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Slate extension onCommit should keep snapshots lazy

docs/solutions/performance-issues/2026-05-18-slate-extension-oncommit-should-keep-snapshot-lazy.md

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Slate extension onCommit should keep snapshots lazy

Problem

Slate v2 moved extension authors from positional commitListeners to object-context onCommit. A naive wrapper would call the internal two-argument listener shape and force a snapshot even when the author only reads commit.

Symptoms

  • onCommit({ commit }) looked cheap at the public API layer.
  • The internal wrapper still had arity 2, so the commit notification path treated it as snapshot-hungry.
  • The API rename risked adding hidden work to history, collaboration, analytics, and debugging extensions.

What Didn't Work

  • Passing (commit, snapshot) through to every onCommit wrapper. That keeps the old listener mechanics but defeats the snapshot laziness built into notifyListeners.
  • Making authors choose between onCommit and a second no-snapshot callback. That would make the public API look split again.

Solution

Keep the internal listener arity at 1, and expose snapshot as a lazy getter on the public context:

ts
registerCommitListener(editor, (commit) => {
  let snapshot: ReturnType<typeof getSnapshot> | null = null

  slots.onCommit?.({
    commit,
    editor,
    get snapshot() {
      snapshot ??= getSnapshot(editor)

      return snapshot
    },
  } as EditorCommitContext<TEditor>)
})

That lets normal handlers stay cheap:

ts
onCommit({ commit }) {
  exportCommit(commit)
}

and keeps snapshot access available when it is actually needed:

ts
onCommit({ snapshot }) {
  cacheChildren(snapshot.children)
}

Why This Works

notifyListeners already uses listener arity to decide whether it should compute the snapshot for commit listeners. A one-argument wrapper preserves that fast path. The getter computes the snapshot only if author code reads it, and caches the result for repeated reads inside the same callback.

Prevention

  • When converting positional listener APIs into object-context callbacks, check whether the old positional arity carried lazy-work semantics.
  • Keep expensive context fields behind getters when most handlers only need the cheap fields.
  • Add or keep contract coverage for both the metadata-only path and the snapshot-reading path when commit callbacks are part of the public extension API.