Swift: how to replace a completion handler with a Future in an element initializer?-CodePudding

I am sorry if this sounds very beginner, but after looking at Apple doc as well as several tutorials I still struggle to understand how Combine's Future works.

I have this very simple code which stores the current date on first button tap, and prints the interval on the second one:

import UIKit

class MyViewController: UIViewController {
    private var startTime: Date = .now
    private var completion: (TimeInterval) -> Void = { _ in }
    private var isOn = false
    
    @IBAction func tapped() {
        if isOn {
            completion(Date.now.timeIntervalSince(startTime))
        } else {
            startTime = .now
        }
        isOn.toggle()
    }

    init(_ completion: @escaping (TimeInterval) -> Void) {
        super.init(nibName: "MyViewController", bundle: .main)
        self.completion = completion
    }
    
    required init?(coder: NSCoder) {
        fatalError()
    }
}

In my AppDelegate:

window?.rootViewController = MyViewController {
    print("Tapped with time interval: \($0)")
}

Now I would like to replace that completion handler with a Future, but I'm confused about what to do. I guess I have to create a function like this in my view controller:

func afterSecondTap() -> Future<TimeInterval, Error> {
    return Future { promise in
        // what to do here?
    }
}

And in the AppDelegate something like this:

window?.rootViewController = MyViewController()
    .afterSecondTap()
    .sink(receiveCompletion: { completion in
            
    }, receiveValue: { value in
            
    })
    .store(in: &subscriptions)

However this would not work because I get an error saying

Cannot assign value of type '()' to type 'UIViewController'

Thank you for helping me understand this

CodePudding user response：

There are two issues here.

First when you assign a rootViewController it expects UIViewController of some sort, as soon as you add .afterSecondTap() and the rest it changes the type. So let's start like this

let controller = MyViewController()
window?.rootViewController = controller

Next let's see how we can publish time interval from the controller, I would think Future is not what you need:

class MyViewController: UIViewController {
    
    private var startTime: Date = .now
    private var isOn = false
    
    var timeInterval: AnyPublisher<TimeInterval, Never> {
        _timeInterval
            .compactMap { $0 }
            .eraseToAnyPublisher()
    }
    private var _timeInterval = ConcurrentValueSubject<TimeInterval?, Never>(nil)
    
    @IBAction func tapped() {
        if isOn {
            _timeInterval.send(Date.now.timeIntervalSince(startTime)
        } else {
            startTime = .now
        }
        isOn.toggle()
    }

    init() {
        super.init(nibName: "MyViewController", bundle: .main)
    }
    
    required init?(coder: NSCoder) {
        fatalError()
    }
}

now you can do this:

let controller = MyViewController()
window?.rootViewController = controller
controller
    .timeInterval
    .sink(
        receiveCompletion: { _ in },
        receiveValue: { _ in })
    .store(in: &subscriptions)

CodePudding user response：

When you store a Cancellable using .store, the value returned will no longer be UIViewController, it is going to be Void.

You shouldn't use Future, instead use PassThroughSubject like so:

class MyViewController: UIViewController {
    private var startTime: Date = .now
    private var isOn = false
    var publisher = PassthroughSubject<TimeInterval, Never>()
    @IBAction func tapped() {
        if isOn {
            publisher.send(Date.now.timeIntervalSince(startTime))
        } else {
            startTime = .now
        }
        isOn.toggle()
    }

    init() {
        super.init(nibName: "MyViewController", bundle: .main)
    }
    
    required init?(coder: NSCoder) {
        fatalError()
    }
}
let viewController = MyViewController()
viewController
    .publisher
    .sink(receiveCompletion: { completion in
                
        }, receiveValue: { value in
                
        })
        .store(in: &subscriptions)
    .
window?.rootViewController = viewController