GCD Timer using DispatchSourceTimer

timer.swift

import Cocoa
import Combine

class Timer: Cancellable, Publisher {
    enum Error: Swift.Error {
        case cancelled
    }
    
    typealias Output = Never
    typealias Failure = Error
    
    private let source: DispatchSourceTimer
    private let subject = PassthroughSubject<Output, Failure>()
    
    init(_ interval: DispatchTimeInterval, leeway: DispatchTimeInterval = .milliseconds(300)) {
        self.source = DispatchSource.makeTimerSource()
        
        let deadline = DispatchTime.now().advanced(by: interval)

        source.schedule(deadline: deadline, repeating: .never, leeway: leeway)
        
        source.setEventHandler { self.fire() }
        source.activate()
    }
    
    func fire() {
        print("fire")
        subject.send(completion: .finished)
    }
    
    func cancel() {
        print("cancel")
        source.cancel()
        subject.send(completion: .failure(.cancelled))
    }
    
    func receive<S>(subscriber: S) where S : Subscriber, Failure == S.Failure, Output == S.Input {
        subject.receive(subscriber: subscriber)
    }
    
    deinit {
        print("deinit")
        cancel()
    }
}

let sub = Timer(.seconds(3)).sink(receiveCompletion: { event in
    switch event {
    case .finished:
        print("finished!")
    case .failure:
        print("cancelled!")
    }
}, receiveValue: { _ in })