Swift Case Studies & Practical Application 3

Question 1

You're designing an API for a payment processor. Multiple concrete types (`CreditCard`, `PayPal`, `ApplePay`) must all be usable wherever a `PaymentMethod` is expected. Which Swift mechanism is most appropriate?

Accepted Answer

A protocol that all types conform to

Answer

A `PaymentMethod` protocol defines the contract (e.g., `charge(amount:)`), and each concrete type independently conforms, enabling polymorphism without inheritance hierarchies.

Question 2

Your app has a `Logger` used across many modules. You want a single shared instance but still allow substituting a mock in tests. Which pattern works best in Swift?

Accepted Answer

Dependency injection with a `LoggerProtocol` passed into each module

Answer

Dependency injection through a protocol lets tests pass a `MockLogger` without changing production code, avoiding the testability trap of a bare singleton.

Question 3

A `Result<[Post], NetworkError>` is returned from a fetch function. You need to map each post's title to uppercase on success. Which approach is most concise?

Accepted Answer

`result.map { posts in posts.map { $0.title.uppercased() } }`

Answer

`Result.map` transforms the success value without unwrapping, keeping the failure case untouched and producing a new `Result<[String], NetworkError>`.

Question 4

You want SwiftUI's `ProfileView` to automatically re-render when `user.name` changes. `User` is a reference type shared across views. What is the correct setup?

Accepted Answer

Make `User` a class conforming to `ObservableObject` with `@Published var name`

Answer

`ObservableObject` + `@Published` triggers SwiftUI's diffing engine; views holding `@ObservedObject` or `@StateObject` references re-render automatically when `name` changes.

Question 5

A team encounters a crash: `Fatal error: Unexpectedly found nil while unwrapping an Optional value`. You are asked to audit the codebase. Which code pattern most directly causes this crash?

Accepted Answer

Force-unwrapping an optional with `!` when the value is nil

Answer

The force-unwrap operator `!` crashes at runtime if the optional contains nil; replacing it with `if let`, `guard let`, or `??` eliminates this crash class.

Swift Practice Test

Swift Practice Test

Swift Case Studies & Practical Application 3