Swift Case Studies & Practical Application 2

Question 1

You're building a banking app where a `Transaction` must always have a non-zero `amount`. Which Swift feature best enforces this at the type level?

Accepted Answer

A failable initializer that returns nil for zero amounts

Answer

A failable initializer (`init?`) prevents creating an invalid `Transaction` by returning nil when the amount is zero, enforcing the invariant at construction time.

Question 2

A networking layer returns `Data?` from a cache. You need to decode it as `User` or fall back to fetching from the server. Which pattern is most idiomatic in Swift?

Accepted Answer

Use `if let data = cachedData, let user = try? JSONDecoder().decode(User.self, from: data)`

Answer

`if let` chaining safely unwraps the optional data and attempts decoding without crashing or leaking errors into the call site.

Question 3

Your app stores user settings with keys like `"theme"`, `"fontSize"`, etc. A teammate suggests using an enum with raw values instead of string literals. What is the primary benefit?

Accepted Answer

Compile-time safety prevents typos in key names

Answer

Enum cases with String raw values let the compiler catch misspelled key names, eliminating a class of runtime bugs common with bare string literals.

Question 4

You have a `Document` class with an expensive `parse()` method. Many view controllers read `parsedContent` but parsing should happen only once. Which Swift pattern solves this?

Accepted Answer

A `lazy var` computed property that caches the result after first access

Answer

`lazy var` defers the expensive computation until first access and then stores the result, so subsequent reads are instant without manual caching logic.

Question 5

A social media feed loads posts asynchronously. Which Swift concurrency construct lets you kick off loading user data and post images simultaneously, then wait for both before rendering?

Accepted Answer

Using `async let` for both fetches then awaiting them together

Answer

`async let` starts both async operations concurrently and the subsequent `await` on both ensures neither renders until both complete.

Swift Practice Test

Swift Practice Test

Swift Case Studies & Practical Application 2