add storeHooks.i to be called when atomState is created

Author: dmaskasky

src/vanilla/internals.ts

@@ -330,6 +330,8 @@ type StoreHookForAtoms = {
 
 /** StoreHooks are an experimental API. */
 type StoreHooks = {
+  /** Listener to notify when the atom is initialized. */
+  readonly i?: StoreHookForAtoms
   /** Listener to notify when the atom is read. */
   readonly r?: StoreHookForAtoms
   /** Listener to notify when the atom value is changed. */
@@ -380,6 +382,7 @@ const createStoreHookForAtoms = (): StoreHookForAtoms => {
 
 function initializeStoreHooks(storeHooks: StoreHooks): Required<StoreHooks> {
   type SH = { -readonly [P in keyof StoreHooks]: StoreHooks[P] }
+  ;(storeHooks as SH).i ||= createStoreHookForAtoms()
   ;(storeHooks as SH).r ||= createStoreHookForAtoms()
   ;(storeHooks as SH).c ||= createStoreHookForAtoms()
   ;(storeHooks as SH).m ||= createStoreHookForAtoms()
@@ -401,6 +404,7 @@ const atomOnMount: AtomOnMount = (_store, atom, setAtom) =>
 const ensureAtomState: EnsureAtomState = (store, atom) => {
   const buildingBlocks = getInternalBuildingBlocks(store)
   const atomStateMap = buildingBlocks[0]
+  const storeHooks = buildingBlocks[6]
   const atomOnInit = buildingBlocks[9]
   if (import.meta.env?.MODE !== 'production' && !atom) {
     throw new Error('Atom is undefined or null')
@@ -409,6 +413,7 @@ const ensureAtomState: EnsureAtomState = (store, atom) => {
   if (!atomState) {
     atomState = { d: new Map(), p: new Set(), n: 0 }
     atomStateMap.set(atom, atomState)
+    storeHooks.i?.(atom)
     atomOnInit?.(store, atom)
   }
   return atomState as never

tests/vanilla/internals.test.tsx

@@ -106,6 +106,17 @@ describe('store hooks', () => {
     return { store, storeHooks }
   }
 
+  describe('init hook (i)', () => {
+    it('should call init hook when atom state is initialized', () => {
+      const { store, storeHooks } = createStoreWithHooks()
+      const baseAtom = atom(0)
+      const initCallback = vi.fn()
+      storeHooks.i.add(baseAtom, initCallback)
+      store.get(baseAtom)
+      expect(initCallback).toHaveBeenCalledTimes(1)
+    })
+  })
+
   describe('read hook (r)', () => {
     it('should call read hook when atom is read', () => {
       const { store, storeHooks } = createStoreWithHooks()