[REQ][dart-dio] Support useOptional / patchOnly with serializationLibrary=freezed

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dart

Rechercherichtung

Read the prior change in #25250 and inspect the Dart Dio generator’s Freezed templates and tests for useOptional and patchOnly; the payload does not name specific files. Prototype the proposed Freezed-specific Optional<T> serialization and check generated code for absent, explicit null, and present values, including the stated defaults and unknown enum behavior. Done when those cases work for wrapped properties while unaffected Freezed output remains unchanged.

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Beschreibung

Enhancement: Feature
Is your feature request related to a problem? Please describe.

Since #25250 added serializationLibrary=freezed (beta), useOptional and patchOnly are ignored under freezed and a warning is logged.

Freezed models declare every optional property as T? with includeIfNull: false. A PATCH body therefore cannot express the three states that partial updates need (JSON Merge Patch, RFC 7396):

Intent JSON
Leave unchanged key omitted
Clear the field "field": null
Set the field "field": "value"

With a plain T?, null can only mean one of the first two. Today it means "omit", so clients cannot clear a field. Switching to includeIfNull: true would instead send null for every unset field and wipe data on the server.

Describe the solution you'd like

Support useOptional / patchOnly with freezed, using the same Optional<T> API (Optional.absent() / Optional.present(value)) as the built_value and json_serializable backends:

UpdateItem(name: 'x')                                    // {"name":"x"}
UpdateItem(name: 'x', imageId: Optional.present(null))   // {"name":"x","imageId":null}
UpdateItem(name: 'x', imageId: Optional.present('i'))    // {"name":"x","imageId":"i"}
UpdateItem.fromJson({'name': 'x', 'imageId': null}).imageId  // Optional.present(null)
UpdateItem.fromJson({'name': 'x'}).imageId                   // Optional.absent()

Proposed design (prototyped with freezed 3.2.5 / json_serializable 6.14, Dart 3.13):

  • A freezed-specific optional.dart in which Optional<T> follows json_serializable's generic-helper protocol, Optional.fromJson(json, fromJsonT) / toJson(toJsonT). json_serializable then generates the inner conversion itself for enums, DateTime, nested models, lists and maps.
  • Optional.absent() serializes to a private sentinel. Affected models use @Freezed(toJson: false) with a factory-level @JsonSerializable(createToJson: true), and their own toJson() strips sentinel keys. Without toJson: false, freezed generates a toJson() that bypasses the stripping.
  • @JsonKey(readValue: readOptionalValue) distinguishes a missing key from an explicit null. json_serializable otherwise reads null as absent.
  • Only models with wrapped properties change. All other freezed output, including the existing samples, is unchanged.

Decisions worth reviewing:

  • Spec defaults. A wrapped property with a default starts as Optional.absent(), not Optional.present(default), because sending the default in a PATCH would overwrite server state. json_serializable currently uses present(default).
  • enumUnknownDefaultCase. It is not applied to wrapped enum fields, because json_serializable rejects unknownEnumValue on Optional<Enum?>. Unknown values throw.
  • Binary properties are never wrapped; they are excluded from JSON anyway.
  • patchOnly scope is unchanged: only schemas $ref'd directly by a PATCH request body are wrapped.
  • Out of scope: operation parameters (path, query, header, form) stay unwrapped, as with the other backends. Clearing a multipart/form-data field is not addressed, because form data has no null.
Describe alternatives you've considered
  • Reusing the shared optional.mustache / OptionalConverter<T>. json_serializable does not apply a generic converter to concrete types such as Optional<String?>. With serializationLibrary=json_serializable and useOptional=true, build_runner currently fails with Could not generate fromJson code for ... To support the type Optional ... (separate bug report to follow).
  • includeIfNull: false on wrapped fields. json_serializable cannot omit a key based on its value, so this also drops Optional.present(null).
  • Sending the full object (PUT-style) with includeIfNull: true. This works only when the client always has the complete current state, sends more data, and can overwrite concurrent edits.
Additional context

Follow-up to #25250. I would like to implement this and can open a PR if the approach looks good.

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