Observer.swift

class ObservationToken {
    let cancellationClosure: (ObservationToken) -> Void
    
    init(cancellationClosure: @escaping (ObservationToken) -> Void) {
        self.cancellationClosure = cancellationClosure
    }
    
    func cancel() {
        cancellationClosure(self)
    }
}

class AudioPlayer {
    var observations = [ObjectIdentifier: (AudioPlayer, Item) -> Void]()

    @discardableResult
    func addObserver<T: AnyObject>(
        _ observer: T, 
        closure: @escaping (T, AudioPlayer, Item) -> Void
    ) -> ObservationToken {
        let token = ObservationToken { [weak self] token in
            self?.observations.removeValue(forKey: ObjectIdentifier(token))
        }

        let id = ObjectIdentifier(token)

        observations[id] = { [weak self, weak observer] player, item in
            guard let observer = observer else {
                self?.observations.removeValue(forKey: id)
                return
            }
            
            closure(observer, player, item)
        }

        return token
    }
}