Question 75042 in Flutter Question posted in
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How can I make 'this' equal to another instance of the same class in Dart? – Flutter

I’m working on a Flutter project in which the architecture forces me to use an instance of a class, say UserModel, as a value attached to the BuildContext. In this way, I can access that class in every part of the app.

Anyway, there are moments in which I want to update the UserModel, and not only by changing the value of some of the fields, but updating all of them. At this moment, I am using an horrible function, let’s call that updateUser(final UserModel newInstance). It follows an example.

class UserModel {
  String name, surname;

  UserModel({required this.name, required this.surname});

  void updateUser(final UserModel newInstance) {
    name = newInstance.name;
    surname = newInstance.surname;
  }
}

The main problem with this solution is that it often happens that I add some fields to the UserModel class, but I forgot to add them to updateUser. Also, the solution I am using looks horrible.

I know that many of you could say that I should enclose the instance of UserModel in a UserModelProvider class and just replace the value of the userModel field. This would not be compatible with the architecture of the software I am developing, so please don’t suggest me that.

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2

Answers


  1. Chosen as BEST ANSWER
    0

    I got it sorted out. The main point to understand is to use reflection, the ability of a program to examine and modify its own structure, behavior, and metadata at runtime.

    To do so, I edit my updateUser function to be able to loop through the class variables and set them in the new instance. Here is the result.

    import 'dart:mirrors';

class UserModel {
  String name, surname;

  UserModel({required this.name, required this.surname});

  void updateUser(final UserModel newInstance) {
    final InstanceMirror thisInstanceMirror = reflect(this);
    final InstanceMirror newInstanceMirror = reflect(newInstance);

    final ClassMirror classMirror = thisInstanceMirror.type;

    for (final DeclarationMirror otherField in classMirror.declarations.values) {
      // We exclude fields that are not variables, like functions
      if (otherField is VariableMirror) {
        // We exclude constant, final and static variables
        if (!otherField.isConst && !otherField.isFinal && !otherField.isStatic) {
          // Finally we update the field with the new value
          thisInstanceMirror.setField(otherField.simpleName,
            newInstanceMirror.getField(otherField.simpleName).reflectee);
        }
      }
    }
  }

  @override
  String toString() {
    return "Name: $name, Surname: $surname";
  }
}

    So now I can use an instance of the class to update itself, for instance by fetching new values from Firestore. It follows an example of the functioning.

    void main() {
  final UserModel user = UserModel(name: "John", surname: "Doe");
  final UserModel user2 = UserModel(name: "Jane", surname: "Doe");

  print(user); // Name: John, Surname: Doe
  print(user2); // Name: Jane, Surname: Doe

  user.updateUser(user2);

  print(user); // Name: Jane, Surname: Doe
}

    And this is crucial if my UserModel is being served by Provider, and therefore attached to the BuildContext.


  2. 0

    To be honest, it’s hard to understand your issue.

    If I’m not wrong, you are looking for copyWith method.

    class UserModel {
  String name, surname;

  UserModel({required this.name, required this.surname});

  UserModal copyWith({
     String? name,
     String? surname,
  }) => UserModel(
          name: name?? this.name,
          surname: surname ?? this.surname);
}

    With this method, you will be able to update one or both variable.

    final  foo = UserModel("foo","foo2");

final boo = foo.copyWith(); // boo = UserModel("foo","foo2");

final yoo = foo.copyWith(name:"yoo"); // yoo = UserModel("yoo","foo2");

// update foo
 foo = foo.copyWith(surname:"update foo"); // foo = UserModel("foo","update foo");

    Updated

    I don’t know why you force to update it, the concept is the same.

    class UserModel {
...
 UserModel updateWITHMODELASYOUWANT( UserModel data){
  return UserModel(
    name: data.name, 
    surname: data.surname,
 }
}

    As I said in the comments, you update the instance, not the object model.

    foo is an instance of UserModel

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