A protocol that abstracts UIViewRepresentable/NSViewRepresentable allowing for a single implementation for UIKit and AppKit platforms

PlatformViewRepresentable.swift

import SwiftUI

#if !os(watchOS)

#if canImport(UIKit)
public typealias PlatformViewRepresentableType = UIViewRepresentable
#else
public typealias PlatformViewRepresentableType = NSViewRepresentable
#endif // canImport(UIKit)

public protocol PlatformViewRepresentable: PlatformViewRepresentableType {
    associatedtype PlatformViewType

    func makePlatformView(context: Context) -> PlatformViewType
    func updatePlatformView(_ platformView: PlatformViewType, context: Context)
}

#if canImport(UIKit)

public extension PlatformViewRepresentable where UIViewType == PlatformViewType {
    func makeUIView(context: Context) -> UIViewType {
        makePlatformView(context: context)
    }

    func updateUIView(_ uiView: UIViewType, context: Context) {
        updatePlatformView(uiView, context: context)
    }
}

#else

public extension PlatformViewRepresentable where NSViewType == PlatformViewType {
    func makeNSView(context: Context) -> NSViewType {
        makePlatformView(context: context)
    }

    func updateNSView(_ nsView: NSViewType, context: Context) {
        updatePlatformView(nsView, context: context)
    }
}

#endif // canImport(UIKit)

#endif // os(watchOS)

Your own custom UIView or NSView has to be platform-independent as well.

Example:

import SwiftUI

#if canImport(UIKit)
typealias PlatformView = UIView
#else
typealias PlatformView = NSView
#endif

final class MyCustomView: PlatformView {
    // Your custom platform view implementation...
}

// Given a custom platform-independent view, implementing support for SwiftUI becomes easy:

struct MyCustomViewRepresentable: PlatformViewRepresentable {
    typealias PlatformViewType = MyCustomView

    func makePlatformView(context: Context) -> MyCustomView {
        MyCustomView()
    }

    func updatePlatformView(_ platformView: MyCustomView, context: Context) {
        // Update platformView, which is an instance of `MyCustomView`...
    }
}