Combine previous value using Combine

Question

How can I rewrite ReactiveSwift/ReactiveCocoa code using Combine framework? I attached screenshot what combinePrevious mean from docs.

let producer = SignalProducer<Int, Never>([1, 2, 3]).combinePrevious(0)
producer.startWithValues { value in
    print(value) // print: (0, 1), (1, 2), (2, 3)
}

Answer 1

These are the custom operators I've come up with (called withPrevious). There are two overloads, one where the initial previous value is nil and the other where you provide the initial previous value so you don't have to deal with optionals.

extension Publisher {

    /// Includes the current element as well as the previous element from the upstream publisher in a tuple where the previous element is optional.
    /// The first time the upstream publisher emits an element, the previous element will be `nil`.
    ///
    ///     let range = (1...5)
    ///     cancellable = range.publisher
    ///         .withPrevious()
    ///         .sink { print ("(\($0.previous), \($0.current))", terminator: " ") }
    ///      // Prints: "(nil, 1) (Optional(1), 2) (Optional(2), 3) (Optional(3), 4) (Optional(4), 5) ".
    ///
    /// - Returns: A publisher of a tuple of the previous and current elements from the upstream publisher.
    func withPrevious() -> AnyPublisher<(previous: Output?, current: Output), Failure> {
        scan(Optional<(Output?, Output)>.none) { ($0?.1, $1) }
            .compactMap { $0 }
            .eraseToAnyPublisher()
    }

    /// Includes the current element as well as the previous element from the upstream publisher in a tuple where the previous element is not optional.
    /// The first time the upstream publisher emits an element, the previous element will be the `initialPreviousValue`.
    ///
    ///     let range = (1...5)
    ///     cancellable = range.publisher
    ///         .withPrevious(0)
    ///         .sink { print ("(\($0.previous), \($0.current))", terminator: " ") }
    ///      // Prints: "(0, 1) (1, 2) (2, 3) (3, 4) (4, 5) ".
    ///
    /// - Parameter initialPreviousValue: The initial value to use as the "previous" value when the upstream publisher emits for the first time.
    /// - Returns: A publisher of a tuple of the previous and current elements from the upstream publisher.
    func withPrevious(_ initialPreviousValue: Output) -> AnyPublisher<(previous: Output, current: Output), Failure> {
        scan((initialPreviousValue, initialPreviousValue)) { ($0.1, $1) }.eraseToAnyPublisher()
    }
}