Step 1 - Simple copyWith

class User {
  final String name;
  final int age;

  User({required this.name, required this.age});

  User copyWith({String? name, int? age}) {
    return User(
      name: name ?? this.name,
      age: age ?? this.age,
    );
  }
}

void main() {
  User a = User(name: 'Asha', age: 20);
  User b = a.copyWith(age: 21);
  print('${a.name} ${a.age}');
  print('${b.name} ${b.age}');
}

• ?? keeps the old value when the argument is omitted (null means “no change”).

Step 2 - Nullable fields need a sentinel (later pattern)

• If String? bio can be null on purpose, copyWith({String? bio}) cannot distinguish “leave bio alone” from “set bio to null” with only ??.
• For this level, prefer non-nullable fields or accept the limitation until you learn wrapper types or Object? sentinels.

Step 3 - When to use

• UI state, value models, configs: copy instead of mutating shared instances.
• Large graphs: consider where copying should be shallow vs deep (manual copyWith is usually shallow).

Good habit

• After copyWith, assert or test that the old instance still has old values (see build guides).

Practice tasks

• Add copyWith to a Product with sku, name, price (all non-nullable String/double).
• Create u2 = u1.copyWith() with no args; confirm u1 and u2 are equal in value but not the same object (identical vs == once you implement equality).