@AutoReducer
public abstract class TodoReducer implements Reducer<List<TodoItem>> {

    @AutoReducer.Action(TodoActions.ADD_ITEM)
    List<TodoItem> addItem(List<TodoItem> items, TodoItem item) {
        return TreePVector.from(items)
                .plus(item);
    }

    @AutoReducer.Action(TodoActions.CHANGE_STATE)
    List<TodoItem> changeState(List<TodoItem> items, long id, boolean isChecked) {
        for (int i = 0; i < items.size(); i++) {
            TodoItem todoItem = items.get(i);
            if (todoItem.id == id) {
                TodoItem changed = new TodoItem(id, todoItem.text, isChecked);
                return TreePVector.from(items)
                        .with(i, changed);
            }
        }
        return items;
    }

    public static TodoReducer create() {
        return new TodoReducerImpl();
    }
}