Best Swift Interview Questions To Ask A Candidate

Finding the right candidate for a Swift developer role can be a challenging task.

To ensure you're selecting the most qualified person for your team, it's essential to conduct a well-structured interview process that includes asking the right questions.

We've put together a list of Swift interview questions designed to test a candidate's theoretical knowledge, practical skills, and understanding of key concepts and practices.

1. Can you explain what Optionals are in Swift and when you would use them?

This question is critical because understanding Optionals is foundational to Swift. Optionals allow for the representation of the absence of a value, preventing potential runtime errors caused by null references, a common issue in many programming languages.

A strong understanding of Optionals will highlight a candidate's knowledge of Swift's unique type safety mechanisms.

2. How do you handle error propagation in Swift? Can you explain the difference between try, try?, and try! ?

Swift provides strong mechanisms for error handling, and a candidate's understanding of these can reveal their ability to write reliable, resilient code.

It's also important to gauge their knowledge of the difference between various 'try' keywords and their respective use cases, as improper usage can lead to different behaviors and potential crashes.

3. How do you handle concurrency in Swift? Have you used GCD or Swift's async/await in your projects?

Concurrency and multi-threading are crucial for modern applications to remain responsive and efficient. Swift's Grand Central Dispatch (GCD) and the new async/await model provide powerful tools for this.

Understanding a candidate's experience with these tools will give insight into their ability to develop high-performing applications.

4. What do you think about SwiftUI? Can you describe a project where you have used it?

SwiftUI is a framework introduced by Apple for building user interfaces across all Apple platforms.

Asking this question allows you to evaluate the candidate's up-to-date knowledge and experience with the latest technologies. Their answer can also reveal their familiarity with declarative programming paradigms.

5. Explain the difference between a struct and a class in Swift. In what scenarios would you prefer one over the other?

This is a fundamental concept in Swift, which has both reference types (classes) and value types (structs and enums). Their usage impacts the design and behavior of the software, especially concerning data management and performance.

By asking this question, you can assess the candidate's understanding of Swift's type system and their ability to make correct architectural decisions.

6. What are Swift protocols and how have you used them in your projects?

Protocols are a fundamental feature of Swift, supporting multiple inheritance and enabling polymorphism. They can provide shared interfaces for classes, structs, and enums.

Understanding a candidate's knowledge of protocols will indicate their competency with Swift's type system and their ability to design flexible, reusable components.

7. Can you describe the difference between escaping and non-escaping closures in Swift?

This question allows you to test a candidate's understanding of memory management and capture semantics in Swift. Incorrect usage of escaping and non-escaping closures can lead to memory leaks or crashes, hence a thorough understanding is necessary to write stable and efficient applications.

8. Can you explain how Swift's type inference works? When might you choose to specify type explicitly?

Type inference is a feature that makes Swift code concise and readable, but it can have performance implications during compilation.

An experienced Swift developer should be able to balance the trade-offs between readability, maintainability, and performance.

9. Could you discuss your experience with unit testing in Swift? What frameworks have you used and why?

Unit testing is vital to ensure the robustness and reliability of the code. It's important to know the candidate's familiarity with testing frameworks like XCTest or third-party frameworks, and their overall approach to testing in Swift.

10. What's your approach to memory management in Swift? Can you explain ARC and how it works?

Automatic Reference Counting (ARC) is Swift's mechanism for managing memory. A developer's understanding of ARC, strong vs weak references, and unowned references is crucial for writing efficient applications and avoiding memory leaks or retain cycles.

This question will provide insight into the candidate's ability to write performance-optimized and bug-free code.

11. What are property observers in Swift? Can you provide an example of when you would use them?

Property observers monitor and respond to changes in a property's value. They're used for managing side effects and for implementing functionality dependent on property changes.

This question will help gauge a candidate's understanding of how Swift allows control over property values and the lifecycle of variables.

12. Can you explain the concept of generics in Swift? How have you used them in your projects?

Generics are one of the most powerful features of Swift, allowing for flexible, reusable code while maintaining type safety.

This question will reveal the candidate's understanding of advanced Swift concepts, their ability to write flexible, reusable code, and their approach to maintaining type safety.

13. Have you ever used Swift Package Manager? How does it compare to other dependency managers?

Swift Package Manager is a tool for managing the distribution of Swift code, and it's integrated with the Swift build system.

Understanding how to use it and how it compares with alternatives can indicate a candidate's familiarity with Swift ecosystem tools and their ability to manage and maintain dependencies effectively.

14. How have you used higher-order functions like map, reduce, and filter in Swift?

Higher-order functions are a feature borrowed from functional programming languages. They can greatly improve the readability and conciseness of code, and they're commonly used in data manipulation.

This question will provide insight into a candidate's familiarity with functional programming concepts within Swift.

15. Can you explain the concept of SwiftUI's State and Binding? When and why would you use them?

State and Binding are property wrappers that play a crucial role in SwiftUI's declarative design. State is a source of truth, while Binding provides a reference to a mutable state. Understanding how these concepts work is vital for building SwiftUI applications.

This question will reveal a candidate's understanding of SwiftUI and its unique approach to state management.

Is there another way to assess Swift skills?

While interview questions provide valuable information, they may not always give a complete picture of a candidate's abilities. This is where incorporating a Swift test can significantly enhance the hiring process.

A Swift test can help identify candidates who not only have theoretical knowledge but can also effectively apply it to practical scenarios. It provides concrete evidence of a candidate's technical capabilities and helps mitigate biases or subjectivity that may arise during the interview process.

Read our Swift test pages to find out how it works and how it can make it easier for you to find the right candidate for the job.