating resources or changing state on the server, like submitting a form.

Q: How would you handle errors during network operations?

Check for an error object in the completion handler of a URLSession task. Display a user-friendly message based on error type. For HTTP, check status codes in the response – 400s are client errors, 500s are server errors. Use Reachability API to check network availability before making the request.

Q: What is URL caching and how would you implement it?

Caching stores network responses locally to avoid unnecessary future requests. iOS provides cache policies like .useProtocolCachePolicy on NSURLRequest. For custom caching, store responses in Core Data and check cache before making new requests. Invalidate stale cache data based on app needs.

Q: How does serialization and deserialization of JSON work in Swift?

Use JSONEncoder to encode Swift objects into JSON by calling encode(). This returns Data convertible into JSON. For parsing JSON back into Swift objects, use JSONDecoder’s decode() method, passing the JSON data and specifying the expected object type.

Advanced Networking Topics

Beyond basics, you may be asked more complex questions around concurrency, real-time communication, and architecture:

Q: How would you handle multiple simultaneous network requests in Swift?

Use DispatchGroup to group tasks and be notified when all complete. Call enter() before each request, leave() in their completion handler, and notify() to run code after all finish. This ensures they complete before further execution.

Q: What are web sockets and how are they used in iOS networking?

Web sockets allow persistent, two-way communication between client and server over TCP. Create a URLSessionWebSocketTask and call resume() to connect. Use send() and receive() methods to exchange messages. Send ping messages periodically to keep the connection alive.

Q: How does Apple Push Notification service work? How is it used for iOS networking?

APNs maintains a permanent IP connection to devices to push remote notifications from a provider server even when the app is inactive. The payload contains the alert text, sound, badge count etc. Notifications can be presented immediately or silently processed in the background.

Q: What considerations are important in implementing a network/API client layer in iOS apps?

• Use proper error handling and HTTP status code checks
• Make network calls asynchronously to avoid blocking UI
• Implement request retries and exponential backoff for recoverable errors
• Authentication and encryption of sensitive data
• Caching and other optimizations to avoid repeated requests
• Abstract API-specific code into a re-usable client layer

Q: How can iOS networking be optimized for performance and efficiency?

• Use background URLSession for large downloads and uploads
• Enable gzip compression in URLRequest to reduce payload size
• Set HTTP cache policies to reuse cached data when applicable
• Pre-fetch data before it’s needed to improve perceived performance
• Throttle and schedule requests to avoid bombarding servers
• Minimize app wakeup for background transfers to increase battery life

Q: What considerations are important when designing iOS networking code?

• Make networking code testable by abstracting it from app code
• Avoid tight coupling between network layer and business logic
• Re-use generic components like authentication, caching, error handling
• Make components modular for easier maintainability
• Ensure thread-safety when accessing shared resources like caches
• Use Swift features like enums and structs for clearer code

Q: How does Alamofire simplify iOS networking compared to URLSession?

Alamofire provides a clean Swift interface and convenient abstractions for common tasks:

• Chainable request methods for each HTTP verb
• Automatic parameter encoding
• Easy uploading of files/data
• Built-in validation and response handling
• Convenient response serialization to Swift types
• Flexible authentication handling via extensions
• Helper methods for common needs like caching

While URLSession is more low-level, Alamofire saves time and lines of code.

Real-World Examples

Interviewers often ask candidates to walk through real networking scenarios:

Q: Imagine you need to fetch a user’s chat messages from a REST API. Walk me through how you would implement this and update the UI.

• Make a GET request to the /messages endpoint, appending user id to the path
• On success, parse the JSON response into a [Message] array
• In case of error, show an alert prompting to retry
• Sort messages by timestamp so they render chronologically
• Dispatch onto main queue before updating UI collection view
• Call reloadData() on collection view once new messages are fetched

Q: Suppose you need to upload a photo captured from the camera. How would you send this over the network?

• When photo is selected, convert UIImage to JPEG Data
• Create URLSessionUploadTask with the request URL
• Set request HTTP method to POST and add headers if needed
• Upload task has a uploadProgress handler to show progress
• In completion handler, check HTTP status code for success
• If upload succeeded, dismiss upload UI and show success message
• Else show error alert allowing user to retry uploading

Q: How would you improve app performance by caching network request results?

• Check cache before making each network request
• Cache policy: Cache data for 1 hour, then re-fetch
• When write data to cache, include timestamp
• Before each network call, check cache timestamp
• If cached data is fresh, return cached data without network request
• Else make network request, cache response, then return data

Being able to walk through realistic examples demonstrates your ability to apply networking principles.

Testing Network Code

Testing networking logic is important for reliable apps. Here are some key questions around testing:

Q: How would you test networking code without making actual requests?

Use dependency injection to pass a custom URLProtocol stub rather than the real protocol. The stub can return mock responses, errors, and track network call details. Unit test request formation, serialization, and response handling logic against these mocked responses.

Q: What kinds of tests would you write for a network client class?

• Unit tests for request formation, serialization, and deserialization
• Response validation tests for common scenarios like 400 or 500 errors
• Tests for edge cases like empty/null/malformed responses
• Tests for correct behavior of caching and retrying logic
• Integration tests against actual API endpoints and payloads

Q: How could you integration test push notifications?

Use a staging/sandbox push server to send test notifications and confirm expected behavior:

• Stub remote notification handling when app is foreground/background/inactive
• Verify notification content is accurately received by app delegate methods
• Test tapping notification launches correct screen when app is inactive
• Confirm badge number, alert text, and sound match notification payload

Thoroughly testing all networking code paths results in robust, bug-free apps able to handle unpredictable real-world networks and remote servers.

Key Takeaways

Here are some key tips for acing the networking portion of your iOS interview:

• Review fundamental networking concepts like HTTP, URLs, caching, serialization formats, etc.
• Understand higher-level APIs like URLSession and Alamofire in addition to the basics
• Be able to explain common scenarios like error handling, background downloading, OAuth authentication
• Focus on clarity and precision over complex language when explaining code
• Practice discussing real-world use cases and walking through implementation
• Highlight testing experience – critical for production apps

Networking is pervasive in iOS development. Demonstrating a strong grasp of its nuances and trade-offs will stand out in your next interview. With diligent preparation, you can master the key iOS networking questions and land your dream mobile dev job!