Checkpoint: Modelling with Types · Swift Essentials for iOS

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

Expense is a struct with var amount: Decimal. What does this print?

Swift

var list = [Expense(amount: 5)]
var first = list[0]
first.amount = 9
print(list[0].amount)

5
9
It doesn’t compile, because array elements are constants.
It crashes, because first is a dangling reference.

What does this print?

Swift

enum Sync {
    case idle
    case syncing(progress: Double)
    case failed(String)
}
let state: Sync = .syncing(progress: 0.4)
switch state {
case .idle: print("Idle")
case .syncing(let p) where p > 0.5: print("Almost")
case .syncing: print("Working")
case .failed(let message): print(message)
}

Almost
Idle
Nothing, because no case matches 0.4.
Working

protocol Exportable { func csvRow() -> String } has five conforming types. Every one of them should also get csvLine, which is csvRow() plus a newline, without writing it five times. What do you add?

A var csvLine: String { get } requirement in the protocol.
extension Exportable { var csvLine: String { csvRow() + "\n" } }
A base class ExportableBase that all five types inherit from.
A global function csvLine(_ item: Exportable) in each file.

A final class ReminderScheduler stores var onFire: (() -> Void)? and sets onFire = { self.notify() }. Memory tools show schedulers are never freed. What’s the fix?

Change onFire from var to let.
Set onFire = nil inside deinit.
Make ReminderScheduler a subclass of NSObject.
Capture weakly: onFire = { [weak self] in self?.notify() }.

load() reads saved expenses and throws if the file is corrupted. What does the user see when that happens?

Swift

let expenses = (try? load()) ?? []

An empty list, with no hint that their data failed to load.
An alert describing the corruption.
A crash with the error’s description.
The expenses that loaded before the corrupted part.

5 questions unanswered