SampleView.swift

import Cocoa
import SwiftUI

/// Preview tool
struct ViewRepresentable<V: NSView>: NSViewRepresentable {
    let provider: () -> V
    
    func makeNSView(context: Context) -> V { provider() }
    func updateNSView(_ nsView: V, context: Context) { }
}

struct ViewControllerRepresentable<C: NSViewController>: NSViewControllerRepresentable {
    let provider: () -> C
    
    func makeNSViewController(context: Context) -> C { provider() }
    func updateNSViewController(_ nsViewController: C, context: Context) { }
}

struct Preview<T: NSResponder, R: View>: View {
    let provider: R
    
    var body: some View { provider }
}

extension Preview where T: NSView, R == ViewRepresentable<T> {
    init(_ provider: @escaping () -> T) {
        self.provider = ViewRepresentable(provider: provider)
    }
    
    init(_ view: T) {
        self.provider = ViewRepresentable(provider: { view })
    }
}

extension Preview where T: NSViewController, R == ViewControllerRepresentable<T> {
    init(_ provider: @escaping () -> T) {
        self.provider = ViewControllerRepresentable(provider: provider)
    }
    
    init(_ controller: T) {
        self.provider = ViewControllerRepresentable(provider: { controller })
    }
}
/// End of preview tool

class DrawOnView: NSView {
    var alternateState: Bool = false {
        didSet { needsDisplay = true }
    }
    
    override func draw(_ rect: CGRect) {
        let leadingPoint = CGPoint(x: bounds.minX, y: alternateState ? bounds.minY : bounds.maxY)
        let trailingPoint = CGPoint(x: bounds.maxX, y: alternateState ? bounds.maxY : bounds.minY)
        
        let context = NSGraphicsContext.current?.cgContext
        context?.setLineWidth(2.0)
        context?.setStrokeColor(NSColor.blue.cgColor)
        context?.move(to: leadingPoint)
        context?.addLine(to: trailingPoint)
        context?.strokePath()
    }
}

class DrawOnViewController: NSViewController {
    var drawOnView: DrawOnView
    
    init() {
        self.drawOnView = .init()
        super.init(nibName: nil, bundle: nil)
        self.wrapAndInstall()
    }
    
    required init?(coder: NSCoder) {
        fatalError("init(coder:) has not been implemented")
    }
    
    @IBAction func toggleState(_ sender: Any?) {
        drawOnView.alternateState.toggle()
    }
    
    private func wrapAndInstall() {
        let button = NSButton(title: "Toggle", target: self, action: #selector(toggleState))
        button.setContentHuggingPriority(.defaultHigh, for: .horizontal)
        
        let stack = NSStackView(views: [button, drawOnView])
        stack.orientation = .vertical
        stack.spacing = 8
        
        view = stack
    }
}

struct SampleView_Previews: PreviewProvider {
    static var previews: some View {
        Preview(DrawOnViewController())
            .frame(width: 120, height: 140)
            .padding()
    }
}