Advanced Swift Interview Questions and Answers
If you’re heading into a senior iOS developer interview or just want to challenge your Swift knowledge, this list of advanced Swift interview questions is for you. We’ve kept the answers clear, real, and practical—like a senior dev would explain to a junior teammate.
1. What is a strong reference cycle (retain cycle) in Swift? How do you fix it?
A strong reference cycle happens when two objects keep strong references to each other, preventing them from ever getting deallocated from memory. For example, if Person has a reference to Apartment, and Apartment has a reference back to Person, neither can be released if both references are strong.
How to fix it:
Use weak or unowned references for one of the properties, usually the one that doesn’t need to own the other. If the reference can become nil, use weak. If it should never become nil during the object’s lifecycle, use unowned.
2. What is the difference between weak and unowned references?
Both help prevent retain cycles by not increasing the reference count, but they behave differently:
weakreferences are optional and becomenilwhen the referenced object is deallocated.unownedreferences are non-optional and will crash your app if accessed after deallocation.
So, use weak when the object might go away. Use unowned only if you’re sure it won’t.
3. How does Swift’s memory management system (ARC) work?
ARC stands for Automatic Reference Counting. Swift automatically tracks how many references exist to a class instance. When there are no more strong references, it deallocates the memory.
But if two objects hold strong references to each other (retain cycle), ARC can’t release them, and you’ll get a memory leak. This is why managing references smartly is important.
4. What’s the difference between escaping and non-escaping closures?
A closure is non-escaping by default, which means it must be executed before the function returns.
An escaping closure might outlive the function—it’s stored and called later (e.g., in completion handlers or async code). You mark these with @escaping.
Example:
5. What are Property Wrappers in Swift? Have you created one?
Property wrappers let you define reusable logic for setting and getting a property. For example, you can create a @UserDefault wrapper that reads/writes to UserDefaults.
Example:
This avoids repeating the logic in every getter/setter.
6. How does Swift handle concurrency in Swift 5.5+ using async/await?
Swift introduced structured concurrency in Swift 5.5 using async/await, making asynchronous code readable and safe.
Example:
It simplifies callback-heavy code and reduces bugs by letting you write async code almost like it’s synchronous.
7. What is an actor in Swift? How is it different from a class?
Actors are like classes, but designed for concurrency. They help you protect mutable state in multithreaded environments.
Only one task can access an actor’s mutable state at a time.
Example:
With classes, you’d have to manually use locks or queues. Actors give you this safety for free.
8. What’s the difference between Struct, Class, Actor, and Enum?
- Struct: Value type (copied on assignment), no inheritance.
- Class: Reference type (shared), supports inheritance.
- Actor: Like class but safe for concurrency.
- Enum: Used for representing finite choices. Can have associated values and computed properties.
Choose struct by default, use class when you need identity/inheritance, and actor when managing shared state across threads.
9. Can you explain the @MainActor attribute?
@MainActor ensures that the marked method or property always runs on the main thread. It’s very useful when dealing with UI code in a multithreaded app.
It’s like a built-in safety net for thread-sensitive code.
10. How does Swift manage inout parameters and what’s their real use case?
inout lets you modify a variable directly inside a function.
Swift creates a temporary copy, modifies it, and writes it back. It’s useful for performance-sensitive or state-modifying operations.
11. How do you avoid thread safety issues in a multithreaded Swift app?
Use:
- Actors (new and safe)
- Serial Dispatch Queues (for controlled access)
- Locks (NSLock, but error-prone)
- Immutable data when possible
The goal is to avoid two threads writing to shared memory at the same time. Start with using actors and GCD.
12. What is function currying? Is it supported in Swift?
Function currying means breaking down a function that takes multiple parameters into a chain of functions, each with a single parameter.
While full currying is not built-in anymore, you can still do partial application:
13. What’s the purpose of @autoclosure in Swift?
It automatically wraps an expression in a closure. It’s useful when you want lazy evaluation without requiring {}.
It improves API readability.
14. What’s the difference between static and dynamic dispatch in Swift?
- Static Dispatch: Function calls are resolved at compile time. Faster. Used with
final,struct,enum. - Dynamic Dispatch: Function calls resolved at runtime. Used with
class, where method might be overridden.
Use final for performance when overriding isn’t needed.
15. What is @available and how do you use it for backward compatibility?
@available lets you mark code as only available in certain OS versions.
It ensures your app won’t crash on older devices.
Also Read: Top Swift Interview Questions and Answers (Beginner to Advanced)
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