How do I update a SwiftUI View in UIKit when value changes?

Question

I am trying to integrate a SwiftUI view that animates on changes to a @State variable (originally progress was @State private progress: CGFloat = 0.5 in the SwiftUI View), into an existing UIKit application. I have read through lots of search results on integrating SwiftUI into UIKit (@State, @Binding, @Environment, etc.), but can't seem to figure this out.

I create a very simple example of what I am attempting to do, as I believe once I see this answer I can adopt it to my existing application.

The Storyboard is simply View controller with a UISlider. The code below displays the SwiftUI view, but I want it to update as I move the UISlider.

import SwiftUI

class ViewController: UIViewController {

    var progress: CGFloat = 0.5

    override func viewDidLoad() {
        super.viewDidLoad()
        // Do any additional setup after loading the view.

        let frame = CGRect(x: 20, y: 200, width: 400, height: 400)

        let childView = UIHostingController(rootView: Animate_Trim(progress: progress))
        addChild(childView)
        childView.view.frame = frame
        view.addSubview(childView.view)
        childView.didMove(toParent: self)
    }

    @IBAction func sliderAction(_ sender: UISlider) {
        progress = CGFloat(sender.value)
        print("Progress: \(progress)")
    }

}

struct Animate_Trim: View {
    var progress: CGFloat

    var body: some View {
        VStack(spacing: 20) {

            Circle()
                .trim(from: 0, to: progress) // Animate trim
                .stroke(Color.blue,
                        style: StrokeStyle(lineWidth: 40,
                                           lineCap: CGLineCap.round))
                .frame(height: 300)
                .rotationEffect(.degrees(-90)) // Start from top
                .padding(40)
                .animation(.default)

            Spacer()

        }.font(.title)
    }
}```

Answer 1

The accepted answer actually doesn't answer the original question "update a SwiftUI View in UIKit..."?

IMHO, when you want to interact with UIKit you can use a notification to update the progress view:

extension Notification.Name {
  static var progress: Notification.Name { return .init("progress") }
}
class ViewController: UIViewController {
  var progress: CGFloat = 0.5 {
    didSet {
      let userinfo: [String: CGFloat] = ["progress": self.progress]
      NotificationCenter.default.post(Notification(name: .progress,
                                                   object: nil,
                                                   userInfo: userinfo))
    }
  }
  var slider: UISlider = UISlider()
  override func viewDidLoad() {
    super.viewDidLoad()
    slider.addTarget(self, action: #selector(sliderAction(_:)), for: .valueChanged)
    slider.frame = CGRect(x: 0, y: 500, width: 200, height: 50)
    // Do any additional setup after loading the view.

    let frame = CGRect(x: 20, y: 200, width: 400, height: 400)

    let childView = UIHostingController(rootView: Animate_Trim())
    addChild(childView)
    childView.view.frame = frame
    view.addSubview(childView.view)
    view.addSubview(slider)
    childView.didMove(toParent: self)
  }

  @IBAction func sliderAction(_ sender: UISlider) {
    progress = CGFloat(sender.value)
    print("Progress: \(progress)")
  }
}

struct Animate_Trim: View {
  @State var progress: CGFloat = 0
  var notificationChanged = NotificationCenter.default.publisher(for: .progress)
  var body: some View {
    VStack(spacing: 20) {
      Circle()
        .trim(from: 0, to: progress) // Animate trim
        .stroke(Color.blue,
                style: StrokeStyle(lineWidth: 40,
                                   lineCap: CGLineCap.round))
        .frame(height: 300)
        .rotationEffect(.degrees(-90)) // Start from top
        .padding(40)
        .animation(.default)
        .onReceive(notificationChanged) { note in
          self.progress = note.userInfo!["progress"]! as! CGFloat
      }
      Spacer()
    }.font(.title)
  }
}

Answer 2

If you do not want to use NotificationCenter. You could use just @Published and assign or sink.

I wrote a working example in a Playground to show the concept:

//This code runs on Xcode playground
import Combine
import SwiftUI

class ObservableSlider: ObservableObject {
    @Published public var value: Double = 0.0
}

class YourViewController {
    var observableSlider:ObservableSlider = ObservableSlider()
    private var cancellables: Set<AnyCancellable> = []
    let hostController = YourHostingController() // I put it here for the sake of the example, but you do need a reference to the Hosting Controller.

    init(){ // In a real VC this code would probably be on viewDidLoad

        let swiftUIView = hostController.rootView

        //This is where values of SwiftUI view and UIKit get glued together
        self.observableSlider.$value.assign(to: \.observableSlider.value, on: swiftUIView).store(in:&self.cancellables)
    }
    func updateSlider() {
        observableSlider.value = 8.5
    }
}

// In real app it would be something like:
//class YourHostingController<YourSwiftUIView> UIHostingController
class YourHostingController {
    var rootView = YourSwiftUIView()

//In a real Hosting controller you would do something like:
//    required init?(coder aDecoder: NSCoder){
//         super.init(coder: aDecoder, rootView: YourSwiftUIView())
//     }
}

struct YourSwiftUIView: View{
    var body: some View {
        EmptyView() // Your real SwiftUI body...
    }
    @ObservedObject var observableSlider: ObservableSlider = ObservableSlider()
    func showValue(){
        print(observableSlider.value)
    }
    init(){
        print(observableSlider.value)
    }
}

let yourVC = YourViewController() // Inits view and prints 0.0
yourVC.updateSlider() // Updates from UIKit to 8.5
yourVC.hostController.rootView.showValue() // Value in SwiftUI View is updated (prints 8.5)