What is NSViewControllerRepresentable

In SwiftUI, there are no View Controllers, only Views. So what should we do if want to use something related to View Controllers? Here comes NSViewControllerRepresentable/UIViewControllerRepresentable. Basically, we can use instance of View Controller Representable as View, contains a view controller.

Since view controllers have their own view, ViewControllerRepresentable has not view body.

protocol NSViewControllerRepresentable : View where Self.Body == Never

A Simple Sample of NSViewControllerRepresentable

1. Create a new SwiftUI app.
2. Add a new file, choose Cocoa Class, name it as TextViewController.swift.
   Choose create xib file as well.
3. Add a close button and label.

4. TextViewController.swift

//
//  TextViewController.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import Cocoa

class TextViewController: NSViewController {
    @IBOutlet weak var label: NSTextField!
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func closeButtonClicked(_ sender: Any) {
        self.dismiss(sender)
    }
}

5. TextView

//
//  TextView.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import AppKit
import SwiftUI

struct TextView:NSViewControllerRepresentable {
    func makeNSViewController(context: Context) -> TextViewController {
        let controller  = TextViewController()        
        return controller
    }    
    
    func updateNSViewController(_ nsViewController: TextViewController, context: Context) {

    }
}

6. ContentView.swift

//
//  ContentView.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import SwiftUI

struct ContentView: View {
    @State var title = ""
    @State var showTextView = false
    
    var body: some View {
        HStack {
            TextField("Title", text: $title)
        
            Button("Show Text View") {
                showTextView = true
            }
            .sheet(isPresented: $showTextView) {
                TextView()
            }
        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

The app ran as below, but the close button didn't work.

Coordinator

View Controllers are class, and View Representables are structs. We use Coordinator to pass changes to SwiftUI.

1. TextView

//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import AppKit
import SwiftUI

struct TextView:NSViewControllerRepresentable {
    @Environment(\.presentationMode) var presentationMode
    @Binding var title:String
    
    class Coordinator: NSObject, TextViewControllerDelegate {
        var parent: TextView

        init(_ parent: TextView) {
            self.parent = parent
        }
        
        func textView(_ tvc: TextViewController) {
            self.parent.presentationMode.wrappedValue.dismiss()
        }
    }
    
    func makeNSViewController(context: Context) -> TextViewController {
        let controller  = TextViewController()
        controller.delegate = context.coordinator
        
        return controller
    }
    
    func updateNSViewController(_ nsViewController: TextViewController, context: Context) {
        nsViewController.label.stringValue = title
    }
    
    func makeCoordinator() -> Coordinator {
        Coordinator(self)
    }
}

2. TextViewController

//
//  TextViewController.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import Cocoa

class TextViewController: NSViewController {
    @IBOutlet weak var label: NSTextField!
    weak var delegate:TextViewControllerDelegate?
    
    override func viewDidLoad() {
        super.viewDidLoad()
    }
    
    @IBAction func closeButtonClicked(_ sender: Any) {
        delegate?.textView(self)
    }
}

protocol TextViewControllerDelegate:AnyObject {
    func textView(_ textView:TextViewController)
}

3. ContentView

//
//  ContentView.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import SwiftUI

struct ContentView: View {
    @State var title = ""
    @State var showTextView = false
    
    var body: some View {
        HStack {
            TextField("Title", text: $title)
        
            Button("Show Text View") {
                showTextView = true
            }
            .sheet(isPresented: $showTextView) {
                TextView(title: $title)
            }
        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}//
//  ContentView.swift
//  ViewControllerRepresentable Test
//
//  Created by zhaoxin on 2022/6/12.
//

import SwiftUI

struct ContentView: View {
    @State var title = ""
    @State var showTextView = false
    
    var body: some View {
        HStack {
            TextField("Title", text: $title)
        
            Button("Show Text View") {
                showTextView = true
            }
            .sheet(isPresented: $showTextView) {
                TextView(title: $title)
            }
        }
        .padding()
    }
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView()
    }
}

App ran like this and the close button worked.

Update Variables of NSViewController

You may aware that there was a func

func updateNSViewController(_ nsViewController: TextViewController, context: Context) {
    nsViewController.label.stringValue = title
}

We couldn't call nsViewController.label.stringValue = title in makeNSViewController, as in the make function the View Controller was created, but the xib was not loaded. We can think this update as viewDidLoad function in View Controller.

Resources

• Above Code Sample
• Wrapping a UIViewController in a SwiftUI view
• Using coordinators to manage SwiftUI view controllers

When developing with Cocoa/UIKit, the most common used architecture is MVC.

• Model-View-Controller

What is the architecture when using SwiftUI? Someone says it is MVVM. Someone says it is MV, MVC without C. I would like say it would be M-VC, Model with View and Controller.

• Stop using MVVM for SwiftUI

M-VC

We want every element to be simple. For MV architecture, the View is too heavy for me. For M-VC, we want the View as simple as possible. The extra parts we think it to be Controller.

Split View into small Views

For a SwiftUI View, when one of the @State properties changed, all @State properties in the same view will be updated. If this is overhead, you can split those @State properties to another view, that will reduce CPU usage and speed up your app.

• Why Timer Change Will Trigger LazyVGrid View Update?
• Demystify SwiftUI

After investigation, I found that this issue was caused by sorted method with keyPath.

List { // this list crashes
    ForEach(dayList.items.sorted(by: \.startDate, ascending: true)) { currentItem in
        ItemRowSwiftUIView(currentItem: currentItem)
    }.onDelete(perform: { indexSet in
        removeItems(atOffsets: indexSet)
    })
}
    
List { // this list doesn't crash
    ForEach(dayList.items.sorted(by: {
        $0.startDate < $1.startDate
    })) { currentItem in
        ItemRowSwiftUIView(currentItem: currentItem)
    }.onDelete(perform: { indexSet in
        removeItems(atOffsets: indexSet)
    })
}

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(dayList: DayList())
    }
}

When using the keyPath to sort, the preview crashed. That because sorted by keyPath used realm and for a unmanaged object dayList, there is no realm.

So we need to create a realm first.

struct ContentView_Previews: PreviewProvider {
    static var previews: some View {
        ContentView(dayList: createRealmAndGetDayList())
    }

    static func createRealmAndGetDayList() -> DayList {
        let realm = try! Realm(configuration: Realm.Configuration(inMemoryIdentifier: "for debug"))
        let dayList = DayList()
        
        try! realm.write({
            realm.add(dayList, update: .all)
        })

        return dayList
    }
}

Then everything will work.