Swift Case Studies & Practical Application 4

Question 1

You're refactoring a legacy codebase where many functions return `-1` to signal errors. Which modern Swift pattern replaces this anti-pattern?

Accepted Answer

Returning `Int?` with nil for errors, or throwing a typed Error

Answer

Optional returns (`Int?`) signal absence without sentinel values, and throwing functions with typed errors give callers structured error information — both are idiomatic replacements for magic-number conventions.

Question 2

A `ProductListViewModel` fetches data and your unit test must verify that `products` is populated after a successful network call. You mock the network layer. What is the key design requirement for this to work?

Accepted Answer

The ViewModel must depend on a protocol, not a concrete network class

Answer

If the ViewModel holds a protocol reference (e.g., `NetworkServiceProtocol`), tests can inject a `MockNetworkService` that returns canned data without real HTTP calls.

Question 3

Your Swift app parses untrusted JSON where a field might be a `String` or an `Int`. Using `Codable`, how do you handle this ambiguity?

Accepted Answer

Write a custom `init(from:)` that tries decoding as String, then falls back to Int

Answer

A custom `init(from decoder:)` can attempt `container.decode(String.self, ...)` and catch the error to retry with `Int.self`, handling the type ambiguity gracefully.

Question 4

You have a Swift actor `DatabaseActor` with a method `func fetchUser(id: Int) async -> User`. On which thread does the body of `fetchUser` execute?

Accepted Answer

On the actor's internal serial executor, isolated from other concurrent calls

Answer

Actors serialize access through their own executor, so `fetchUser`'s body never runs concurrently with other methods on the same actor, preventing data races.

Question 5

A colleague writes `var items: [Item] = []` as a property of a SwiftUI `View` struct to hold fetched data. Why won't changes to `items` cause the view to re-render?

Accepted Answer

Plain `var` properties don't participate in SwiftUI's state management; `@State` is needed

Answer

SwiftUI only re-renders a view when a property wrapped with `@State`, `@ObservedObject`, or similar triggers a change notification — plain `var` assignments are invisible to the framework.

Swift Practice Test

Swift Practice Test

Swift Case Studies & Practical Application 4