Swiftpack.co - Package - manmal/CombineProperty


When using Combine, a challenge sometimes is: How can a Publisher's current value be retrieved?

Combine.CurrentValueSubject would be an option since it stores a current value, but this is a dead end - you cannot map() a subject and retrieve the mapped current value. It's also sometimes not ideal to leave the current value mutable.

Properties (inspired by ReactiveSwift) take either a CurrentValueSubject or an arbitrary Publisher plus initial value, and provide:

  • The current value
  • A Publisher that returns all future values, including the current one
  • A Publisher that returns all future values, excluding the current one
  • Operators to transform to other Properties (map, flatMap, boolean and/or/negate,...)


There are at least two scenarios where Properties shine:

  • Slicing global state (e.g. AppState) into sub-states (e.g. AuthState) for easier testing. Maybe a ViewModel only needs the AuthState to operate - if it gets passed the AppState, then the whole state has to be mocked for every test.
  • "Storing" dynamic data in a struct (like the ViewModel in the example below).


Here's a not-so-academical example of how Properties can be used:

struct ViewModel {
    let viewState: Property<ViewState>
    let cellContents: Property<[CellContent]>
    let isActivityIndicatorVisible: Property<Bool>
    let emptyListPlaceholder: Property<String?>

        loadImpulses: PassthroughSubject<APIParams, Never>,
        fetchFromAPI: @escaping (APIParams) -> AnyPublisher<APIResult, APIError>
    ) {
        let loadingStates = loadImpulses
            .flatMap { params -> AnyPublisher<ViewState, Never> in
                let loadingState = Just(ViewState.loading(earlierResult: nil))
                let fetchStates = fetchFromAPI(params)
                    .catch { Just(ViewState.error($0)) }
                return loadingState.append(fetchStates).eraseToAnyPublisher()
        viewState = Property(initial: .initial, then: loadingStates)
        isActivityIndicatorVisible = viewState.map {
            switch $0 {
            case .initial, .loading(.none):
                return true
            case .loading, .loaded, .error:
                return false
        cellContents = viewState.map {
            switch $0 {
            case let .loaded(result), let .loading(earlierResult: .some(result)):
                return result.lines
            case .initial, .error, .loading:
                return []
        emptyListPlaceholder = viewState.map {
            switch $0 {
            case let .loaded(result), let .loading(earlierResult: .some(result)):
                return result.lines.isEmpty ? "No items found" : nil
            case .initial, .error, .loading:
                return nil

enum ViewState {
    case initial
    case loading(earlierResult: APIResult?)
    case loaded(APIResult)
    case error(APIError)

// Simple placeholder types so the example compiles:
struct APIResult {
    let lines: [String]
typealias CellContent = String
typealias APIParams = Int
enum APIError: Error {
    case wrapped(Error)

State of this Lib

This is an early beta. Crucial tests exist now, e.g. for lifetime, and the central flatMap operator. All public components and methods are documented.


I'm happy to accept pull requests, and my DMs are open @manuelmaly.


0.2.0 (2020/10/25):

  • Fixed lifetime problems for when a Property is not retained, but its Publishers are still being observed.
  • Added crucial tests for Property lifetime and flatMap
  • Added documentation to all public-facing components and methods
  • Added operators:
    • map with 1, 2, or 3 keypaths
    • map with a fixed value
    • filter
    • compactMap
    • zip with one Property

0.1.0 (2020/10/17):

  • Initial implementation. Early alpha.


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