ViewExtensions.OnUnloaded causes app crash if the view is already disposed, this happens for web view in Blazor Hybrid apps

[alexyakunin]

Description

We frequently see production app crashes associated the following stack trace in our Blazor Hybrid app for Android:

Exception android.runtime.JavaProxyThrowable: [System.ObjectDisposedException]: ObjectDisposed_Generic
ObjectDisposed_ObjectName_Name, Microsoft.AspNetCore.Components.WebView.Maui.BlazorAndroidWebView
  at Java.Interop.JniPeerMembers.AssertSelf (Unknown Source)
  at Java.Interop.JniPeerMembers+JniInstanceMethods.InvokeVirtualVoidMethod (Unknown Source)
  at Android.Views.View.RemoveOnAttachStateChangeListener (Unknown Source)
  at Android.Views.View+<>c__DisplayClass3033_0.<remove_ViewDetachedFromWindow>b__0 (Unknown Source)
  at Java.Interop.EventHelper.RemoveEventHandler (Unknown Source)
  at Android.Views.View.remove_ViewDetachedFromWindow (Unknown Source)
  at Microsoft.Maui.Platform.ViewExtensions+<>c__DisplayClass43_0.<OnUnloaded>b__1 (Unknown Source)
  at Microsoft.Maui.Platform.ActionDisposable.Dispose (Unknown Source)
  at Microsoft.Maui.Platform.ViewExtensions+<>c__DisplayClass43_0.<OnUnloaded>b__3 (Unknown Source)
  at Java.Lang.Thread+RunnableImplementor.Run (Unknown Source)
  at Java.Lang.IRunnableInvoker.n_Run (Unknown Source)
  at Android.Runtime.JNINativeWrapper.Wrap_JniMarshal_PP_V + 0x5 (Unknown Source)
  at mono.java.lang.RunnableImplementor.n_run (RunnableImplementor.java)
  at mono.java.lang.RunnableImplementor.run (RunnableImplementor.java:29)
  at android.os.Handler.handleCallback (Handler.java:1000)
  at android.os.Handler.dispatchMessage (Handler.java:104)
  at android.os.Looper.loopOnce (Looper.java:242)
  at android.os.Looper.loop (Looper.java:362)
  at android.app.ActivityThread.main (ActivityThread.java:8393)
  at java.lang.reflect.Method.invoke
  at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run (RuntimeInit.java:552)
  at com.android.internal.os.ZygoteInit.main (ZygoteInit.java:992)

As you may notice, there are no own methods in this stack trace.

As far as I can judge, it's an attempt to remove native event handler on already disposed native web view triggered by the following code in ViewExtensions.OnUnloaded(...):

EventHandler<AView.ViewDetachedFromWindowEventArgs>? routedEventHandler = null;
ActionDisposable? disposable = new ActionDisposable(() =>
{
	if (routedEventHandler != null)
		view.ViewDetachedFromWindow -= routedEventHandler; // HERE
});
routedEventHandler = (_, __) => ...;

There is a number of ways to address it, but even suppression of an exception for view.ViewDetachedFromWindow -= ... would be fine for the logic in OnUnloaded.

We couldn't find a workaround for this specific issue: no own methods on call stack in this case + it seems pretty hard to replace internal BlazorAndroidWebView with our own implementation. So I'd appreciate any suggestions for Blazor Hybrid apps here in the meantime, i.e. until the moment this issue is fixed.

Steps to Reproduce

Sorry, no exact steps - the issue is happening fairly rarely in production, but frequently enough to be our #1 app crash cause.

Link to public reproduction project repository

No response

Version with bug

9.0.40 SR4

Is this a regression from previous behavior?

Not sure, did not test other versions

Last version that worked well

No response

Affected platforms

Android

Affected platform versions

No response

Did you find any workaround?

No.

Relevant log output

[jfversluis]

I understand you're saying that you don't know exactly what causes this and all you have is this to go on, but that makes it very hard for us to look into. We don't know anything about your codebase. So without any relevant code or a reproduction this will be very hard, if not impossible to diagnose. At least not without spending a lot of time on this which is not something we can just do.

Please keep investigating and if you have anything more concrete to share, add it here or open something new when this gets closed. Thanks for understanding!

[javiercn]

@jfversluis I've untagged Blazor as this seems to be related to the webview itself and not something Blazor specific.

[alexyakunin]

I understand you're saying that you don't know exactly what causes this and all you have is this to go on, but that makes it very hard for us to look into. We don't know anything about your codebase. So without any relevant code or a reproduction this will be very hard, if not impossible to diagnose. At least not without spending a lot of time on this which is not something we can just do.

Please keep investigating and if you have anything more concrete to share, add it here or open something new when this gets closed. Thanks for understanding!

Okey, not sure if I am fully clear. Here is what I am saying:

EventHandler<AView.ViewDetachedFromWindowEventArgs>? routedEventHandler = null;
ActionDisposable? disposable = new ActionDisposable(() =>
{
	if (routedEventHandler != null)
		view.ViewDetachedFromWindow -= routedEventHandler; // The exception crashing the app can be thrown here on WebView disposal / GC
});
routedEventHandler = (_, __) => ...;

And the fix like this would be fine:

EventHandler<AView.ViewDetachedFromWindowEventArgs>? routedEventHandler = null;
ActionDisposable? disposable = new ActionDisposable(() =>
{
	if (routedEventHandler != null) 
	try {
		view.ViewDetachedFromWindow -= routedEventHandler; 
	}
	catch (ObjectDisposedException) {
		// It's ok to suppress it: view could be disposed at this point
	}
});
routedEventHandler = (_, __) => ...;

More preferrable fix would be, of course, to suppress this exception in any of such scenarios (i.e. when an event handler is removed from a disconnected .NET proxy of Java object).

[alexyakunin]

As for the repro, unfortunately I don't have time for this. As I said, this issue is fairly frequent in Google Play Store crash logs, but reproducing it reliably may require unpredictable amount of time.

[dhindrik]

We also see this. We get this error:
System.AggregateException - TaskExceptionHolder_UnhandledException (ObjectDisposed_Generic ObjectDisposed_ObjectName_Name, Microsoft.AspNetCore.Components.WebView.Maui.BlazorAndroidWebView)

Unfortunately, I don't have a stacktrace. But it only happens for users from Norway. The users has Android 9, 10 and 14. And it only happens on tablets.

[mfeingol]

Seeing this as well.

Like @alexyakunin I'm only seeing the unhandled ObjectDisposedException in crash reports and do not have a live repro:

void JniPeerMembers.AssertSelf(IJavaPeerable)
void JniInstanceMethods.InvokeVirtualVoidMethod(string, IJavaPeerable, JniArgumentValue*)
Called from: void View.RemoveOnAttachStateChangeListener(IOnAttachStateChangeListener)
void EventHelper.RemoveEventHandler<IOnAttachStateChangeListener, IOnAttachStateChangeListenerImplementor>(ref WeakReference, Func<IOnAttachStateChangeListenerImplementor, bool>, Action<IOnAttachStateChangeListener>, Action<IOnAttachStateChangeListenerImplementor>)
Called from: void View.remove_ViewDetachedFromWindow(EventHandler<ViewDetachedFromWindowEventArgs>)
void RunnableImplementor.Run()
void IRunnableInvoker.n_Run(IntPtr jnienv, IntPtr native__this)
Called from: void JNINativeWrapper.Wrap_JniMarshal_PP_V(_JniMarshal_PP_V callback, IntPtr jnienv, IntPtr klazz)