[Research Spike] Distribute over Swift Package Manager

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As a wallet developer I would like to be able to import ZcashLightClientKit as a Swift Package instead of being forced to use Cocoapods

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https://developer.apple.com/videos/play/wwdc2019/416/

Binary Frameworks in Swift
Xcode 11 now fully supports using and creating binary frameworks in Swift. Find out how to simultaneously support devices and Simulator with the new XCFramework bundle type, how Swift module interfaces work, and how to manage changes to your framework over time.

Idea:
Extract Rust code to its own project and create an XCFramework with it. LibrustZcash does not change often enough that it's actually a burden to maintain long term

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Making Cocoapods support XCFrameworks as a way of transitioning to SPM
https://anuragajwani.medium.com/distributing-universal-ios-frameworks-as-xcframeworks-using-cocoapods-699c70a5c961

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Experiences on XCFramework and Rust
https://developer.apple.com/forums/thread/650511

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Rust + XCFramework + SPM
rust-lang/rust#79408

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about subfolders https://stackoverflow.com/questions/48666237/swift-package-manager-dependency-in-a-repository-subdirectory/50095567

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There are two approaches to distribute the package using SPM

1. Building a static framework of the whole SDK so that developers add the SDK as a binary dependency
   this entails creating a repo for the sdk for this purpose and uploading the binary with the same semver than the cocoapods version.
   the downsides of this approach are:

• local development is probably harder
• access to the source code is lost, only generated interfaces are available which limits learning opportunities and could hurt the ability of developers to send PRs upstream on easy fixes.

2. work on a layered solution that allows the SDK to be distributed as an open sourced repo that uses a binary dependency containing the rust build.
   we need to abstract away the rust layer on a separate repo with the rust welding layer now being a public interface.
   A binary framework is built and published from that repo using dependency managers (SPM and Cocoapods). This framework has its own semantic versioning and can be consumed by other tools as is.
   The ZcashLightClientKit framework depends on this other framework
   the downsides of this approach are:

• code is spread apart on many places, implementing a this with the code as is could add complexity to the daily dev environment and make refactoring more difficult
• requires careful handling of the rust code versioning

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Interesting finding by Hasan Ali Atak

https://github.com/mozilla/rust-components-swift