Get previous & current value of a Observable

ObservableType+Previous.swift

extension ObservableType {
    func withPrevious() -> Observable<(previous: Element?, current: Element?)> {
        return scan([]) { (previous, current) in
            return Array(previous + [current]).suffix(2)
        }
        .map { array -> (previous: Element?, current: Element?) in
            let previous = array.count > 1 ? array.first : nil
            let current = array.last
            return (previous, current)
        }
    }

    func withPrevious(startWith first: Element) -> Observable<(Element, Element)> {
        return scan((first, first)) { ($0.1, $1) }.skip(1)
    }
}