core

@stainless-code/layers

Headless, UI-agnostic engine for opening any layer from anywhere and managing it as an ordered, named stack. An app-wide LayerClient owns stack state and lifecycle; awaiting a typed result is optional, and void client.open(...) is equally first-class.

Adapters bind the engine to a specific UI library or framework — @stainless-code/react-layers, @stainless-code/preact-layers, @stainless-code/svelte-layers, @stainless-code/vue-layers, @stainless-code/solid-layers, @stainless-code/angular-layers — and each depends on and re-exports this package.

Install

bun add @stainless-code/layers

Usually you install an adapter instead; it pulls this package in as a dependency.

When to use

Reach for the core directly when building a library or framework adapter, or driving overlay or stack state in a headless, non-UI context. App developers usually install their library or framework adapter instead; it re-exports this core.

Use it to open layers imperatively from anywhere and manage ordered, named stacks. Await a typed result or fire-and-forget; both are first-class. Singletons via upsert with live update, serial one-at-a-time queues, nested or child stacks, transitions, dismissal blockers, validation, and gcTime each stand on their own.

Skip the core directly when an app only needs overlay UI and an adapter exists—install the adapter. Skip the whole library for a single, always-local overlay with no return, stacking, queue, animation, or guard needs.

Quick start

import {
  LayerClient,
  layerOptions,
  layerKey,
  createCallContext,
  isPayloadValidationError,
} from "@stainless-code/layers";

type ConfirmPayload = { title: string };
type ConfirmResponse = boolean;

const client = new LayerClient({
  defaultStackOptions: {
    confirm: { scope: { strategy: "serial" }, gcTime: 5_000 },
  },
});

// Brand a reusable definition — response type (boolean) flows into open()
const confirm = layerOptions<ConfirmPayload, ConfirmResponse>({
  stack: "confirm",
  key: ["confirm", "remove"],
  enteringDelay: 200,
  exitingDelay: 200,
});

// Or brand a key alone
const removeKey = layerKey<ConfirmResponse>()(["confirm", "remove"]);

// Observe stack snapshots (adapters bind this to UI reactivity)
const stack = client.getStack("confirm");
const unsubStack = stack.subscribe(() => {
  for (const state of stack.getSnapshot()) {
    const layer = stack.getLayer(state.id)!;
    const call = createCallContext(stack, layer, state);
    // render state + call in your UI; call.end(response) resolves the caller's await
  }
});

client.subscribeStacks((stackId) => {
  /* new stack materialized */
});

try {
  const ok = await client.open({
    ...confirm,
    payload: { title: "Remove item?" },
    validate: (input: unknown) => {
      const title =
        typeof input === "object" && input !== null && "title" in input
          ? input.title
          : undefined;
      if (typeof title !== "string" || !title.trim()) {
        throw new Error("Title required");
      }
      return { title };
    },
  });
  // ok: boolean — inferred from layerOptions DataTag
} catch (err) {
  if (isPayloadValidationError(err)) {
    /* err.issues */
  }
}

unsubStack();

Payload and response are both optional. This fire-and-forget layer has neither; its renderer resolves the call with call.dismiss().

const maintenanceNotice = layerOptions<void>({
  stack: "notice",
  key: ["notice", "maintenance"],
});
const noticeStack = client.getStack("notice");

noticeStack.subscribe(() => {
  for (const state of noticeStack.getSnapshot()) {
    const layer = noticeStack.getLayer(state.id)!;
    const call = createCallContext(noticeStack, layer, state);
    void call.dismiss();
  }
});

void client.open(maintenanceNotice);

Engine model

LayerClient
  ├── ensureStack / getStack / getStackIds / subscribeStacks / dismissAll
  └── LayerStack (per named surface)
        ├── getSnapshot / getQueuedSnapshot / subscribe
        └── Layer[] (ordered; each has state + promise)

1. Declare — layerOptions<P, R>({ stack, key, ... }) brands the key with a DataTag for response inference.
2. Observe — subscribe to a stack snapshot; use createCallContext(stack, layer, state) to wire call.end / call.dismiss in your renderer.
3. Call — await client.open({ ...options, payload }); resolution happens when something calls call.end(response) or stack.dismiss(layer, response).

Public API

Classes

Export	Role
LayerClient	App-wide orchestrator. open(), getStack(), getStackIds(), subscribeStacks(), dismissAll(), ensureStack(), bindChildStack()
LayerStack	One named surface. getSnapshot(), getQueuedSnapshot(), subscribe(), getLayer(), find(), open(), dismiss(), dismissAll(), addBlocker(), update(), setRunning(), settle(), cancelQueued()
Layer	One frame: id, key, state, promise (ControlledPromise<R>), abortController, addBlocker(), setRunning(), resolve() / reject() / abort()
Subscribable	Base subscription primitive (subscribe(), size) used by stacks
ControlledPromise<T>	Promise with external resolve / reject (Resolve, Reject types)
PayloadValidationError	Thrown when validate fails at open

Functions

Export	Role
layerOptions()	Brand a reusable options object; key carries a DataTag so open() infers R / E
layerKey()	Brand a key alone: layerKey<R>()(key)
createCallContext()	Build the imperative call handle (end, dismiss, update, settle, addBlocker, …)
createLayerGroup()	Child stack scoped to a parent layer's lifetime (open, dismissAll, dispose)
childStackId()	Derive a collision-free child stack id from a parent call
hashKey() / keySignature()	Stable string identity for a LayerKey
isPayloadValidationError()	Narrow PayloadValidationError
notifyManager	{ batch, batchCalls } — dedupe notifications inside a batch

Key inference (DataTag)

layerOptions / layerKey brand keys at compile time only. Helpers: DataTag, InferDataTagResponse, InferDataTagError, ResponseOf, ErrorOf.

Payload validation

Optional validate on layerOptions or open — a StandardSchemaV1 schema or sync Validator fn ((input: unknown) => output, throws on invalid). Parsed synchronously at open; failure rejects with PayloadValidationError (issues: ReadonlyArray<ValidationIssue>). Input/output inference: InferValidatorInput, InferValidatorOutput.

App-wide error typing

Augment Register to set DefaultLayerError once:

declare module "@stainless-code/layers" {
  interface Register {
    defaultError: AppError;
  }
}

Layer & stack options (types)

LayerOptions, OpenLayerOptions, StackOptions, StackDefaults, LayerClientOptions, LayerGroupOptions, LayerGroupHandle, OmitKeyof.

Notable per-layer options: enteringDelay / exitingDelay, upsert, loadFn, validate, rootProps, component.

Notable per-stack options: scope: { strategy: "serial" | "parallel" }, gcTime, dismissAllMode.

Snapshots & lifecycle (types)

LayerState, LayerPhase (pending | queued | active | dismissed | error), LayerTransition (entering | settled | exiting), LayerActionStatus, LayerCallContext, LayerComponentProps, LayerComponent, LayerKey.

Transitions complete on whichever fires first of { enteringDelay/exitingDelay elapsed, call.settle() }. Serial stacks queue later opens (phase: "queued", visible via getQueuedSnapshot()).

Blockers & bulk dismiss (types)

BlockerFn, StackBlockerFn, DismissOptions (force?), DismissAllOptions / DismissAllMode ("skipBlocked" | "stopAtBlocked" | "force"). call.end / call.dismiss return Promise<boolean> (false when vetoed). dismissing is true while blockers run.

Standard Schema (type)

StandardSchemaV1 — vendored Standard Schema v1 interface (+ InferInput / InferOutput namespace helpers).

---

Full guide & adapter examples: repo README. Architecture: docs/architecture.md.