How to make Swift Package Manager package for multiple platforms

Issue #504

https://twitter.com/NeoNacho/status/1181245484867801088?s=20

There’s no way to have platform specific sources or targets today, so you’ll have to take a different approach. I would recommend wrapping all OS specific files in #if os and just having one target. For tests, you could do something similar, one test target, but conditional tests

Every files are in Sources folder, so we can use platform and version checks. For example Omnia is a Swift Package Manager that supports iOS, tvOS, watchOS, macOS and Catalyst.

For macOS only code, need to check for AppKit and Catalyst

https://github.com/onmyway133/Omnia/blob/master/Sources/macOS/ClickedCollectionView.swift

#if canImport(AppKit) && !targetEnvironment(macCatalyst)

For SwiftUI feature, need to check for iOS 13 and macOS 10.15

https://github.com/onmyway133/Omnia/blob/master/Sources/SwiftUI/Utils/ImageLoader.swift

@available(iOS 13.0, OSX 10.15, tvOS 13.0, watchOS 6.0, *)