updates.swift

/*
 * Copyright (C) 2020 by Will Johansson
 *
 * Permission to use, copy, modify, and/or distribute this software for any purpose with or without fee is hereby granted.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES WITH REGARD TO THIS SOFTWARE INCLUDING
 * ALL IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY SPECIAL,
 * DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS,
 * WHETHER IN AN ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN CONNECTION WITH THE
 * USE OR PERFORMANCE OF THIS SOFTWARE.
 */

enum Change<T> {
    case initial(T)
    case delta(previous: T, current: T)
}

extension ObservableType {   
    /* 
     * Get latest two from an observable.
     */
    func updates() -> Observable<(Element, Element)> {
        scan([]) { ($0 + [$1]).suffix(2) }
            .skip { $0.count < 2 }
            .map { ($0[0], $0[1]) }
    }
    
    /*
     * Get state of the change from an observable.
     */
    func change() -> Observable<Change<Element>> {
        scan([]) { ($0 + [$1]).suffix(2) }
            .map { $0.count == 1 ? .initial($0[0]) : .delta(previous: $0[0], current: $0[1]) }
    }
}