unionKey property

Determines what key should be used to de/serialize union types.

Consider:

@freezed
class Union with _$Union {
  factory Union.first() = _First;
  factory Union.second() = _Second;

  factory Union.fromJson(Map<String, Object> json) => _$UnionFromJson(json);
}

When serializing or deserializing Union, Freezed will ask/demand for an extra json key, which represents which constructor should be used.

More specifically, when calling Union.toJson, we will have:

void main() {
  print(Union.first().toJson()); // { 'runtimeType': 'first' }
  print(Union.second().toJson()); // { 'runtimeType': 'second' }
}

This variable allows customizing the key used ("runtimeType" by default).

For example, we could change our previous Union implementation to:

@Freezed(unionKey: 'type')
abstract class Union with _$Union {
  // ...
}

which changes how fromJson/toJson behaves:

void main() {
  print(Union.first().toJson()); // { 'type': 'first' }
  print(Union.second().toJson()); // { 'type': 'second' }
}

Implementation

Freezed class