# `Writer.apply`

> 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 [`Writer`](Writer.md) · inherited by [`Tx`](Tx.md), [`Session`](Session.md), [`testing.FakeSession`](testing.FakeSession.md) · stable · since 2.0.0 · called as `session.apply`

```ts
apply(scene: SceneSpec, options?: C & SignalOption): Promise<Report>;
```

Replaces the look's `apply()` block atomically (spec section 6.10, R2). Each key of `scene`
that is present replaces that part: the named materials, `apply()`'s
ordered rules, and the listed collections. An omitted key leaves that part unchanged;
`rules: []` clears only `apply()`'s rules. Rules made through [`find().set`](PartActions.set.md) form a later block
it never removes.
Outcome: one commit; look-only, so no undo step unless labelled. Timing: resolves at the
commit point, after its loads and readiness (≤ readyTimeoutMs); in the write queue. New content
loads hidden while the old scene keeps drawing; everything swaps in one frame.
Merging and in the write queue: never superseded, so pending calls commit in call order.
Not for: a priced material choice ([`select`](Writer.select.md)) or a saved position ([`setSceneModel`](Writer.setSceneModel.md),
[`applyPatch`](Writer.applyPatch.md)); nor for standing rules that later `apply()` calls must leave alone, which are
[`find(q).set`](PartActions.set.md).
Errors: rejects HoloInputError on an invalid spec ([`validate('scene', spec)`](validate.md) checks its
shape and its collections' keys and parents; `{ dryRun: true }` checks all this call checks
and changes nothing), and 'duplicate-rule-id' when two of `rules` share an id (spec section 6.10, R3).
Asset failures are [`problems`](ReportFields.problems.md) ('asset-failed'); a rule that matches nothing records
'target-unmatched'.

## 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. |
| `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). |
| `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… |
| `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 |
| --- | --- | --- | --- |
| `signal` | `AbortSignal` | undefined | Cancels this call; the member's JSDoc says what an abort does. |

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/05-Writer.apply.ts`):

```ts
import type { MaterialSpec, Session } from '@holostep-configurators/runtime-client/contract';
declare const session: Session;
declare const signal: AbortSignal;
declare const finishes: Readonly<Record<string, MaterialSpec>>;      // 'oak', 'walnut', …
// The find() block: set once. apply() never removes or reorders it.
await session.find([{ name: 'mp-l*' }, { name: 'mp-r*' }]).set({ visible: false });
// The apply() block: the whole look, built from app state and replaced on every call.
export const applyLook = (finish: string) => session.apply({
  materials: finishes,
  rules: [
    // in array order
    { query: { tag: 'carcass' }, set: { material: { ref: finish } } },
    {
      query: { node: 'cab:42', tag: 'front' },
      set: { material: { baseColor: '#2b2b2b', maps: { baseColor: 'none' } } },
    },
  ],
});
await applyLook('oak');
// omitted keys are unchanged: both blocks of rules stay
await session.apply({ materials: finishes });
```

## Generic-API test (spec 3.1)

| Rule | Result |
| --- | --- |
| 1. Neutral name | pass: `apply` names no product |
| 2. A precedent | pass: [CSS cascade (ordered, persistent rules)](https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_cascade/Cascade) (row `G32`) |
| 3. Two apps, or labs | pass: 6 apps (ledger: app-fc306ec3dc; fleet cross-check: app-378d190f1b, app-4f16d2f6da, app-510d750508, app-ceb4eccc72, app-fb16dd1985) |
| 4. The cheaper fix first | decided per need in Appendix B: rows `G32` at tier stable (a member only where no rule, data, asset or app code serves the need) |