# `Assembly.attach`

> Contract 2.0.0 · package sha256 `a857eca95b35921109faf997094bb4192f12527d9d0fcd4be7e97f06425bad6e` · generated from `configurator-runtime.d.ts` by `services/onsite-scripter/scripts/build-contract-docs.mjs`; edit the source, not this file.

Method of [`Assembly`](Assembly.md) · stable · since 2.0.0 · called as `session.assembly.attach`

```ts
attach(node: string, options?: RecordedOptions<C> & AttachOptions): Promise<AttachReport>;
```

Attaches `node` by its connectors (recorded; merging; in the write queue): moves its model root
and records the connection. Spec section 6.12 is the normative table. In short: placement
honours the stored plane config (`align`, `frontDir`, `allowRotatedSnap`) exactly as 1.4
`snapTo` did; it refuses only a disabled connector or one occupied by another pair
(re-attaching a connected pair re-places `node`), an `accepts` mismatch where
`accepts` is declared, or planes that don't meet; it never moves another node, and it drops
`node`'s other connections that no longer meet after the move ('connection-stale'). A refused
attach changes nothing. `dryRun` reports without changing. Through a `tx`: 'staged', and a
dropped connection that the same transaction attaches again is not a problem.
Not for: moving nodes by pose, which is never recorded as a connection.
Errors: a refusal resolves (`attached: false`), never rejects, and so does an internal
failure ('rolled-back', 'internal-error'; spec section 6.4); invalid input rejects
HoloInputError. Other connections it drops are [`problems`](ReportFields.problems.md) ('connection-stale').

## Options

Through [`Session`](Session.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `actor` | `Actor` | the `expectGroup`'s actor when one is given, else 'app' | Who made the write. |
| `checkpoint` | `JsonValue` | unchanged | Page state stored atomically with this step; returned in [`state().checkpoint`](SessionState.checkpoint.md) and the feed. |
| `dryRun` | `boolean` | false | Runs and reports, then rolls back by design ('dry-run'). |
| `expectGroup` | `HistoryGroup` | undefined | A guard, checked at the call: this write belongs to the undo group you are running. |
| `expectRevision` | `number` | undefined (no check) | If-match on [`session.revision`](Session.revision.md). |
| `from` | `string` | the best free one | The connector of `node` to use (a [`ConnectorRef.connector`](ConnectorRef.connector.md)). |
| `near` | `Point3` | the node's position | Which of `node`'s connectors to prefer when several have partners: the one nearest this world point. |
| `orient` | `'auto' \| 'keep'` | 'auto' | 'auto': yaw `node` about world +Y so the two normals oppose, then translate. |
| `origin` | `string` | null | Your tag, echoed unchanged in [`Report.origin`](ReportFields.origin.md) and `cause.origin`. |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |
| `supersede` | `boolean` | false (the write is queued behind the transaction and applies after it closes) | While a transaction is open: roll it back (it resolves 'superseded'); the writes queued behind it apply in call order, then this one, which supersedes an… |
| `to` | `ConnectorRef \| { node: string }` | the best free connector of any other node | A connector (`{ node, connector }`: used at any distance), or a node (its best free connector). |
| `undo` | `'rebase' \| (string & {})` | a recorded-state change gets one step labelled 'Change'; a look-only write gets none; the session's first model (cause 'load') gets none, whatever the label: it is the floor of history | An undo label makes this write one undo step. |

Through [`Tx`](Tx.md):

| Option | Type | Default | What it does |
| --- | --- | --- | --- |
| `from` | `string` | the best free one | The connector of `node` to use (a [`ConnectorRef.connector`](ConnectorRef.connector.md)). |
| `near` | `Point3` | the node's position | Which of `node`'s connectors to prefer when several have partners: the one nearest this world point. |
| `orient` | `'auto' \| 'keep'` | 'auto' | 'auto': yaw `node` about world +Y so the two normals oppose, then translate. |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |
| `to` | `ConnectorRef \| { node: string }` | the best free connector of any other node | A connector (`{ node, connector }`: used at any distance), or a node (its best free connector). |

Refused here (typed `never`, so passing one fails to compile): `actor`, `checkpoint`, `dryRun`, `expectGroup`, `expectRevision`, `origin`, `supersede`, `undo`.

## Abort (`signal`)

This call is a write (`SignalOption.signal`, [the kinds of call](../read-this-first.md#conventions)).

> A write: before the commit point it rolls back and rejects AbortError; after it, no effect, except that a running camera move stops where the last frame drew it (`CameraMoveOptions`).
>
> Through a `tx` the commit point is the transaction's: aborting a write still loading rejects AbortError and stages nothing (catch it: unobserved, it rolls the transaction back, spec section 6.1, O6); aborting one already staged unstages it, its paths keeping the latest other write to them as after a failed savepoint (O7), and its Promise stays resolved 'staged'.

## Example

Imports: values (`has`, `validate`, the `Holo*Error` classes, the pure helpers) from `@holostep-configurators/runtime-client`, types from `@holostep-configurators/runtime-client/contract`, `/math` and `/testing` names from those entries; `boot` from the runtime module the root's `loadRuntimeAtFloor` resolves to. A type your page defines is your app's: declare it there, never import it from the package.

Compiled under the contract's strict settings (`examples/declaration/12-Assembly.attach.ts`):

```ts
import type { DeepReadonly, IModelNode, Reason, SceneModel, Session } from '@holostep-configurators/runtime-client/contract';
declare const session: Session;
declare const signal: AbortSignal;
// Add an arm and attach it, in one undo step; a refusal rolls the add back too. For an app whose
// scene model is its source of truth: a store app adds in its store and lets its one sync write
// the model (spec section 11.2), or a sync rolls this back, or its next sync drops the arm.
class AttachRefused extends Error {
  constructor(readonly reason: Reason) { super(`attach refused: ${reason}`); }
}
declare function withNode(model: DeepReadonly<SceneModel> | null, node: IModelNode): SceneModel;
export async function addArm(arm: IModelNode, seatId: string): Promise<Reason | 'superseded' | null> {
  try {
    const r = await session.transaction(async (tx) => {
      // staged and loaded, hidden
      await tx.setSceneModel(withNode(tx.getSceneModel(), arm));
      const a = await tx.assembly.attach(arm.nodeId, {
        from: 'mp-left',
        to: { node: seatId, connector: 'mp-right' },
      });
      // 'occupied' | 'rejected' | 'no-candidate' | 'not-loaded'
      if (!a.attached) throw new AttachRefused(a.reason ?? 'other');
    }, { undo: 'Add arm', origin: 'arm-ui' });
    if (r.outcome === 'superseded') return 'superseded';    // a newer write won: not a refusal
    return r.outcome === 'committed' ? null : (r.reason ?? 'other');
  } catch (error) {
    // the transaction rolled back
    if (error instanceof AttachRefused) return error.reason;
    throw error;
  }
}
```

## Generic-API test (spec 3.1)

| Rule | Result |
| --- | --- |
| 1. Neutral name | pass: `attach` names no product |
| 2. A precedent | pass: [Onshape mate connectors](https://cad.onshape.com/help/Content/mate_connector.htm) (row `K63`) |
| 3. Two apps, or labs | pass: 6 apps (ledger: app-04c2774145, app-37356bf09f, app-4762caacdc, app-c1384707fc, app-f9a32142f1; fleet cross-check: app-04c2774145, app-37356bf09f, app-4762caacdc, app-ceb4eccc72, app-f9a32142f1) |
| 4. The cheaper fix first | decided per need in Appendix B: rows `K63` at tier app, stable (a member only where no rule, data, asset or app code serves the need) |