Hook

Marker for the Litho hooks.

Carrying this annotation is what makes a function a hook, and it drives both hook rules: KComponentHookCallsite requires the call to come from render() or another @Hook function, and KComponentHookInDeferredLambda requires it to run synchronously during render. Neither rule restricts control flow — the compiler plugin gives each call site a structural identity, so hooks may be called in conditions, loops, and after early returns. Two consequences are worth knowing: a hook's state lives exactly as long as the hook keeps being called, so a hook that stops running loses its state and starts from its initial value when it runs again; and repeated invocations of one call site are positional unless wrapped in withNewScope with a stable product key. See the Hooks Overview doc.

Calling a hook requires opting in, and the only supported way to do that is the -opt-in argument that litho_hook_identity_decorator emits alongside the Litho compiler plugin. A module that calls a hook from an un-annotated function without the plugin therefore fails to compile instead of throwing on its first render — a compiler plugin cannot detect its own absence, so the always-present annotation has to carry the requirement.

Never hand-write the opt-in. @OptIn(Hook::class) or a manual -opt-in argument silences the requirement without adding the plugin, which is precisely the state this marker exists to prevent. Both are what kotlinc suggests when the requirement fires, which is why message spells out the real fix — the diagnostic is the only place the reader is looking.