description	React runtime — useScreenFlow, applyTransitionResult, loading strategies, and lifecycle callbacks.

Runtime

The runtime bridges the pure flow engine to React. It owns screen state, applies TransitionResults, runs lifecycle callbacks, and exposes the useScreenFlow hook that most apps use directly.

`useScreenFlow`

The primary React integration. Creates an engine, holds the active screen in state, and returns navigation helpers plus derived affordances.

import { useScreenFlow } from "@hbarkallah/react-screen-flow";

function FlowView({ state }: { state: AppState }) {
  const flow = useScreenFlow(flowDefinition, state, {
    initialScreen: "login",       // optional override on mount
    autoNavigate: true,           // sync screen when state changes
    enableTransitionDetails: true
  });

  const Screen = flow.ScreenComponent;

  return (
    <main>
      {flow.isTransitioning ? <Spinner /> : null}
      <Screen />
      <button onClick={() => void flow.goNext()} disabled={!flow.canGoNext}>
        Next
      </button>
    </main>
  );
}

Return value

Property	Type	Description
`screenId`	`ScreenId`	Active screen
`ScreenComponent`	`ComponentType`	Component for the active screen
`goNext`	`() => Promise<void>`	Navigate along the `next` edge
`goPrev`	`() => Promise<void>`	Navigate along the `prev` edge
`goTo`	`(target) => Promise<void>`	Direct navigation
`reset`	`() => void`	Clear transition state and return to `resolveStart(state)`
`canGoNext`	`boolean`	From `engine.evaluate`
`canGoPrev`	`boolean`	From `engine.evaluate`
`nextTarget`	`ScreenId \| null`	Resolved `next` target
`prevTarget`	`ScreenId \| null`	Resolved `prev` target
`loadingStrategy`	`LoadingStrategy`	Resolved from definition
`isTransitioning`	`boolean`	Async guard in flight
`isBlocking`	`boolean`	`isTransitioning && loadingStrategy === "block-ui"`
`lastBlocked`	`string \| null`	Human-readable block message
`transition`	`ScreenFlowTransitionDetails \| null`	Structured details when `enableTransitionDetails` is true

Options

`initialScreen`

Overrides engine.resolveStart(state) on the first render. Useful for bootstrapping from a URL segment or deep link before route sync runs.

`autoNavigate`

When true, watches the state argument. On change, calls engine.syncToState(currentScreen, state) and applies the result if the target differs. Keeps the visible screen aligned with auth, profile, or permission state without manual goTo calls.

// User logs in → state.user becomes truthy → flow moves to dashboard
useScreenFlow(flow, state, { autoNavigate: true });

`enableTransitionDetails`

Opt-in structured transition state for production UI, debugging, and analytics. When disabled, transition is null and the hook keeps a smaller surface area.

const flow = useScreenFlow(definition, state, {
  enableTransitionDetails: true,
});

flow.transition?.status;   // "idle" | "transitioning" | "blocked" | "error"
flow.transition?.blocked;  // full BlockedTransition object
flow.transition?.error;    // async failure details

`applyTransitionResult`

The core runtime function. Takes a TransitionResult from the engine and mutates React state through injected handlers. Also fires lifecycle callbacks on the FlowDefinition.

import { applyTransitionResult } from "@hbarkallah/react-screen-flow";

const moved = await applyTransitionResult(
  engine.transition("login", { type: "next" }, state),
  {
    definition: flowDefinition,
    state,
    currentScreen: "login",
    handlers: {
      setScreen,
      setLastBlocked,
      setIsTransitioning,
      setTransitionStatus,      // optional
      setBlockedTransition,     // optional
      setTransitionError,       // optional
    },
  },
);

// Returns true if navigation succeeded, false if blocked

useScreenFlow calls this internally. Export it when building a custom runtime (e.g. Zustand store, React Native navigator, or test harness) that still wants the same callback and loading-strategy behavior.

Application flow

sequenceDiagram
  participant Caller
  participant Apply as applyTransitionResult
  participant Def as FlowDefinition
  participant Handlers

  Caller->>Apply: TransitionResult
  Apply->>Def: onBeforeTransition

  alt success
    Apply->>Def: onLeave(from)
    Apply->>Handlers: setScreen(to)
    Apply->>Def: onEnter(to)
    Apply->>Def: onTransitionSuccess
    Apply->>Def: onAfterTransition(status: success)
    Apply-->>Caller: true
  else blocked
    Apply->>Handlers: setLastBlocked, optional redirect
    Apply->>Def: onBlocked / onTransitionBlocked
    Apply->>Def: onAfterTransition(status: blocked | error)
    Apply-->>Caller: false
  else pending
    Apply->>Handlers: setIsTransitioning(true)
    Note over Apply: Apply loading strategy
    Apply->>Apply: await promise
    Apply->>Apply: Re-enter with resolved result
    Apply->>Handlers: setIsTransitioning(false)
  end

Success path

On status: "success":

onBeforeTransition fires
onLeave runs for the departing screen (if screen changed)
setScreen(target) updates React state
onEnter runs for the arriving screen (if screen changed)
onTransitionSuccess and onAfterTransition({ status: "success" }) fire
Blocked/error state is cleared

Blocked path

On status: "blocked":

onBeforeTransition fires
If blockedBehavior === "redirect" and redirectTo is set, navigates to redirectTo. Otherwise the screen stays put.
For optimistic loading, rolls back to previousScreen when the async guard eventually blocks.
setLastBlocked receives message ?? guardReason ?? reason
onBlocked and onTransitionBlocked fire
If the block carried an error (thrown/rejected guard), onTransitionError fires and status becomes "error". Otherwise status is "blocked".
onAfterTransition fires with the final status

Pending path (async guards)

When status: "pending":

onBeforeTransition fires
setIsTransitioning(true) and status "transitioning"
Loading strategy is applied before awaiting the promise:

Strategy	While pending	On success	On block
`deferred`	Stay on current screen	Move to target	Stay on current screen
`optimistic`	Move to target immediately	Stay on target	Roll back to previous screen
`block-ui`	Stay on current screen; `isBlocking` is true	Move to target	Stay on current screen

result.promise is awaited
Resolved result is handled recursively (success or blocked)
setIsTransitioning(false) in finally

If the promise rejection is not caught by the engine (unexpected throw in finally path), onTransitionError fires and the error is re-thrown.

Loading strategies

Configured on FlowDefinition.loadingStrategy. Defaults to "deferred".

import {
  resolveFlowLoadingStrategy,
  resolveLoadingStrategy,
  DEFAULT_LOADING_STRATEGY,
} from "@hbarkallah/react-screen-flow";

resolveFlowLoadingStrategy(definition); // reads definition.loadingStrategy
resolveLoadingStrategy(undefined);      // "deferred"
DEFAULT_LOADING_STRATEGY;                // "deferred"

The loading strategy only affects async guards. Sync guards resolve instantly and never set isTransitioning.

Lifecycle callbacks

All callbacks live on FlowDefinition and are invoked by applyTransitionResult.

Callback	When
`onBeforeTransition`	Before any result is applied (success, blocked, or pending)
`onTransitionSuccess`	After a successful screen change
`onTransitionBlocked`	After a blocked transition (guard failure)
`onBlocked`	Alias-style hook; also called on blocked transitions
`onTransitionError`	Guard threw or async guard rejected
`onAfterTransition`	Terminal callback with `status: "success" \| "blocked" \| "error"`

onEnter / onLeave on individual TransitionRules fire around successful screen changes only.

const flow: FlowDefinition<State, typeof screens> = {
  // ...
  blockedBehavior: "redirect",
  onBeforeTransition: ({ from, to, state }) => {
    console.log(`attempting ${from} → ${to}`);
  },
  onTransitionSuccess: ({ from, to }) => {
    analytics.track("flow_step", { from, to });
  },
  onTransitionBlocked: (result) => {
    toast.error(result.message ?? "Navigation blocked");
  },
  onAfterTransition: ({ from, to, status }) => {
    logger.info({ from, to, status });
  },
};

For router-backed flows, keep URLs aligned with the active screen in your app. See the React Router guide for a copy-paste route sync helper.

Custom runtimes

You can bypass useScreenFlow and wire the engine to any state container:

import { createFlowEngine, applyTransitionResult } from "@hbarkallah/react-screen-flow";

const engine = createFlowEngine(definition);

async function navigate(intent: NavigationIntent) {
  const result = engine.transition(currentScreen, intent, appState);

  await applyTransitionResult(result, {
    definition,
    state: appState,
    currentScreen,
    handlers: {
      setScreen: (screen) => store.setState({ screen }),
      setLastBlocked: (msg) => store.setState({ lastBlocked: msg }),
      setIsTransitioning: (v) => store.setState({ isTransitioning: v }),
    },
  });
}

Keep the split:

Engine — decide what should happen
Runtime — apply the decision, run callbacks, handle async loading UX

Testing the runtime

applyTransitionResult is async and can be tested with mock handlers:

import { describe, expect, it, vi } from "vitest";
import { applyTransitionResult } from "@hbarkallah/react-screen-flow";

it("applies a successful transition", async () => {
  const setScreen = vi.fn();
  const onSuccess = vi.fn();

  const ok = await applyTransitionResult(
    { status: "success", screen: "dashboard", from: "login", to: "dashboard" },
    {
      definition: { ...flow, onTransitionSuccess: onSuccess },
      state: appState,
      currentScreen: "login",
      handlers: {
        setScreen,
        setLastBlocked: vi.fn(),
        setIsTransitioning: vi.fn(),
      },
    },
  );

  expect(ok).toBe(true);
  expect(setScreen).toHaveBeenCalledWith("dashboard");
  expect(onSuccess).toHaveBeenCalled();
});

See src/runtime/applyTransitionResult.test.ts and src/useScreenFlow.test.ts for full coverage of loading strategies, redirects, and transition details.

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