fix(core): commit state before finalize publishes (#38983)

packages/core/src/state.ts

@@ -59,7 +59,11 @@ export interface Options<State, DraftApi> {
   readonly initial: () => State
   /** Wraps mutable state in a domain-specific draft API. */
   readonly draft: MakeDraft<State, DraftApi>
-  /** Runs after all active transforms and before the rebuilt state becomes visible. */
+  /**
+   * Runs after the rebuilt state becomes visible. Update events published here
+   * act as read barriers: subscribers refetching on the event observe the
+   * committed state.
+   */
   readonly finalize?: (draft: DraftApi) => Effect.Effect<void>
 }
 
@@ -81,9 +85,8 @@ export function create<State, DraftApi>(options: Options<State, DraftApi>): Inte
   const semaphore = Semaphore.makeUnsafe(1)
 
   const commit = Effect.fn("State.commit")(function* (next: State) {
-    const api = options.draft(next)
-    if (options.finalize) yield* options.finalize(api)
     state = next
+    if (options.finalize) yield* options.finalize(options.draft(next))
   })
 
   const apply = (transform: TransformCallback<DraftApi>, draft: DraftApi) =>

packages/core/test/state.test.ts

@@ -36,6 +36,25 @@ describe("State", () => {
     }),
   )
 
+  it.effect("commits rebuilt state before finalize runs", () =>
+    Effect.gen(function* () {
+      const observed: string[][] = []
+      const state: State.Interface<{ values: string[] }, { add: (item: string) => void }> = State.create({
+        initial: () => ({ values: [] as string[] }),
+        draft: (draft) => ({ add: (item: string) => draft.values.push(item) }),
+        finalize: () => Effect.sync(() => observed.push([...state.get().values])),
+      })
+
+      yield* state.transform((draft) => {
+        draft.add("value")
+      })
+
+      // Update events publish from finalize, so consumers reading on the event
+      // must observe the rebuilt state, not the previous one.
+      expect(observed).toEqual([["value"]])
+    }),
+  )
+
   it.effect("runs transforms during every reload", () =>
     Effect.gen(function* () {
       let value = "first"