Screen Saver in SwiftUI - simple colour rotation

SSaver.swift

//
import SwiftUI
import ScreenSaver

struct Defaults {
   /// Use bundleID for the Module name. It is found in the project properties, of target, General Tab.
   let storage = ScreenSaverDefaults(forModuleWithName: "com.ajc.SSaver")!
   let refreshRateKey = "refreshRate"
   var refreshRate: Double { storage.double(forKey: refreshRateKey) }

   func saveRefreshRate(value: Double) -> Bool {
      storage.set(value, forKey: refreshRateKey)
      return refreshRate == value ? true : false
   }
   func setDefault(refreshRate: Double) -> Bool {
      guard self.refreshRate == 0 else { return true }
      storage.register(defaults: [refreshRateKey: refreshRate])
      return refreshRate == self.refreshRate ? true : false
   }
   static var shared = Defaults()
}

class SSaver: ScreenSaverView {
   var viewModel: ViewModel         /// An observable object for sharing with SwiftUI
   var preferencesSheet: NSWindow   /// Maintain reference to the screensaver Preferences sheet

   required init?(coder: NSCoder) { fatalError("Coder not implemented") }
   override init?(frame: NSRect, isPreview: Bool) {
      preferencesSheet = NSWindow()
      viewModel = ViewModel(defaults: Defaults.shared )
      super.init(frame: frame, isPreview: isPreview)
      setup()
   }
   /// Update screensaver state, and request redraw when needed
   override func animateOneFrame() {
      viewModel.animationTick()
      needsDisplay = true
   }
   /// Called just after screen is drawn
   //   override func startAnimation() { super.startAnimation() }
   /// Stops animation - also called when showing  Preferences
   //   override func stopAnimation() { super.stopAnimation() }
   override var hasConfigureSheet: Bool { true }
   override var configureSheet: NSWindow? { return preferencesSheet }
}

extension SSaver {

   func setup() {
      animationTimeInterval = 1/30.0
      // Previews don't animate by default, so kickstart it
      if isPreview {
         DispatchQueue.main.asyncAfter(deadline: .now() + 0.5, execute: startAnimation)
      }
      // Attach Preferences SwiftUI view to a NSWindow (preferencesSheet)
      let preferencesView = NSHostingView(rootView:
         Preferences(viewModel: viewModel) { // pass a function to SwiftUI to allow it to close the Preferences sheet
            self.window!.endSheet(self.preferencesSheet) } )
      preferencesView.frame = bounds

      preferencesSheet.contentView = preferencesView

      let saverView = NSHostingView(rootView: Saver(viewModel: viewModel))
      saverView.frame = bounds
      /// setting to window?.contentView doesnt work at all. If set in startAnimation, the view appears, but does not animate
      addSubview(saverView)
   }
}
// SwiftUI from here on out :-
@MainActor
class ViewModel: ObservableObject {

   @Published var defaults: Defaults
   @Published var viewColors: RotatingColors
   var lastUpdate = Date.now

   func animationTick() {
      if Date.now > lastUpdate + defaults.refreshRate {
         lastUpdate = Date.now
         viewColors.next()
      }
   }
   init(defaults: Defaults) {
      self.defaults = defaults
      viewColors = RotatingColors([.red, .orange, .yellow, .green, .blue, .indigo, .purple] )
      _ = defaults.setDefault(refreshRate: 4.0)
   }
}

struct RotatingColors { // Initialise with array of colors, and the getColor function will rotate through them every "refresh" seconds.
   private let colours: Array<Color>
   private var index: Array<Color>.Index
   private var nextIndex: Array<Color>.Index = 0
   init(_ colours: [Color]) {
      self.colours = colours.count == 0 ? [.black] : colours
      index = self.colours.startIndex
      nextIndex = getNextIndex()
   }
   var color: Color { colours[index] }
   var nextColor: Color { colours[nextIndex]}
   private func getNextIndex() -> Array<Color>.Index {
      let n = colours.index(after: index)
      return n == colours.endIndex ? colours.startIndex : n
   }
   mutating func next()  {
      let i = nextIndex;
      nextIndex = getNextIndex()
      index = i
   }
}

struct Saver: View {
   @ObservedObject var viewModel: ViewModel
   var body: some View {
(color: viewModel.viewColors.nextColor, location: location) ])
      ZStack(alignment: .bottomLeading) {
         viewModel.viewColors.color
            .animation(.linear(duration: viewModel.defaults.refreshRate), value: viewModel.viewColors.color)
         Text("\(Int(viewModel.defaults.refreshRate))")
            .foregroundColor(viewModel.viewColors.nextColor)
      }
   }
}
struct Preferences: View {
   @ObservedObject var viewModel: ViewModel
   var close: () -> Void
   @State private var refreshRate = -2.0
   var body: some View {
      VStack {
         HStack(alignment: .center) {
            Text("Refresh colour rate: \(Int(refreshRate))").padding()
            Slider(value: $refreshRate, in: 1...20) }
         .frame(width: 300)
         Divider().frame(width: 300)
         Spacer()
         HStack {
            Button("Cancel") { close() }
            Button("OK") { saveDefault(); close() }.buttonStyle(.borderedProminent) }
      }
      .padding()
      .onAppear { loadSavedDefault() }
      .frame(minWidth: 300, idealWidth: 400, maxWidth: .infinity,  minHeight: 200, idealHeight: 210, maxHeight: .infinity)
      .fixedSize()
   }

   func loadSavedDefault() {
      self.refreshRate = viewModel.defaults.refreshRate
   }

   func saveDefault() {
      let value = Double(Int(self.refreshRate))
      _ = viewModel.defaults.saveRefreshRate(value: value)
   }
}