Recently, I was involved in a couple of recruitment processes. During each of them there was of course a technical part, involving a technical interview and/or an online task to complete (eg. DevSkiller).

Below you can find most of the questions I was asked, grouped by topic.

Language / Foundation

• Q: What is ARC and how it works (compared to manual memory management)?
  A: Transitioning to ARC (Obj-C) and Swift ARC
  • Q: (ObjC) Is release/autorelease still used in ARC?
    A: Not explicitly, it is used under the hood.
  • Q: What is Auto Release Pool? Is it still used in Swift?
    A: Rock Bruno’s blog post
• Q: Describe strong vs weak (vs unowned) reference. What are reference cycles?
  A: TL;DR: Strong references increment reference counter (by 1), and weak (also unowned) references not. More in already linked Swift ARC.
• Q: Obj-C Category vs Swift extension – are they related? how? what are the differences?
  A: Brief comparison of categories and extensions
• Q: What is the difference between static and class methods?
  A: Subclasses can override class methods (and static not).
• Q: Describe enum type – its capabilities and example use cases
  A: Enumerations on Swift.org
  • Q: Associated values / raw values ?
  • Q: What is a recursive enum? (indirect keyword?)
• Q: What are reference and value types? describe, which types are what, memory?
  A: Classes and structures or Reference vs value types. Also functions, closures and actors are reference types (Closures).
• Q: What is a race condition? And data race?
  A: Race condition vs data race
  • Q: How to detect and debug them?
    A: Using Xcode’s Thread and Address sanitizers
• Q: Generics, protocol with generics (where can be used?)
  A: Generics
  • Q: Associated type in protocol

Foundation

• Q: Collections – basic types, differences
  A: Collection Types
  • Hashable protocol – for what it is used?
  • Subscript access performance (O(1))
• Q: What is GCD? What are the other ways to achieve concurrency?
  A: Dispatch, Modern Concurrency (async/await + Tasks), OperationQueue, Threads
  • Dispatch Queue types (serial vs concurrent)
  • What is a barrier?
  • Difference between async and sync calls?
• Q: Higher order functions – what it is? What standard HOFs you know?
  A: Higher order functions in Swift
  • Q: How reduce() works?
  • Q: flatMap() vs compactMap()
    A: Differences explained
  • Q: What is a lazy array (Swift)?
    A: Lazy Collections

UI Kit

• Q: Describe UIView.frame vs bounds
  A: Difference between frame and bounds
  • Example of view changing bounds.origin? UIScrollView
• Q: What are view transformations?
  A: CGAffineTransform
• Q: How can you animate a UIView property?
• A: UIViewPropertyAnimator, UIView.animate()
• Q: Should @IBOutlet be declared weak or strong? And why is it var and implicitly unwrapped optional?
  A: Weak vs strong IBOutlet and Optionals vs implicitly unwrapped optionals

Core Bluetooth

I was asked if I am familiar with below topics:

• GATT
• MTU
• CTKD, encryption
• pairing and bonding
• L2CAP

Design/Architecture

Describe and compare: MVC vs MVVM vs MVP.