An equivalent to computed properties using @Published in Swift Combine?

Question

In imperative Swift, it is common to use computed properties to provide convenient access to data without duplicating state.

Let's say I have this class made for imperative MVC use:

class ImperativeUserManager {
    private(set) var currentUser: User? {
        didSet {
            if oldValue != currentUser {
                NotificationCenter.default.post(name: NSNotification.Name("userStateDidChange"), object: nil)
                // Observers that receive this notification might then check either currentUser or userIsLoggedIn for the latest state
            }
        }
    }

    var userIsLoggedIn: Bool {
        currentUser != nil
    }

    // ...
}

If I want to create a reactive equivalent with Combine, e.g. for use with SwiftUI, I can easily add @Published to stored properties to generate Publishers, but not for computed properties.

    @Published var userIsLoggedIn: Bool { // Error: Property wrapper cannot be applied to a computed property
        currentUser != nil
    }

There are various workarounds I could think of. I could make my computed property stored instead and keep it updated.

Option 1: Using a property observer:

class ReactiveUserManager1: ObservableObject {
    @Published private(set) var currentUser: User? {
        didSet {
            userIsLoggedIn = currentUser != nil
        }
    }

    @Published private(set) var userIsLoggedIn: Bool = false

    // ...
}

Option 2: Using a Subscriber in my own class:

class ReactiveUserManager2: ObservableObject {
    @Published private(set) var currentUser: User?
    @Published private(set) var userIsLoggedIn: Bool = false

    private var subscribers = Set<AnyCancellable>()

    init() {
        $currentUser
            .map { $0 != nil }
            .assign(to: \.userIsLoggedIn, on: self)
            .store(in: &subscribers)
    }

    // ...
}

However, these workarounds are not as elegant as computed properties. They duplicate state and they do not update both properties simultaneously.

What would be a proper equivalent to adding a Publisher to a computed property in Combine?

Answer 1

EDIT:

Although I think this answer has merits, nowadays I never use it and instead use the same technique that @lassej described in their answer.

I'd advise considering it first, and then check for other answers.

Create a new publisher subscribed to the property you want to track.

@Published var speed: Double = 88

lazy var canTimeTravel: AnyPublisher<Bool,Never> = {
    $speed
        .map({ $0 >= 88 })
        .eraseToAnyPublisher()
}()

---

You will then be able to observe it much like your @Published property.

private var subscriptions = Set<AnyCancellable>()


override func viewDidLoad() {
    super.viewDidLoad()
 
    sourceOfTruthObject.$canTimeTravel.sink { [weak self] (canTimeTravel) in
        // Do something…
    })
    .store(in: &subscriptions)
}

Not directly related but useful nonetheless, you can track multiple properties that way with combineLatest.

@Published var threshold: Int = 60
    
@Published var heartData = [Int]()
    
/** This publisher "observes" both `threshold` and `heartData`
 and derives a value from them.
 It should be updated whenever one of those values changes. */
lazy var status: AnyPublisher<Status,Never> = {
    $threshold
       .combineLatest($heartData)
       .map({ threshold, heartData in
           // Computing a "status" with the two values
           Status.status(heartData: heartData, threshold: threshold)
       })
       .receive(on: DispatchQueue.main)
       .eraseToAnyPublisher()
}()