init

state/store.ts

@@ -0,0 +1,34 @@
+type Listener = () => void
+type OnChange<T> = (args: { newState: T; oldState: T }) => void
+
+export type Store<T> = {
+  getState: () => T
+  setState: (updater: (prev: T) => T) => void
+  subscribe: (listener: Listener) => () => void
+}
+
+export function createStore<T>(
+  initialState: T,
+  onChange?: OnChange<T>,
+): Store<T> {
+  let state = initialState
+  const listeners = new Set<Listener>()
+
+  return {
+    getState: () => state,
+
+    setState: (updater: (prev: T) => T) => {
+      const prev = state
+      const next = updater(prev)
+      if (Object.is(next, prev)) return
+      state = next
+      onChange?.({ newState: next, oldState: prev })
+      for (const listener of listeners) listener()
+    },
+
+    subscribe: (listener: Listener) => {
+      listeners.add(listener)
+      return () => listeners.delete(listener)
+    },
+  }
+}