How to use Swift Protocols with Generic methods and Generic Types

Question

I have a class StateMachine<A>

final class StateMachine<A> {

    private var previousState: State? = nil
    private var currentState: State
    private var content: A?
    var delegate: StateMachineDelegate?
    var state: State = .loading {
        didSet {
            previousState = currentState
            currentState = state
        }
    }

    init(currentState: State, delegate: StateMachineDelegate?) {
        self.currentState = currentState
    }
}

and a delegate protocol StateMachineDelegate

protocol StateMachineDelegate {
    func updateWith(content: A)
}

I'm trying to express that if the StateMachine is created with type A, the delegate should implement the method func updateWith(content: A) which accepts a parameter of the same type A. Is this possible?

Your code has a lot of problems now: you can't put nested types in generics (enum State); you must initialize all non-optional properties in your init; you must specify associatedtype in your delegate protocol. — user28434'mstep
– user28434'mstep, Commented Dec 7, 2016 at 14:42

Rob Napier · Accepted Answer · 2016-12-07 14:52:26Z

2

You'd implement what you're asking for by adding another type parameter:

final class StateMachine<A, Delegate: StateMachineDelegate> where Delegate.A == A {

    private var previousState: State? = nil
    private var currentState: State
    private var content: A?
    var delegate: Delegate?
    var state: State = .loading {
        didSet {
            previousState = currentState
            currentState = state
            delegate?.updateWith(content: state)
        }
    }

    init(currentState: State, delegate: Delegate?) {
        self.currentState = currentState
    }
}

protocol StateMachineDelegate {
    associatedtype A
    func updateWith(content: A)
}

But I wouldn't do it this way. If your delegate really is just an update method, then a closure is a better solution:

final class StateMachine<A> {    
    // ...
    private var content: A?
    var notify: (A) -> Void

    var state: State = .loading {
        didSet {
            previousState = currentState
            currentState = state
            notify(state)
        }
    }

    init(currentState: State, notify: @escaping (A) -> Void) {
        self.currentState = currentState
        self.notify = notify
    }
}

answered Dec 7, 2016 at 14:52

Rob Napier

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1 Comment

Darren Findlay Over a year ago

Excellant answer! Thank you.

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