@async/flow

@async/flow

Portable store, resource, and handler runtime for Async packages.

Flow is useful when an app needs signal-like state, event handlers, async resources, and small workflow helpers without adopting a full statechart engine.

Pick the smallest layer that solves the problem:

• L1 primitives: use createSignal, createComputed, createResource, and createStore when an adapter or library needs explicit refs and controllers.
• L2 Flow: use flow(...) when state changes should run through named events and batched handlers.
• L3 helpers: use compose(...), parallel(...), remember(...), status(...), transition(...), and guards when a workflow benefits from reusable steps and finite status helpers.

Install

pnpm add @async/flow

Quick Start

import { flow, status } from "@async/flow";

const counter = flow({
  store: {
    count: 0,
    phase: status("idle", ["idle", "active"])
  },

  on: {
    increment(store, input = {}) {
      store.count += input.by ?? 1;
      store.phase = "active";
    },

    reset(store) {
      store.count = 0;
      store.phase = "idle";
    }
  }
});

counter.dispatch("increment", { by: 2 });

counter.store.count; // 2
counter.refs.phase.get(); // "active"

A Flow instance combines:

• store: author-facing values with getter/setter behavior.
• refs: explicit signal, status, and computed refs for adapters.
• resources: mounted async resources and their controllers.
• dispatch(name, input): event execution.
• can(name, input?): event availability checks.
• explain(name, input?): structured blocked-event reasons.
• describe(): public inspection metadata for stores, resources, handlers, transitions, and guards.
• compose([...]): ordered handler steps from @async/flow/compose.
• parallel(...): fan-out/fan-in effects inside a composed handler.
• remember(...): explicit previous-value copies around scoped work.

Store Values

Plain primitives and arrays become writable store refs. Computed values are read-only. Plain record values stay explicit; use signal(value) when an object should be a single writable value.

import { computed, flow, signal, status } from "@async/flow";

const cart = flow({
  store: {
    items: [],
    settings: signal({ currency: "USD" }),
    count: (store) => store.items.length,
    isEmpty: computed((store) => store.count === 0),
    phase: status("idle", ["idle", "ready"])
  },

  on: {
    add(store, input) {
      store.items = [...store.items, input.item];
      store.phase = "ready";
    }
  }
});

cart.dispatch("add", { item: { id: "sku_123" } });

cart.store.count; // 1
cart.refs.items.get(); // [{ id: "sku_123" }]
cart.store.settings = { currency: "EUR" };

Resources

resource(loader) declares a lazy async value with lifecycle state and explicit controls. The loader receives the store plus tools containing a native abort signal, the load input, and the resource version.

import { flow, resource } from "@async/flow";

const greeting = flow({
  store: {
    name: "World",
    greeting: resource(async (store, { signal }) => {
      const response = await fetch(`/api/greeting/${store.name}`, { signal });
      return response.text();
    })
  },

  on: {
    fetch(store) {
      return store.greeting.load();
    },

    reload(store) {
      return store.greeting.reload();
    },

    cancel(store, reason) {
      return store.greeting.cancel(reason);
    }
  }
});

await greeting.dispatch("fetch");

greeting.store.greeting.status; // "ready"
greeting.store.greeting.value; // loaded text

Lazy resources stay as controller objects in store. Immediate resources read as values through store and keep their controller under flow.resources.

const profile = flow({
  store: {
    user: resource({ immediate: true }, async (_store, { signal }) => {
      const response = await fetch("/api/user", { signal });
      return response.json();
    })
  },

  on: {
    refreshUser() {
      return this.resources.user.reload();
    }
  }
});

profile.store.user; // current value
profile.resources.user.status; // "loading", "ready", or "error"

More detail: Resource Lifecycle.

Compose And Status Workflows

Use compose(...) for ordered steps that should share one Flow handler input. Each step receives (store, input, previous). Use parallel(...) when one ordered step should run independent effects before continuing. Use remember(...) when a handler should copy previous store values into explicit store fields after scoped work succeeds.

import { flow, parallel, remember, set, status, transition, when } from "@async/flow";
import { compose } from "@async/flow/compose";

const checkout = flow({
  store: {
    step: status("shipping", ["shipping", "payment", "review"]),
    previousStep: null,
    canSubmit: true,
    loading: false,
    orderId: null
  },

  on: {
    next: remember(["step", "previousStep"], [
      transition("step", {
        shipping: "payment",
        payment: "review"
      })
    ]),

    submit: compose([
      when((store) => store.step === "review" && store.canSubmit),
      set("loading", true),
      parallel({
        inventory(_store, input) {
          return reserveInventory(input.form);
        },
        tax(_store, input) {
          return calculateTax(input.form);
        }
      }),
      async (_store, input) => {
        const order = await submitOrder(input.form);
        return order.id;
      },
      (store, _input, orderId) => {
        store.orderId = orderId;
      },
      set("loading", false)
    ])
  }
});

compose stays synchronous until a step returns a promise-like value. Flow then flushes the current synchronous batch and resumes later steps in a fresh batch. That lets loading = true render before async work settles.

More detail: Compose And Status Helpers.

Event Availability And Inspection

Flow can answer whether an event is callable now without dispatching it.

checkout.can("next"); // true
checkout.explain("submit");
// { event: "submit", allowed: false, reason: "guard_failed", source: "guard" }

Use describe() when adapters or tests need stable public metadata:

const description = checkout.describe();

description.handlers; // ["next", "submit"]
description.store.step.kind; // "status"
description.transitions.next.status; // "step"

Descriptions expose names, current values, lifecycle state, and safe metadata. They do not expose raw handlers or predicates.

Runtime Options

The top-level authoring helper accepts either config or options plus config.

flow(config);
flow({ scheduler, context }, config);

With two arguments, the first object is always runtime options and the second is always Flow config.

Handlers receive (store, input). Runtime capabilities are available through method syntax or normal functions:

const appFlow = flow(
  {
    context() {
      return { logger: console };
    }
  },
  {
    store: {
      count: 0
    },

    on: {
      increment(store, input) {
        store.count += input.by;
        this.logger.log(store.count);
        return this.dispatch("read");
      },

      read(store) {
        return store.count;
      }
    }
  }
);

Receiver capabilities include this.store, this.refs, this.resources, this.dispatch(name, input), this.can(name, input), this.explain(name, input), this.describe(), this.after(ms, eventName, input), and this.dispose(cleanup).

Public Subpaths

import { flow, signal, status, computed, resource } from "@async/flow";
import { compose, parallel, remember } from "@async/flow/compose";
import { createFlow, createResource, createStore, createSignal } from "@async/flow/runtime";
import { defineFlow, defineResource } from "@async/flow/define";
import { createResource as createStandaloneResource } from "@async/flow/resource";

The old @async/flow/run subpath is not public. Use @async/flow/compose.

Docs

• Docs Index
• Layer Guide
• Signals, Computed, Resources, And Store
• Resource Lifecycle
• Compose And Status Helpers

Package Checks

pnpm test
pnpm run typecheck
pnpm run pack:check