CopyWith that works for "resetting" nullable fields

copyWithFunction.dart

class Foo {
  int bar;
  int? baz;

  Foo(this.bar, this.baz);

  // The "normal" copyWith does not allow to "reset" nullable
  // value to `null`.
  // Passing `null` means "keep the value"
  Foo copyWith({
    int? bar,
    int? baz,
  }) => Foo(
    bar ?? this.bar,
    // This means we can never "reset" baz to `null`
    baz ?? this.baz, 
  );

  // Using callback for the copy means we get more options
  // - dont pass the function (`null`) => keep the old value
  // - pass the function (non `null`) => use the returned value
  //   - can be non-null
  //   - can be null
  Foo copyWithFunc({
    int Function(int barOld)? bar,
    // null function => no change; null return from function => reset to null
    int? Function(int? bazOld)? baz,
  }) => Foo(
    bar == null ? this.bar : bar(this.bar),
    // can now be reset to null
    baz == null ? this.baz : baz(this.baz), 
  );
}

void main() {
  final foo = Foo(0,0);
  
  // 1) change value
  final one = foo.copyWithFunc(baz: (_) => 1);
  print(one.baz);
  

  // 2) keep value 
  final old = foo.copyWithFunc(baz: null);
  print(old.baz);

  // 3) reset to null
  final nul = foo.copyWithFunc(baz: (_) => null);
  print(nul.baz);

  // For nested objects we could even do the following
  // foo.copyWithFunc(nested: (old) => old.copyWithFunc(baz: (_) => newBaz));
}