Another approach for simplified wrapping of AppKit and UIKit views in SwiftUI.

SwiftUI WrappableView.swift

//
//  WrappableView.swift
//  macOS-Wordflower
//
//  Created by Marcus Westin on 1/24/20.
//  Copyright © 2020 Marcus Westin. All rights reserved.
//

// My preferred approach to wrapping AppKit and UIKit views in SwiftUI

import SwiftUI

protocol WrappableView: OSView {
    typealias ViewType = Self
    typealias Context = OSViewRepresentableContext<WrappedView<Self>>
    
    /// Optional: update view
    func update(context: Context)
    
    /// Optional: wrapped has a functional default implementation and should not
    /// have to be overriden by protocol implementations
    var wrapped: WrappedView<Self> { get }
}

// MARK: Default implementations
extension WrappableView {
    /// Default empty implementation of optional update function
    func update(context: Context) {}
    
    /// Implementation of wrapped, which should not be overriden by classes
    internal var wrapped: WrappedView<Self> {
        WrappedView { self }
    }
}

struct WrappedView<Wrappable: WrappableView>: OSViewRepresentable {
    let wrap: () -> Wrappable
    
    internal func makeCoordinator() -> Wrappable {
        return wrap()
    }

    #if os(macOS)
    internal func makeNSView(context: NSViewRepresentableContext<WrappedView<Wrappable>>) -> Wrappable.ViewType {
        return context.coordinator
    }
    internal func updateNSView(_ view: Wrappable.ViewType, context: NSViewRepresentableContext<WrappedView<Wrappable>>) {
        context.coordinator.update(context: context)
    }

    #elseif os(iOS)
    internal func makeUIView(context: NSViewRepresentableContext<WrapView>) -> Wrapper.ViewType {
        return context.coordinator
    }
    internal func updateUIView(_ view: Wrapper.ViewType, context: NSViewRepresentableContext<WrapView>) {
        context.coordinator.update(context: context)
    }
    #endif
}


// MARK: - Example usage

struct WrappableView_Previews: PreviewProvider {
    static var previews: some View {
        Group {
            ExampleFirstResponderTextField().wrapped
            ExampleFirstResponderTextField().wrapped
        }
    }
    
    
    final class ExampleFirstResponderTextField: NSTextField, WrappableView, NSTextFieldDelegate {

        override func viewDidMoveToWindow() {
            DispatchQueue.main.async {
                self.becomeFirstResponder()
            }
        }
    }
}