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@Invoker
public class TodoListInvoker extends InvokerWrapper {

    @InvocationHandler
    public String createTodoList(String name, String user) throws Exception {
        var identifier = "TDLS_" + UUID.randomUUID();
        getCommandGateway()
                .send(new TodoListCreateCommand(identifier, name),
                      toUserMetadata(user))
                .get(30, TimeUnit.SECONDS);
        return identifier;
    }

    @InvocationHandler
    public CompletableFuture<TodoListView> findTodoListByIdentifier(String id) {
        return getQueryGateway()
                .query(new TodoListViewFindByIdentifierQuery(id))
                .thenApply(Single::getData);
    }
}

@Saga(version = 1)
public class ErpTodoListActivityRegistrationSaga {

    @SagaEventHandler(init = true, associationProperty = "identifier")
    public SagaState on(TodoListCreatedEvent event) { /* start tracking */ }

    @SagaEventHandler(associationProperty = "identifier")
    public void on(TodoListTodoCheckedEvent event, SagaState state,
                   CommandGateway commandGateway) throws Exception {
        if (event.isAllChecked()) {
            commandGateway.send(new ErpUserActivityRegisterCommand(
                    "TodoList", event.getIdentifier(), state.getUsages()
            )).get();
        }
    }
}

@Projection
public class TodoListProjection {

    @QueryHandler
    public Single<TodoListView> handle(TodoListViewFindByIdentifierQuery q) {
        return Single.of(repository.findById(q.getIdentifier())
                .map(TodoList::toView).orElseThrow());
    }

    @QueryHandler
    public Multiple<TodoListListItemView> handle(TodoListListItemViewSearchQuery q) {
        return Multiple.of(repository.search("%" + q.getNameLike() + "%",
                PageRequest.of(q.getPage(), q.getSize()))
                .map(TodoList::toListItemView).toList());
    }
}

@Projector(version = 2)
public class TodoListProjector {

    @EventHandler
    public void on(TodoListTodoCheckedEvent event, Metadata metadata,
                   Instant timestamp, ProjectorStatus status) {
        var list = repository.findById(event.getIdentifier()).orElseThrow();
        var todo = list.todo(event.getTodoIdentifier());
        todo.setCompletedAt(timestamp.atZone(ZoneId.systemDefault()));
        todo.setCompletedBy(metadata.get("user"));
        repository.save(list);
        if (status.isHeadReached()) sse.notifyListChanged(list.getId());
    }
}