Swift Generator that produces Fibonacci numbers

FibonacciGenerator.swift

import UIKit

struct FibonacciGenerator: GeneratorType, SequenceType {

    var (f1, f2) = (1, 1)

    mutating func next() -> Int? {
        let result = f1
        (f1, f2) = (f2, f1 + f2)
        return result
    }

    func generate() -> FibonacciGenerator {
        return FibonacciGenerator()
    }
}

var generator = FibonacciGenerator().generate()

for _ in 1...20 {
    generator.next()
}