todo_vm_add.md

void add(String title, String subtitle) {
  var currentState = state;
  if (currentState == TodoState.empty()) {
    currentState = TodoState();
  }
  //check fot current state
  if (currentState is TodoStateData) {
    //object
    final todo = Todo(
        id: rand.nextInt(100).toString(), title: title, subtitle: subtitle);
    //update
    final todos = currentState.todos.toList()..add(todo);
    //localstorage
    read<LocalStorage>().saveTodo(todo);
    //update state
    state = TodoState(
      todos: todos,
    );
  }
}

You should create another one of this file as a dart file for better readability on the medium post.