kobbok
commented
4 months ago I'm experiencing the same problem, for me it's happening with a CollectionView when scrolling it up and down repeatedly.
It seems the InnerGestureListener gets disposed, then the internal InnerGestureListener constructor gets called, judging from the comment above that function the gesture listener is still required by the App but since it was disposed the _dragAndDropGestureListener is now null causing the NullReferenceException.
It looks like the void GestureDetector.IOnGestureListener.OnLongPress(MotionEvent e) function in Xamarin.Forms.Platform.Android\InnerGestureListener.cs is missing a disposed check. Something like the following should fix the problem:
void GestureDetector.IOnGestureListener.OnLongPress(MotionEvent e)
{
+ if (_disposed)
+ return;
+
SetStartingPosition(e);
_dragAndDropGestureHandler.OnLongPress(e);
}Edit: Learned that CollectionView and DragGestureRecognizer and DropGestureRecognizer are way harder to combine than seems at first glance. Fixed my problem by using https://github.com/billvenhaus/ReorderableCollectionView.Forms amazing package
Sadly this probably doesn't fix the problem for OP
Description
DragGestureRecognizer is disposed before long press is processed
System.NullReferenceException: Object reference not set to an instance of an object. IOnGestureListener.OnLongPress (Android.Views.MotionEvent e) D:\a\1\s\Xamarin.Forms.Platform.Android\InnerGestureListener.cs:120
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``` Microsoft Visual Studio Professional 2022 Version 17.5.1 VisualStudio.17.Release/17.5.1+33424.131 Microsoft .NET Framework Version 4.8.04084 Installed Version: Professional Visual C++ 2022 00483-10700-27530-AA705 Microsoft Visual C++ 2022 C# Tools 4.5.0-6.23123.11+271ccd71554f7d28d2f90551aafd0bdeb5d327aa C# components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used. Microsoft JVM Debugger 1.0 Provides support for connecting the Visual Studio debugger to JDWP compatible Java Virtual Machines Mono Debugging for Visual Studio 17.5.9 (11975e6) Support for debugging Mono processes with Visual Studio. NuGet Package Manager 6.5.0 NuGet Package Manager in Visual Studio. For more information about NuGet, visit https://docs.nuget.org/ Open Command Line 2.5.238 2.5.238 Visual C++ for Cross Platform Mobile Development (Android) 17.0.33312.129 Visual C++ for Cross Platform Mobile Development (Android) Xamarin 17.5.0.173 (d17-5@33e727c) Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android. Xamarin Designer 17.5.3.46 (remotes/origin/d17-5@e4dd80b2bb) Visual Studio extension to enable Xamarin Designer tools in Visual Studio. Xamarin Templates 17.5.41 (ba80d05) Templates for building iOS, Android, and Windows apps with Xamarin and Xamarin.Forms. Xamarin.Android SDK 13.2.0.0 (d17-5/797e2e1) Xamarin.Android Reference Assemblies and MSBuild support. Mono: 6dd9def Java.Interop: xamarin/java.interop/main@149d70fe SQLite: xamarin/sqlite/3.40.0@fdc1e34 Xamarin.Android Tools: xamarin/xamarin-android-tools/main@9f02d77 Xamarin.iOS and Xamarin.Mac SDK 16.2.0.5 (7738c90c9) Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support. ```Build Logs
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Workaround