Extension for a Combine-Publisher that returns the current and previous value.

Combine+Pairwise.swift

//
//  Combine+Pairwise.swift
//
//  Created by Felix Mau on 17.05.21.
//  Copyright © 2021 Felix Mau. All rights reserved.
//

import Combine

extension Publisher {

  typealias Pairwise<T> = (previous: T?, current: T)

  /// Emits a tuple containing the previous and current elements from the upstream publisher.
  ///
  /// The first emitted tuple has `nil` as the `previous` value because there is no element
  /// before the first one.
  ///
  /// ### Example
  ///
  /// ```swift
  /// let subscription = (1...5).publisher
  ///   .pairwise()
  ///   .sink { print("(\($0.previous), \($0.current))", terminator: " ") }
  /// ```
  ///
  /// Output:
  /// ```
  /// (nil, 1) (Optional(1), 2) (Optional(2), 3) (Optional(3), 4) (Optional(4), 5)
  /// ```
  ///
  /// - Returns: A type-erased publisher that emits `(previous, current)` tuples
  ///   for each element produced by the upstream publisher.
  ///
  /// Based on: <https://stackoverflow.com/a/67133582/3532505>
  func pairwise() -> AnyPublisher<Pairwise<Output>, Failure> {
    // `scan` requires an initial value, which is `nil` in this case.
    // We therefore emit an optional tuple and remove the optional using `compactMap`.
    scan(nil) { previousPair, currentElement -> Pairwise<Output>? in
      Pairwise(previous: previousPair?.current, current: currentElement)
    }
    .compactMap { $0 }
    .eraseToAnyPublisher()
  }
}

Amazing!! Thank you so much for this 🙌