Checkpoint: Shipping a Real App

Five questions on this section. Answer all of them, then see your score and why each answer is right or wrong.

Each fetch takes about a second. How do you make this take about one second in total?

Swift

func loadAll() async throws -> (Decimal, Decimal) {
    let eur = try await service.fetchRate(from: "USD", to: "EUR")
    let gbp = try await service.fetchRate(from: "USD", to: "GBP")
    return (eur, gbp)
}

Wrap each call in Task { } and return right away.
Mark the function @MainActor.
Call fetchRate from a DispatchQueue.main.async block.
Use async let eur = … and async let gbp = …, then try await (eur, gbp).

A nonisolated helper function assigns to model.state on a @MainActor model, and Swift 6 reports “main actor-isolated property ‘state’ can not be mutated from a nonisolated context”. What’s a correct fix?

Mark the helper @MainActor, so it runs where the model’s state lives.
Mark the model @unchecked Sendable.
Switch the target back to the Swift 5 language mode.
Copy the model into a local variable before assigning.

The rates API returns HTTP 404 with an HTML page for an unknown currency. Your code decodes data right after URLSession.shared.data(from:). What do users see?

A URLError saying the page wasn’t found.
A decoding error, such as “data couldn’t be read because it isn’t in the correct format”.
An empty list of rates and no error.
The HTML page rendered in the app.

An edit screen binds fields straight to a SwiftData model with @Bindable var expense: Expense. Product wants a Cancel button that discards changes. What’s the cleanest design?

Keep binding directly, and call context.insert(expense) on Cancel.
Keep binding directly; edits only save when the screen closes.
Turn Expense into a struct so edits are copies.
Edit @State draft copies of the fields, and write them to the model only on Save.

Which test verifies that record throws BudgetError.overLimit(by: 7)?

#expect(throws: BudgetError.overLimit(by: 7)) { try food.record(title: "Dinner", amount: 45) }
#expect(try food.record(title: "Dinner", amount: 45) == .overLimit(by: 7))
do { try food.record(title: "Dinner", amount: 45) } catch { }
try #require(food.spent == 57)

5 questions unanswered