Shows how a multi-platform SwiftUI can bridge to UIKit and AppKit.

AppKitTextView.swift

// Implementation of the view using AppKit.
#if os(macOS)
import AppKit
import SwiftUI

final class AppKitTextView: NSView {
    let textView: NSTextView = {
        let this = NSTextView()
        this.translatesAutoresizingMaskIntoConstraints = false
        return this
    }()

    init() {
        super.init(frame: .zero)
        addSubview(textView)
        NSLayoutConstraint.activate([
            textView.leadingAnchor.constraint(equalTo: leadingAnchor),
            textView.trailingAnchor.constraint(equalTo: trailingAnchor),
            textView.topAnchor.constraint(equalTo: topAnchor),
            textView.bottomAnchor.constraint(equalTo: bottomAnchor)
        ])
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

struct AppKitTextViewRepresentable: NSViewRepresentable {
    @Binding var text: String

    func makeNSView(context: Context) -> some NSView {
        let textView = AppKitTextView()
        textView.textView.delegate = context.coordinator
        return textView
    }

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

    func makeCoordinator() -> Coordinator {
        Coordinator(text: $text)
    }
}

extension AppKitTextViewRepresentable {
    final class Coordinator: NSObject, NSTextViewDelegate {
        private var text: Binding<String>

        init(text: Binding<String>) {
            self.text = text
        }

        func textDidChange(_ notification: Notification) {
            guard let textView = notification.object as? NSTextView else {
                return
            }
            text.wrappedValue = textView.string
        }
    }
}
#endif

ContentView.swift

// This uses our multi-platform view and has no knowledge of the different platforms.
struct ContentView: View {
    @State private var text: String = ""

    var body: some View {
        TextView(text: $text)
            .onChange(of: text) { newValue in
                print(newValue)
            }
    }
}

TextView.swift

// A wrapper that chooses either the UIKit or AppKit implementation depending on which platform we are compiling for.
struct TextView: View {
    @Binding var text: String

    var body: some View {
        #if os(iOS)
        UIKitTextViewRepresentable(text: $text)
        #else
        AppKitTextViewRepresentable(text: $text)
        #endif
    }
}

UIKitTextView.swift

// Implementation of the view using UIKit.
#if os(iOS)
import UIKit
import SwiftUI

final class UIKitTextView: UIView {
    let textView: UITextView = {
        let this = UITextView()
        this.translatesAutoresizingMaskIntoConstraints = false
        return this
    }()

    init() {
        super.init(frame: .zero)
        addSubview(textView)
        NSLayoutConstraint.activate([
            textView.leadingAnchor.constraint(equalTo: leadingAnchor),
            textView.trailingAnchor.constraint(equalTo: trailingAnchor),
            textView.topAnchor.constraint(equalTo: topAnchor),
            textView.bottomAnchor.constraint(equalTo: bottomAnchor)
        ])
    }

    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

struct UIKitTextViewRepresentable: UIViewRepresentable {
    @Binding var text: String

    func makeUIView(context: Context) -> some UIView {
        let textView = UIKitTextView()
        textView.textView.delegate = context.coordinator
        return textView
    }

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

    func makeCoordinator() -> Coordinator {
        Coordinator(text: $text)
    }
}

extension UIKitTextViewRepresentable {
    final class Coordinator: NSObject, UITextViewDelegate {
        private var text: Binding<String>

        init(text: Binding<String>) {
            self.text = text
        }

        func textViewDidChange(_ textView: UITextView) {
            text.wrappedValue = textView.text
        }
    }
}
#endif