Add dart_mappable parser support for type conversion

[Copilot]

Implements Parser.DartMappable to support dart_mappable as an alternative to json_serializable for type conversion, addressing the feature request in the issue.

Changes

Core (retrofit/lib/http.dart)

• Added Parser.DartMappable enum value

Generator (generator/lib/src/generator.dart)

• Deserialization: uses <ClassName>Mapper.fromMap(data)
• Serialization: uses object.toMap()
• Handles single objects, lists, maps, nullable types, and extension types

Documentation & Example

• Added usage section to README with setup instructions
• Created example_dartmappable/ with working example

Usage

@RestApi(
  baseUrl: 'https://api.example.com',
  parser: Parser.DartMappable,
)
abstract class ApiService {
  factory ApiService(Dio dio) = _ApiService;

  @GET('/tasks')
  Future<List<Task>> getTasks();
}

@MappableClass()
class Task with TaskMappable {
  const Task({this.id, this.name});
  final String? id;
  final String? name;
}

Generated code uses TaskMapper.fromMap() for responses and task.toMap() for request bodies.

Warning

Firewall rules blocked me from connecting to one or more addresses (expand for details)

I tried to connect to the following addresses, but was blocked by firewall rules:

• dl-ssl.google.com
  • Triggering command: wget -qO- REDACTED (dns block)
• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)
• https://storage.googleapis.com/download.dartlang.org/linux/debian/dists/stable/InRelease
  • Triggering command: /usr/lib/apt/methods/https (http block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

Original prompt

---

This section details on the original issue you should resolve

<issue_title>dart_mappable for type conversion</issue_title>
<issue_description>Can I use dart_mappable for type conversion?</issue_description>

<agent_instructions>dart_mappable support</agent_instructions>

Comments on the Issue (you are @copilot in this section)

@trevorwang Not yet.

PRs are appreciated. </comment_new>

• Fixes dart_mappable for type conversion #618

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