Currently OpenAPIRuntime has a lot of dependencies on methods, types etc from Foundation. So, currently its not possible to just replace all import Foundation with import FoundationEssentials as recently suggested by the ESG.

I have made a POC of what it would take to remove all dependencies from Foundation, to allow us to use openapi-generator on platforms where including the ICU is not acceptable, due to binary sizes.

The changes can be seen in:
apple/swift-openapi-runtime@main...madsodgaard:swift-openapi-runtime:foundation-essentials

Note: The above changes are just a POC and are not my final suggestion for a solution.

I am starting this issue to discuss whether this is feasible and would be something the swift-openapi-generator maintainers would be open to going forward with.

Below is a list of the issues that we need to address if this were to work out:

NSLock

Currently NSLock is used a few different places. But we should be able to just replace this with the Lock from swift-nio.

String(format:)

This function is used in Acceptable.swift to convert Double to String with 3 decimals. I think we should be able to write a simple Swift impl of this.

trimmingCharacters(in:)

This is only used to remove whitespaces and quotes. This is relatively simple to write a pure Swift impl for.

NSNull, NSNumber etc...

The OpenAPIValue supports CoreFoundation types like NSNull and NSNumber. I propose that we add a default trait FullFoundationTrait, that hides the support for these behind them.

ISO8601DateFormatter

My proposal for solving this is to hide this type behind FullFoundationSupport. Then we make a new implementation of DateTranscoder which uses the .formatted() APIs from FoundationEssentials.

Unfortunately, these APIs are only available on:

@available(macOS 12.0, iOS 15.0, tvOS 15.0, watchOS 8.0, *)

So, that means that if people disable the FullFoundationSupport and run on Apple Platforms before the above, they loose access to the default iso date formatter and corresponding initializers on Client. Imo, this is fine, since the FullFoundationSupport is probably only for non-apple platforms. (but this could be source-breaking)

localizedDescription

This is used quite a few places when logging the errors. I am not sure if this is a hard requirement or if we could use "\(error)" instead?

localizedCompare and localizedCaseInsensitiveCompare

This function has been replaced by a pure < Swift comparison.

Percent-based encoding/decoding

The URISerializer and URIParser previously used addingPercentEncoding and replacingOccurrences. These have been replaced by pure Swift implementation inspired by implementations in swift-foundation.

Checklist

• NSLock (Use lock from NIO instead of NSLock swift-openapi-runtime#172)
• ISO8601DateFormatter (Introduce FullFoundation trait swift-openapi-runtime#177)
• localizedDescription (Remove localizedDescription swift-openapi-runtime#173)
• localizedCompare and localizedCaseInsensitiveCompare (Remove localized compares swift-openapi-runtime#174)
• Percent-based encoding/decoding (Replace Foundation replacingOccurrences(of:with:) and percent encoding swift-openapi-runtime#176)
• CoreFoundation types: NSNumber, NSNull etc... (Introduce FullFoundation trait swift-openapi-runtime#177)
• String(format:) (Replace String(format:) for Double to String conversions swift-openapi-runtime#178)
• trimmingCharacters (Remove trimmingCharacters(in:) swift-openapi-runtime#175)
• Add CI step to check for regress (add CI for checking Foundation linkage regression swift-openapi-runtime#179)