[BUG][Swift] JSON not deserialized if expected type is "just" a string

[lemoinem]

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)? Yes
• What's the version of OpenAPI Generator used? master
• Have you search for related issues/PRs?
• What's the actual output vs expected output?

Description

I have an API returning JSON (always, the Content-Type on the answer is JSON, the documentation specified JSON, etc.

openapi-generator version

master

OpenAPI declaration file content or url

openapi: 3.0.1
info:
  title: Test string API
  version: 1.0.0
components:
  responses:
    OK_200:
      description: OK
      content:
        application/json:
          schema:
            type: "string"
paths:
  /ping:
    get:
      summary: Endpoint
      description: Returns a JSON-encoded string
      responses:
        '200':
          $ref: "#/components/responses/OK_200"

Command line used for generation

generate -i string-api.yaml -g swift4 -o output

Steps to reproduce

1. Implement the API and return the following full HTTP Response:

HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8

"API String Response"

2. Query the API using the code generated
3. The result of the API is "\"API String Response\"" instead of "API String Response"

The JSON is not deserialized. Despite the appropriate Content-Type and documentation.

Related issues/PRs

None I could find.

Suggest a fix

The issue seems to arise from this special case:
https://github.com/OpenAPITools/openapi-generator/blob/master/modules/openapi-generator/src/main/resources/swift4/AlamofireImplementations.mustache#L362-L380

Although I do understand the use case (sometimes, we just want the response's body as string, not interpreted), this is preventing proper use of the code when we have an API returning a simple string.

Obviously changing the API to wrap the value inside an object or an array isn't acceptable (nor always possible if the API isn't under our control).

I would suggest to use the AlamofireRequestBuilder (returned by getNonDecodableBuilder) for this use case. While the AlamofireDecodableRequestBuilder should always try to decode its content.

Moreover, because of https://bugs.swift.org/browse/SR-6163, We won't be able to rely on JSONDecoder for String values (same is true for plain numbers (int/float), boolean and null). In all these cases, we should add a case in the AlamofireDecodableRequestBuilder to use JSONSerialization.jsonObject(with: json, options: .allowFragments)) as? T instead. The existing special case for String should be removed and replaced by this as well.

Note: There might be many issues linked to https://forums.swift.org/t/top-level-t-self-encoded-as-number-json-fragment/11001 as well.
Maybe switching to JSONSerialization with .allowFragments as the JSON Serializer/Deserializer would be a good idea...

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