SwiftUI: In SplitView, how can I detect if the master view is visible?

Question

When SwiftUI creates a SplitView, it adds a toolbar button that hides/shows the Master view. How can I detect this change so that I can resize the font in the detail screen and use all the space optimally?

I've tried using .onChange with geometry but can't seem to get that to work.

Answer 1

After thinkering for a while on this I got to this solution:

struct ContentView: View {
@State var isOpen = true

var body: some View {
    NavigationView {
        VStack{
            Text("Primary")
                .onUIKitAppear {
                    isOpen.toggle()
                }
                .onAppear{
                print("hello")
                    isOpen.toggle()
                }
                .onDisappear{
                    isOpen.toggle()
                    print("hello: bye")
                }
       .navigationTitle("options")
        }
        Text("Secondary").font(isOpen ? .body : .title)

    }.navigationViewStyle(.columns)
}


}

The onUIKitAppear is a custom extension suggested by apple to be only executed once the view has been presented to the user https://developer.apple.com/forums/thread/655338?page=2

  struct UIKitAppear: UIViewControllerRepresentable {
     let action: () -> Void
     
     func makeUIViewController(context: Context) -> UIAppearViewController {
        let vc = UIAppearViewController()
        vc.delegate = context.coordinator
        return vc
     }
     
     func makeCoordinator() -> Coordinator {
        Coordinator(action: self.action)
     }
     
     func updateUIViewController(_ controller: UIAppearViewController, context: Context) {}
     
     class Coordinator: ActionRepresentable {
        var action: () -> Void
        init(action: @escaping () -> Void) {
              self.action = action
        }
        func remoteAction() {
              action()
        }
     }
  }

  protocol ActionRepresentable: AnyObject {
     func remoteAction()
  }

  class UIAppearViewController: UIViewController {
     weak var delegate: ActionRepresentable?
     var savedView: UIView?
     
     override func viewDidLoad() {
        self.savedView = UILabel()
        
        if let _view = self.savedView {
              view.addSubview(_view)
        }
     }
     
     override func viewDidAppear(_ animated: Bool) {
        delegate?.remoteAction()
     }
     
     override func viewDidDisappear(_ animated: Bool) {
        view.removeFromSuperview()
        savedView?.removeFromSuperview()
     }
  }

  public extension View {
     
     func onUIKitAppear(_ perform: @escaping () -> Void) -> some View {
        self.background(UIKitAppear(action: perform))
     }
  }

Answer 2

If you're using iOS 16 you can use NavigationSplitView with NavigationSplitViewVisibility

Example:

struct MySplitView: View {
    @State private var columnVisibility: NavigationSplitViewVisibility = .all
    
    var bothAreShown: Bool { columnVisibility != .detailOnly }
        
    var body: some View {
        NavigationSplitView(columnVisibility: $columnVisibility) {
            Text("Master Column")
        } detail: {
            Text("Detail Column")
            Text(bothAreShown ? "Both are shown" : "Just detail shown")
        }
    }
}