Setting state's variable to null

[kkizlaitis]

Good afternoon!

I have taken the TodoState from the repository's example and added a variable named selectedTodoId (which can be null):

class TodoState {    

  final List<Todo> todos;
  final int selectedTodoId;            

  TodoState({this.todos, this.selectedTodoId});

  TodoState copy({List<Todo> todos, int selectedTodoId}) {
    return TodoState(          
      todos: todos ?? this.todos,
      selectedTodoId: selectedTodoId ?? this.selectedTodoId);
  }             

  static TodoState initialState() => TodoState(todos: const [], selectedTodoId: null);
}

What is the best way to set copy the state by setting selectedTodoId to null? If I call the state's state.copy(selectedTodoId: null), the previous selectedTodoId is assigned (well, because Dart interprets as if selectedTodoId was simply not passed.

I may call TodoState.initialState().copy(todos: previousState.todos, selectedTodoId: null), but if I later add some more variables to the TodoState, I will most definitely forget about adding them to that call (which will lead to regressions).

What is your take on this problem, @marcglasberg ? :) Thank you!

[marcglasberg]

That's a problem with the copy pattern in Dart. There are many solutions, none of them perfect. I'll give you 2:

The following works because selectedTodoId can't be -1:

TodoState copy({List<Todo> todos, int selectedTodoId = -1}) {
    return TodoState(          
      todos: todos ?? this.todos,
      selectedTodoId: (selectedTodoId != -1) ? selectedTodoId : this.selectedTodoId);
  }      

The following is a special method just to remove the selected todo:

TodoState withoutSelectedTodo() {
    return TodoState(          
      todos: todos,
      selectedTodoId: null);
  }      

[kkizlaitis]

Thanks @marcglasberg for your suggestions!

TodoState withoutSelectedTodo() {
    return TodoState(          
      todos: todos,
      selectedTodoId: null);
  }

This suggestion doesn't do much because you still have not to forget to add a parameter for new variable in the TodoState.

TodoState copy({List<Todo> todos, int selectedTodoId = -1}) {
    return TodoState(          
      todos: todos ?? this.todos,
      selectedTodoId: (selectedTodoId != -1) ? selectedTodoId : this.selectedTodoId);
  }

I like this more because it is less prone to regressions. Unless your argument can be negative :) Also, what would you do if the argument was a String? Define a "NO_VALUE" private variable that user would hopefully never use?

[marcglasberg]

I'd use an empty string. If you don't want to differentiate between null and empty string.

Or I'd use a combination of characters from the private use areas: https://en.wikipedia.org/wiki/Private_Use_Areas