generate fibonacci numbers using Swift

FibonacciGenerator.swift

protocol GeneratorType {
    associatedtype Element
    mutating func next() -> Element?
}

class FibsGenerator: GeneratorType {
    var state = (0, 1)
    func next() -> Int? {
        let upcomingNumber = state.0
        state = (state.1, state.0 + state.1)
        return upcomingNumber
    }
}

let gen = FibsGenerator()
while (true) {
    print(gen.next())
}