App.js

// useEffect(..., []);

useEffect(() => {
  const subscription = API
    .graphql(graphqlOperation(onCreateMessage))
    .subscribe({
      next: (event) => {
        setMessages([...messages, event.value.data.onCreateMessage]);
      }
    });
  
  return () => {
    subscription.unsubscribe();
  };
}, [messages]);