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madyanov/FlowKit v0.2.0
Declarative Flow Coordinators
⭐️ 5
🕓 2 weeks ago
.package(url: "https://github.com/madyanov/FlowKit.git", from: "v0.2.0")


A library that allows you to describe flow coordinators as FSM.



  • Separation of responsibilities: navigation implementation, navigation logic and state transformation are described and can be tested independently of each other.
  • No shared state.
  • Declarativity, DSL out of the box, generation of flow graph description in the DOT language.
  • Server-driven flow ready.


The main idea is a vertical separation of responsibilities of the flow:

  • Navigation implementation (TransitionNavigator) – changing the tree of screens.
  • Navigation logic (TransitionProvider) – determining which screen to go next.
  • State transformation (StateReducer) – transformation of data as the flow progresses and determining when there is enough data to complete the flow.

These components are asynchronous and operate with entities such as Step, StepResult, State.


Flow steps. In fact, these are screens or some kind of navigation actions.

According to the step the TransitionNavigator makes transition to the screen.


The results of the flow steps. Either the data that came from the previous screen, or some kind of flag indicating a completion of the step.

Based on the result of the step, the StateReducer creates a new state, and the TransitionProvider determines which transition to make next.


The state of the flow collected from the results of the previous steps. I.e., this is the data collected as the flow progresses, and used either in the flow itself, or returned at its completion.

Unlike the step and the result of the step, the state is passed to all the main components of the flow: TransitionNavigator, StateReducer, TransitionProvider. All of them can use the state for their logic to work correctly, but only the StateReducer can change the state.


Add a dependency:

.package(url: "https://github.com/madyanov/FlowKit.git", .upToNextMinor(from: "0.2.0")),

:warning: Until 1.0.0 minor versions may be breaking.

Determine the flow steps, the results of the steps and the possible states:

enum MyFlowStep {
    case amount
    case invalidAmount
    case tariffs

enum MyFlowStepResult {
    case amount(Int)
    case tariffs(Tariff)
    case confirmation(ConfirmationResult)

enum MyFlowState {
    case country(Country)
    case amount(Int, country: Country)
    case tariff(Tariff, amount: Int, country: Country)

Implement TransitionNavigator, StateReducer and TransitionProvider:

final class MyFlowTransitionNavigator: TransitionNavigator { ... }

final class MyFlowStateReducer: StateReducer { ... }

final class MyFlowTransitionProvider: TransitionProvider { ... }

Create and start the flow:

final class MyFlow {
    private lazy var transitionNavigator = MyFlowTransitionNavigator(...)
    private lazy var stateReducer = MyFlowStateReducer(...)
    private lazy var transitionProvider = MyFlowTransitionProvider(...)

    private lazy var flow = Flow(transitionNavigator: transitionNavigator,
                                 stateReducer: stateReducer,
                                 transitionProvider: transitionProvider)

    func start(with country: Country) -> Promise<Transfer> {
        return flow.start(from: .amount, with: .country(country))

Example of flow implementation


Instead of implementing the TransitionProvider, you can use the existing DeclarativeTransitionProvider, which allows you to describe flow using DSL:

let dsl = FlowDSL {
    emit(using: emitter)
    step {
        on(.error) { forward(to: .alert) }
    step(.amount) {
        on(.invalidAmount) { forward(to: .invalidAmount) }
        next { forward(to: .tariffs) }
    step(.tariffs) {
        next { forward(to: .confirmation) }
    step(.confirmation) {
        on(.confirmationContinue) { forward(to: .success) }
        on(.confirmationEditAmount) { back(to: .amount) }
        on(.confirmationEditTariff) { back(to: .tariffs) }
    step(.success) {
        next { forward(to: .finish) }

let transitionProvider = DeclarativeTransitionProvider(flowDSL: dsl)



Create a DSL builder that implements the FlowDSLBuilder protocol:

final class MyFlowDSLBuilder: FlowDSLBuilder { ... }

To describe the state change reaction, the type defining possible events (Event) and the emitter function of these events are used.

Define the Event type inside the builder:

final class MyFlowDSLBuilder: FlowDSLBuilder {
    enum Event {
        case invalidAmount
        case confirmationContinue
        case confirmationEditAmount
        case confirmationEditTariff


Define the event emitter:

static func emitter(_ stepResult: MyFlowStepResult, _ state: MyFlowState) -> Event? {
    switch stepResult {
    case .amount(let amount) where amount < 100:
        return .invalidAmount
    case .confirmation(.continue, _):
        return .confirmationContinue
        return nil

Describe the flow using this emitter:

let dsl = FlowDSL {
    emit(using: emitter)
    step(.amount) {
        on(.invalidAmount) { forward(to: .invalidAmount) }
        next { forward(to: .tariffs) }


Complete example where FlowDSLBuilder is implemented in the TransitionProvider


  • step(Step) { ... } – event handling for a specific step
  • step { ... } – event handling for any step
  • on(Event) { ... } – handle specific event
  • next { ... } – handle when no events were emitted


The DOTBuilder allows you to convert DSL to the DOT graph description language:

let dot = DOTBuilder()
let dsl = FlowDSL { ... }



Result for the example flow:

strict digraph {
	node [shape=box]

	"*" -> alert [label="on error"]
	amount -> invalidAmount [label="on invalidAmount"]
	amount -> tariffs
	tariffs -> confirmation
	confirmation -> success [label="on confirmationContinue"]
	confirmation -> amount [label="on confirmationEditAmount"]
	confirmation -> tariffs [label="on confirmationEditTariff"]
	success -> finish


Flow Graph


Stars: 5
Last commit: 2 weeks ago
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