Xamarin.Forms调用iOS原生控制器遇空值及导航创建咨询

Let's break down your two issues step by step to get things working smoothly:

Your initial code uses UIApplication.SharedApplication.KeyWindow, but this can return null on iOS 13+ due to the introduction of multi-scene support. Here's a more reliable approach, plus a clean way to integrate it with Xamarin.Forms:

Step 1: Use DependencyService to Encapsulate Native Code #

First, define a shared interface in your Xamarin.Forms project so you don't tie shared code to iOS specifics:

// Shared project
public interface IDocumentScannerService
{
    void LaunchScanner();
}

Step 2: Implement the Service in iOS #

In your iOS project, create the native implementation and find the correct top-level view controller (not just the root, since modals might be active):

[assembly: Dependency(typeof(DocumentScannerService))]
namespace YourAppNamespace.iOS
{
    public class DocumentScannerService : IDocumentScannerService
    {
        public void LaunchScanner()
        {
            // Get the active scene's key window (iOS 13+ compatible)
            var activeScene = UIApplication.SharedApplication.ConnectedScenes
                .OfType<UIWindowScene>()
                .FirstOrDefault(scene => scene.ActivationState == UISceneActivationState.ForegroundActive);
            
            var keyWindow = activeScene?.Windows.FirstOrDefault(w => w.IsKeyWindow);
            if (keyWindow == null) return;

            // Traverse to the top-most presented view controller
            var topViewController = keyWindow.RootViewController;
            while (topViewController.PresentedViewController != null)
            {
                topViewController = topViewController.PresentedViewController;
            }

            // Launch the scanner
            RGLDocReader.Shared.ShowScanner(topViewController, HandleScannerCompletion);
        }

        private void HandleScannerCompletion(RGLDocumentReaderResults results)
        {
            // Send results back to Xamarin.Forms (use MessagingCenter or an event)
            MessagingCenter.Send(this, "ScannerResultsReceived", results);
        }
    }
}

Step 3: Call from Xamarin.Forms #

In your Forms page, trigger the scanner with:

DependencyService.Get<IDocumentScannerService>().LaunchScanner();

If you're hitting null values when working with navigation controllers, here are the most common fixes:

• Check Your Navigation Controller Setup
  Make sure you're initializing the navigation controller properly and setting it as the window's root. A common mistake is setting a plain UIViewController as the root instead of wrapping it in a UINavigationController:

  // Correct setup in SceneDelegate/AppDelegate
  var mainVc = new YourMainViewController();
  var navController = new UINavigationController(mainVc);
  window.RootViewController = navController;
  window.MakeKeyAndVisible();
  

• Avoid Accessing ViewControllers Too Early
  Don't try to access navigation controllers or child view controllers in a UIViewController's constructor. Wait until ViewDidLoad or ViewWillAppear—this is when the view hierarchy is fully initialized:

  public override void ViewDidLoad()
  {
      base.ViewDidLoad();
      // Safe to access NavigationController or other view elements here
      Console.WriteLine($"Navigation Controller: {NavigationController}");
  }
  

• For Xamarin.Forms Integration
  If you're mixing Forms and native iOS navigation, get the native view controller corresponding to your current Forms page:

  var currentFormsPage = Application.Current.MainPage.Navigation.NavigationStack.LastOrDefault();
  var nativeViewController = currentFormsPage?.GetViewController();
  
  if (nativeViewController != null)
  {
      // Push a native view controller
      nativeViewController.NavigationController.PushViewController(new YourNativeDetailVc(), true);
  }
  

• Debug with Logs
  Add quick console logs to pinpoint where the null is coming from:

  Console.WriteLine($"Root VC: {window.RootViewController}");
  Console.WriteLine($"Nav Controller: {window.RootViewController?.NavigationController}");
  

内容的提问来源于stack exchange，提问作者brucaqs