Support for XML request body and response body

[ugocottin]

Motivation

I want to be able de generate client and server from an OpenAPI specification, for an API which use XML as content type for request and response body.

Currently, application/xml contents for request and response are not generated by swift-openapi-generator, and only OpenAPIRuntime.HTTPBody enum associated value is available for xml content.

A Workaround is to use an external XML encoder and decoder, like CoreOffice XMLCoder, and make the encoding from / decoding to HTTPBody.

struct Credentials: Encodable {
    var username: String
    var password_hash: String
}

struct Token: Decodable {
    var token: String
}
let credentials = Credentials(username: "johnapplessed", password_hash: "d2hhdCBkaWQgeW91IGV4cGVjdD8=")
let encoder = XMLEncoder()
let encoded = try encoder.encode(credentials)
let response = try await client.authenticate(body: .xml(HTTPBody(encoded)))
let xmlBody = try response.ok.body.xml
let xmlData = try await Data(collecting: xmlBody, upTo: .max)
let decoder = XMLDecoder()
let decoded = try decoder.decode(Token.self, from: xmlData)
let token = decoded.token

Proposed solution

One quick solution could be to implement a new CodingStrategy for xml in swift-openapi-generator.
For swift-openapi-runtime, we could add the following methods to Converter:

Client/server	Set/get	Schema location	Coding strategy	Optional/required	Method name
client	set	request body	XML	optional	setOptionalRequestBodyAsXML
client	set	request body	XML	required	setRequiredRequestBodyAsXML
client	get	response body	XML	required	getResponseBodyAsXML
server	get	request body	XML	optional	getOptionalRequestBodyAsXML
server	get	request body	XML	required	getRequiredRequestBodyAsXML
server	set	response body	XML	required	setResponseBodyAsXML

Encoding / Decoding logic can be implemented into these methods, with an XML encoder / decoder.
As Converter have encoder and decoder properties as type JSONEncoder and JSONDecoder, we could:

1. Rename encoder to jsonEncoder and decoder to jsonDecoder, and instantiate a xmlEncoder and xmlDecoder in Converter init.
2. Instantiate a XMLEncoder or XMLDecoder in call of methods above.

Example:

extension Converter {
    public func setRequiredRequestBodyAsXML<T: Encodable>(
        _ value: T,
        headerFields: inout HTTPFields,
        contentType: String
    ) throws -> HTTPBody {
        try setRequiredRequestBody(
            value,
            headerFields: &headerFields,
            contentType: contentType,
            convert: convertBodyCodableToXML
        )
    }
}

Alternatives considered

I found that Converter struct is not suited for adding support of new content type. With the proposed solution, the Converter struct will have types of <#HTTPMediaType#>Encoder and <#HTTPMediaType#>Decoder, while not necessarily needed.
For XML only API, Converter still need a JSONEncoder and JSONDecoder.

I thought that, maybe, and I would like to have your opinion on this, we must implement a Converter for a specific http media type, like Vapor is doing with ContentEncoder and ContentDecoder. Each http media type would be assigned to a specific Converter.

What's your thoughts about this?

Additional information

No response

[czechboy0]

Hi @ugocottin,

yes XML is officially supported by the OpenAPI specification, so it'd be reasonable for us to support it here.

Since we already generate Codable types, I think the main piece of work would be to create an XMLEncoder and XMLDecoder that can work with Codable types, and then we can update the generator.

Once that's in place, your plan above with extending the Converter, adding a CodingStrategy is exactly right.

A heads up - this would need to be contributed by you, and we'd help steer the code through pull request review.

If that sounds good, go ahead! 🙏

[czechboy0]

Oh I missed that you already linked to such encoder/decoder in https://github.com/CoreOffice/XMLCoder.

The only concern there is that it's a dependency that isn't API-stable.

I wonder if we could somehow offer this as explicitly opt-in, where only adopters who want XML would include that dependency.

Maybe some new optional conversion methods on OpenAPIRuntime.Configuration, able to convert between HTTPBody and Codable using a custom encoder/decoder.

[ugocottin]

Hello @czechboy0

Thanks for the feedback. I've submitted a first pull request to apple/swift-openapi-runtime#102.

As you suggested, I added the possibility to define custom encoding / decoding behaviour through OpenAPIRuntime.Configuration, with CustomCoder protocol.

Thanks!

[czechboy0]

Looks good - feel free to open a PR for the generator as well (even if it won't pass until the runtime change lands and is released), it's easier to review both changes together.

[czechboy0]

Ok @ugocottin, the runtime changes landed in swift-openapi-runtime 1.4.0, so please bump the dependency of the generator's Package.swift and you should be able to finish the generator work.

[czechboy0]

As part of the generator PR, it'd be great to also add an example project that shows how to bring your own XML encoder/decoder and plug it all together (see the Examples directory).

[ugocottin]

Hi @czechboy0, thanks for letting me know. I will make a pr soon