Why do I get an error when setting rootViewController to UIHostingController in the SwiftUI Lifecycle?

Question

I am trying to set the status bar style within the latest SwiftUI Lifecycle. Here's my code that performs the logic to do this, adapted from here.

private class HostingController<Content: View>: UIHostingController<Content> {
  override var preferredStatusBarStyle: UIStatusBarStyle {
    return UIApplication.statusBarStyleHierarchy.last ?? UIApplication.defaultStatusBarStyle
  }
}

/// By wrapping views in a StatusBarControllerView, they will become the app's main / primary view.
/// This will enable setting the statusBarStyle.
struct StatusBarControllerView<Content: View>: View {
  var content: Content

  init(@ViewBuilder content: () -> (Content)) {
    self.content = content()
  }

  var body:some View {
    EmptyView()
      .onAppear {
        UIApplication.shared.setHostingController(rootView: AnyView(content))
      }
  }
}

extension View {
  /// Sets the status bar style color for this view.
  func statusBar(style: UIStatusBarStyle) -> some View {
    UIApplication.statusBarStyleHierarchy.append(style)

    return self
      // Once this view appears, set the style to the new style.
      .onAppear {
        UIApplication.hostingController?.setNeedsStatusBarAppearanceUpdate()
      }
      // Once it disappears, set it to the previous style.
      .onDisappear {
        UIApplication.statusBarStyleHierarchy.removeLast()
        UIApplication.hostingController?.setNeedsStatusBarAppearanceUpdate()
    }
  }
}

private extension UIApplication {
  static var hostingController: HostingController<AnyView>?

  static var statusBarStyleHierarchy: [UIStatusBarStyle] = []
  static let defaultStatusBarStyle: UIStatusBarStyle = .lightContent

  /// Sets the App to start at rootView
  func setHostingController(rootView: AnyView) {
    let hostingController = HostingController(rootView: AnyView(rootView))
    windows.first?.rootViewController = hostingController
    UIApplication.hostingController = hostingController
  }
}

I then set up the StatusBarControllerView by placing it in my root App, as shown below:

@main
struct App: SwiftUI.App {
  var body: some Scene {
    WindowGroup {
      StatusBarControllerView {
        Text("Content goes here")
      }
    }
  }
}

While it's working great visually, I am now getting the following error in the console:

Unbalanced calls to begin/end appearance transitions for <_TtGC7SwiftUI19UIHostingControllerGVS_15ModifiedContentVS_7AnyViewVS_12RootModifier__: 0x13ee0d9d0>.

Does anyone know how to modify the provided code to avoid this error message? The problem appears to occur when setting the root view controller in private extension UIApplication: windows.first?.rootViewController = hostingController

Clarification 1/3/2021

The reason I am trying to change the status bar color this way instead of by editing the plist is because I would like to be able to change the status bar color dynamically in any view of my app, rather than app-wide, as shown below:

struct MyView: View {
  var body: some View {
    MySubview()
      .statusBar(style: .darkContent)
  }
}

Answer 1

Before diving into the weeds, I’ve seen some answers that lead me to believe this isn’t necessary, and can be solved by changing your plist: here and here.

Assuming you do want a hosting controller, I believe the issue is that you are setting the view two different ways. One is the empty view that’s being called from the app body (through StatusBarControllerView), and the other is the hosting UIView that is being assigned control through the window rootViewController. This can be demonstrated by changing out EmptyView() with some visible view; it doesn’t appear, because the HostingController is replacing it. Swift is able to replace it, but throws the error that you’re seeing to let you know that this is happening. If you do want the HostingController to be the main view, you could set it directly by changing StatusBarControllerView to a UIViewRepresentable:

struct StatusBarControllerView<Content: View>: UIViewRepresentable {
    var content: () -> (Content)
    
    func makeUIView(context: Context) -> some UIView {
        UIApplication.shared.setHostingController(rootView: AnyView(content()))
        return UIApplication.hostingController?.view ?? UIView()
    }
    
    func updateUIView(_ uiView: UIViewType, context: Context) {}
}

And removing the line windows.first?.rootViewController = hostingController so that it’s only set once.

You could also set the HostingController up in the scene delegate, based on this answer. However, that would require adding a scene delegate and app delegate to your project (here’s a decent tutorial on that).