MyLittleSuite/dart_mapperdart_mapper

Defaults & Constants

Provide fallback values for null source fields and fixed constant values.

Defaults & Constants

Two @Mapping parameters let you inject values that don't come from the source object:

  • defaultValue: — used as the fallback when the source field is null
  • constant: — always used, regardless of the source

Default Value

Provide a Dart expression string as the fallback for a nullable source field:

class UserInput {
  final String name;
  final String? status;  // nullable
  final String? bio;     // nullable
  const UserInput({required this.name, this.status, this.bio});
}

class UserOutput {
  final String name;
  final String status;  // non-nullable
  final String bio;     // non-nullable
  final String version;
  const UserOutput({
    required this.name,
    required this.status,
    required this.bio,
    required this.version,
  });
}

@Mapper()
abstract class UserMapper {
  @Mapping(target: 'status', defaultValue: 'active')
  @Mapping(target: 'bio', defaultValue: 'No bio provided')
  @Mapping(target: 'version', constant: '1.0')
  UserOutput toOutput(UserInput input);
}

Generated:

@override
UserOutput toOutput(UserInput input) {
  return UserOutput(
    name: input.name,
    status: input.status ?? 'active',
    bio: input.bio ?? 'No bio provided',
    version: '1.0',
  );
}

Constant Value

constant: always emits the given expression — the source field is not read:

@Mapping(target: 'apiVersion', constant: "'v2'")
ResponseDto toDto(ResponseData data);

Generated:

apiVersion: 'v2',

Because the value is emitted verbatim, string constants require inner quotes:

  • constant: "'active'" → emits 'active'
  • constant: "42" → emits 42
  • constant: "true" → emits true

Difference: defaultValue vs constant

ParameterWhen appliedSource field read?
defaultValueSource field is nullYes (checked for null)
constantAlwaysNo

Mutual Exclusion Rules

  • constant cannot be combined with source, defaultValue, or callable
  • defaultValue cannot be combined with callable

See Also

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Subclass Mapping
Callable Mapping