Issues with Swift's Combine framework CombineLatest

Question

I went through the WWDC video of "Introducing Combine" where it was said that whenever a publisher value gets updated the CombineLatest gets called and updated. But the snippet I created works oddly.

class Mango {
    var enableButton = false
    @Published var userName = "admin"
    @Published var password = "poweruser"
    @Published var passwordAgain = "poweruser"
    var validatePassword: AnyCancellable {
        Publishers.CombineLatest($password, $passwordAgain).map { (password, reenterpass) -> String? in
            print("Is Password Same to \(password)? :", password == reenterpass)
            guard password == reenterpass else { return nil }
            return password
        }.eraseToAnyPublisher()
            .map { (str) -> Bool in
            print("In Map", str != nil)
            guard str != nil else { return false }
            return true
        }.assign(to: \.enableButton, on: self)
    }

    init() {
        validatePassword
    }

    func checkSub() {
        print("1. Is password same? ->",enableButton)
        password = "nopoweruser"
        print("2. Is password same? ->",enableButton)
    }
}

When I initialize and call the function checkSub() where the publisher 'password' is updated the CombineLatest does not get called. Why is it behaving oddly?

Input:

let mango = Mango()<br>
mango.checkSub()

Output:

Is Password Same to poweruser? : true  
In Map true  
1. Is password same? -> true  
2. Is password same? -> true

Answer 1

It seems like the issue is with memory management. The validatePassword cancellable is autoreleased, meaning that the subscription is completed as soon as you create it, since you do not retain it. Make it a property instead of computed property, using lazy var and it should work fine.

lazy var validatePassword: AnyCancellable = {
    Publishers.CombineLatest($password, $passwordAgain).map { (password, reenterpass) -> String? in
        print("Is Password Same to \(password)? :", password == reenterpass)
        guard password == reenterpass else { return nil }
        return password
    }.eraseToAnyPublisher()
        .map { (str) -> Bool in
        print("In Map", str != nil)
        guard str != nil else { return false }
        return true
    }.assign(to: \.enableButton, on: self)
}()

With lazy you are retaining the cancellable which gets released only after the object is released. So, this should work properly.