[Bug] [iOS] [Shell] "Nav Stack consistency error"

[dmariogatto]

Description

I am getting an exception report in AppCenter, "Nav Stack consistency error". Occurring in all versions of XForms (including nightly, was hoping #8135 might have helped but no effect).

Multiple versions of iOS (13, 12 & 11), affecting about ~5% of users. I have not been able to replicate and have no way to reach out to users to get retry steps.

IShellSectionController.SendPopping (Xamarin.Forms.Page page)
ShellSectionRenderer.SendPoppedOnCompletion (System.Threading.Tasks.Task popTask)
AsyncMethodBuilderCore+<>c.<ThrowAsync>b__7_0 (System.Object state)
NSAsyncSynchronizationContextDispatcher.Apply ()
(wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate)
UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName)
Application.Main (System.String[] args)

Steps to Reproduce

I have a TabBar setup in my AppShell. Three tabs with only a single content page as the child. In the Shell's constructor I also register some modal routes, which I navigate to using Shell.GoToAsync(). I do not trigger navigation in any other way, nor do I pop anything off the stack manually.

Expected Behavior

Obviously I could be at fault here, but it would be helpful if the exception contained some more information to aid in debugging. Possibly the name of the page it's trying to pop (when available) and the current state of the navigation stack.

Actual Behavior

Generic exception, which is difficult to debug/troubleshoot.

Basic Information

• Version with issue: All (including nightly)

[roycornelissen]

I got one too yesterday while debugging my app. I think it happened after changing the orientation of my device to landscape and back to portrait, but I haven't been able to reproduce it.

[dmariogatto]

At least it's not just me 😊. My app is portrait only (no rotation enabled).

[karlsson-mvm]

Same issue for me in an unpredictable way, but when it occours is during Popping to AppShell (Tabbed Page) event, from a page where Shell.TabBarIsVisible="False"

[dfandler]

I have the same issue, 100 % reproduceable with following stacktrace.
My setup is following:
Shell with many ShellItems, each with only 1 childView
User takes a picture ->
opens a custom CameraView takes picture (modal push)
opens Custom Displayview (another modal push)
closes both pages ( 2x popasync)
crashes with exception.

I have to put a small delay between the popasyncs (200 ms) or the app will also crash #6484

System.Exception: Nav Stack consistency error
at Xamarin.Forms.ShellSection.Xamarin.Forms.IShellSectionController.SendPopping (Xamarin.Forms.Page page) [0x0000e] in D:\a\1\s\Xamarin.Forms.Core\Shell\ShellSection.cs:137
at Xamarin.Forms.Platform.iOS.ShellSectionRenderer.SendPoppedOnCompletion (System.Threading.Tasks.Task popTask) [0x0004f] in D:\a\1\s\Xamarin.Forms.Platform.iOS\Renderers\ShellSectionRenderer.cs:404
at System.Runtime.CompilerServices.AsyncMethodBuilderCore+<>c.b__7_0 (System.Object state) [0x00000] in /Users/builder/jenkins/workspace/xamarin-macios/xamarin-macios/external/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/AsyncMethodBuilder.cs:1021
at Foundation.NSAsyncSynchronizationContextDispatcher.Apply () [0x00000] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.6.0.12/src/Xamarin.iOS/Foundation/NSAction.cs:178
at (wrapper managed-to-native) UIKit.UIApplication.UIApplicationMain(int,string[],intptr,intptr)
at UIKit.UIApplication.Main (System.String[] args, System.IntPtr principal, System.IntPtr delegate) [0x00005] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.6.0.12/src/Xamarin.iOS/UIKit/UIApplication.cs:86
at UIKit.UIApplication.Main (System.String[] args, System.String principalClassName, System.String delegateClassName) [0x0000e] in /Library/Frameworks/Xamarin.iOS.framework/Versions/13.6.0.12/src/Xamarin.iOS/UIKit/UIApplication.cs:65

[simonemarra]

I've got the same exception in my app when user starts a "back" swipe from a page, but do not complete the movement.

I don't know how to solve it since I have no control on such navigation (or at least it is what I think...)

[rs-mobitech]

I am having the same problem now. I anyone looking at this? My app keeps crashing.

[dmariogatto]

Is anyone able to create a small reproducible example?

In theory, a possible fix would be changing,

void IShellSectionController.SendPopping(Page page)
{
	if (_navStack.Count <= 1)
		throw new Exception("Nav Stack consistency error");
        
	_navStack.Remove(page);
	SendAppearanceChanged();
}

to,

void IShellSectionController.SendPopping(Page page)
{
	if (_navStack.Contains(page))
		_navStack.Remove(page);

        SendAppearanceChanged();			
}

But without something reproducible it's a stab in the dark and could cause its own issues.

[TimBoccaleri]

I get this same issue when I try to swipe back from a detail page but stop in the middle of the swipe then I press the back button. It works fine on XF4.2.0.70949 but does not work on XF4.3.0.991221. I have not tried any other versions.

I have attached two GIFs. One with it working and another with it crashing.

Working

Crashing

[dmariogatto]

Thanks @TimBarham!

Looks like PopViewController is called on the start of the back swipe gesture, even if it is cancelled. This removes the page from the Shell's navigation stack, but it still exists for the UINavigationController.

[simonemarra]

I get this same issue when I try to swipe back from a detail page but stop in the middle of the swipe then I press the back button. It works fine on XF4.2.0.70949 but does not work on XF4.3.0.991221. I have not tried any other versions.

This is exactly the same problem with my app.