InjectedNSView.swift

struct InjectedNSView<Content: View, Injected: NSView>: NSViewRepresentable {
    let content: Content
    let injected: () -> Injected
    func makeNSView(context: Context) -> Injected {
        let injected = injected()
        let hostingView = NSHostingView(rootView: content)
        hostingView.translatesAutoresizingMaskIntoConstraints = false
        injected.addSubview(hostingView)
        hostingView.topAnchor.constraint(equalTo: injected.topAnchor).isActive = true
        hostingView.bottomAnchor.constraint(equalTo: injected.bottomAnchor).isActive = true
        hostingView.leftAnchor.constraint(equalTo: injected.leftAnchor).isActive = true
        hostingView.rightAnchor.constraint(equalTo: injected.rightAnchor).isActive = true
        return injected
    }
    func updateNSView(_ nsView: Injected, context: Context) {
        if let view = nsView.subviews.first as? NSHostingView<Content> {
            view.rootView = content
        }
    }
}

// Example usage

extension View {
    func onKeyDown(perform action: @escaping OnKeyDown.Action) -> some View {
        return InjectedNSView(content: self) { OnKeyDown(action: action) }
    }
}

class OnKeyDown: EventHandlerView {
    override func keyDown(with event: NSEvent) {
        if !action(event) {
            super.keyDown(with: event)
        }
    }
}

class EventHandlerView: NSView {
    typealias Action = (NSEvent) -> Bool
    let action: Action
    init(action: @escaping Action) {
        self.action = action
        super.init(frame: .zero)
    }
    @objc required dynamic init?(coder aDecoder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
}

private let characterSet = CharacterSet.init()
    .union(.alphanumerics)
    .union(.punctuationCharacters)
    .union(.symbols)
    .union(.whitespacesAndNewlines)
    .subtracting(.init(charactersIn: "\t"))

struct BasicTextInput: View {
    @Binding var text: String
    var body: some View {
        ZStack(alignment: .topLeading) {
            RoundedRectangle(cornerRadius: 8.0)
                .strokeBorder(style: .init(lineWidth: 2))
                .frame(maxWidth: .infinity, maxHeight: .infinity)
            Text(text)
                .padding(8)
        }
        .padding(.horizontal, -0.5)
        .padding(.bottom, -0.25)
        .contentShape(RoundedRectangle(cornerRadius: 7.0))
        .focusable()
        .padding(2)
        
        // *******************************************************
        .onKeyDown { event in
            if let specialKey = event.specialKey,
               specialKey == .delete,
               text.count > 0 {
                let index = text.index(text.endIndex, offsetBy: -1)
                text = String(text.prefix(upTo: index))
                return true
            }
            else if let characters = event.characters,
               let unicodeScalar = characters.unicodeScalars.first {
                if characterSet.contains(unicodeScalar) {
                    text.append(characters)
                    return true
                }
            }
            return false
        }
        // *******************************************************

        .padding(-2)
    }
}

struct BasicTextInput_PreviewContainer: View {
    @State private var text: String = ""
    var body: some View {
        VStack {
            Spacer()
            Text("Enter some text:").font(.title).bold()
                .frame(maxWidth: .infinity, alignment: .leading)
            BasicTextInput(text: $text)
            Button("Clear") {
                text = ""
            }
            .frame(maxWidth: .infinity, alignment: .trailing)
            Spacer()
        }
        .padding()
    }
}

struct BasicTextInput_Previews: PreviewProvider {
    static var previews: some View {
        BasicTextInput_PreviewContainer()
    }
}