NavigationView / ItemsRepeater Crash with FluentAvalonia 2.1.0-preview4

[robloo]

Describe the bug

The preview NavigationView crashes in the ItemsRepeater ItemsRepeater.OnElementPrepared / ViewportManager.OnElementPrepared code. This was previously working in preview2 and earlier versions of FluentAvalonia 2.0+.

Screenshots

Desktop/Platform (please complete the following information):

• OS: Windows 10 Pro
• FluentAvalonia Version 2.1.0-preview4
• Avalonia Version 11.1.0-beta1

Additional context

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=FluentAvalonia
  StackTrace:

 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ViewportManager.OnElementPrepared(Avalonia.Controls.Control element, FluentAvalonia.UI.Controls.VirtualizationInfo vInfo)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ItemsRepeater.OnElementPrepared(Avalonia.Controls.Control element, int index, FluentAvalonia.UI.Controls.VirtualizationInfo vInfo)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ViewManager.GetElementFromElementFactory(int index)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ViewManager.GetElement(int index, bool forceCreate, bool suppressAutoRecycle)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ItemsRepeater.GetElementImpl(int index, bool forceCreate, bool suppressAutoRecycle)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.RepeaterLayoutContext.GetOrCreateElementAtCore(int index, FluentAvalonia.UI.Controls.ElementRealizationOptions options)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.VirtualizingLayoutContext.GetOrCreateElementAt(int index, FluentAvalonia.UI.Controls.ElementRealizationOptions options)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.FlowLayoutAlgorithm.GetAnchorIndex(Avalonia.Size availableSize, bool isWrapping, double minItemSpacing, bool disableVirtualization, string layoutId)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.FlowLayoutAlgorithm.Measure(Avalonia.Size availableSize, FluentAvalonia.UI.Controls.VirtualizingLayoutContext context, bool isWrapping, double minItemSpacing, double lineSpacing, int maxItemsPerLine, FluentAvalonia.UI.Controls.ScrollOrientation orientation, bool disableVirtualization, string layoutId)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.StackLayout.MeasureOverride(FluentAvalonia.UI.Controls.VirtualizingLayoutContext context, Avalonia.Size availableSize)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.Layout.Measure(FluentAvalonia.UI.Controls.LayoutContext context, Avalonia.Size availableSize)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.ItemsRepeater.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Grid.MeasureCell(int cell, bool forceInfinityV)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Grid.MeasureCellsGroup(int cellsHead, Avalonia.Size referenceSize, bool ignoreDesiredSizeU, bool forceInfinityV, out bool hasDesiredSizeUChanged)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Grid.MeasureCellsGroup(int cellsHead, Avalonia.Size referenceSize, bool ignoreDesiredSizeU, bool forceInfinityV)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Grid.MeasureOverride(Avalonia.Size constraint)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.LayoutHelper.MeasureChild(Avalonia.Layout.Layoutable control, Avalonia.Size availableSize, Avalonia.Thickness padding)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.NavigationView.MeasureTopNavigationViewDesiredWidth(Avalonia.Size availableSize)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.NavigationView.HandleTopNavigationMeasureOverrideNormal(Avalonia.Size availableSize)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.NavigationView.HandleTopNavigationMeasureOverride(Avalonia.Size availableSize)	Unknown
 	FluentAvalonia.dll!FluentAvalonia.UI.Controls.NavigationView.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Grid.MeasureOverride(Avalonia.Size constraint)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.LayoutHelper.MeasureChild(Avalonia.Layout.Layoutable control, Avalonia.Size availableSize, Avalonia.Thickness padding, Avalonia.Thickness borderThickness)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Presenters.ContentPresenter.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.LayoutHelper.MeasureChild(Avalonia.Layout.Layoutable control, Avalonia.Size availableSize, Avalonia.Thickness padding)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Decorator.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Primitives.VisualLayerManager.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Window.MeasureOverride(Avalonia.Size availableSize)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.WindowBase.MeasureCore(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.Layoutable.Measure(Avalonia.Size availableSize)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.LayoutManager.Measure(Avalonia.Layout.Layoutable control)	Unknown
 	Avalonia.Base.dll!Avalonia.Layout.LayoutManager.ExecuteInitialLayoutPass()	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Window.ShowCore(Avalonia.Controls.Window owner)	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.Window.Show()	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.ShowMainWindow()	Unknown
 	Avalonia.Controls.dll!Avalonia.Controls.ApplicationLifetimes.ClassicDesktopStyleApplicationLifetime.Start(string[] args)	Unknown
 	Avalonia.Controls.dll!Avalonia.ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime(Avalonia.AppBuilder builder, string[] args, System.Action<Avalonia.Controls.ApplicationLifetimes.IClassicDesktopStyleApplicationLifetime> lifetimeBuilder)	Unknown

[amwx]

Do you have a repro, or if not an actual project at least a description of how the NavView is set up so I can repro this?

[robloo]

I was afraid you were going to say that. Reducing down to a repro in a full app is always tricky. Might be faster to debug with a local build of FluentAvalonia itself. Will spend more time on this later in the week.

[amwx]

If you can't get a repro, does the error message have the line number - I didn't see it above.

Also, populating the NavView, are you binding or using raw NavigationViewItems? If binding, what does your data template look like and what is the source collection type? Is SelectedItem bound? Is it hierarchical? PaneDisplayMode? Basically anything you can tell me about how its set up and I can try to repro it myself. Does it crash upon load or do you have to do anything?

All I know is just from a quick look at the code and stepping back from how that final method is called, nothing should be null at that point. The only thing I can think of is maybe GetElement is being called too early from a binding (like SelectedItem being set) and layout hasn't run so the ScrollViewer isn't available yet, but that should repro in more cases if that's the case.

[robloo]

Edit: Somehow it submitted while I was typing... so I finished it.

@amwx I'm running with FluentAvalonia source directly now so can step into the code.

Also, populating the NavView, are you binding or using raw NavigationViewItems?

NavigationViewItem are added directly in XAML -- there is no binding here.

Is SelectedItem bound?

No

Is it hierarchical?

No

PaneDisplayMode?

Top

Does it crash upon load or do you have to do anything?

It crashes on initial load. It's crashing as soon as Show() is called on the Window and the first layout/arrange pass is running. This is before the Window is even loaded and before and more complex code is run. As far as I can tell it's still constructing the visual tree and not even running real app code yet.

ScrollViewer isn't available yet

That is indeed what is happening. I notice most usages of _scroller have a null check as well. The area of the crash there is no check.

[robloo]

@amwx The below does work around the issue. App starts up and runs normally after this change adding the null checks. Somehow the code path is getting run before EnsureScroller() or something. Will look further.

    public void OnElementPrepared(Control element, VirtualizationInfo vInfo)
    {
        // If we have an anchor element, we do not want the
        // scroll anchor provider to start anchoring some other element.
        vInfo.CanBeScrollAnchor = true;
-       _scroller.RegisterAnchorCandidate(element);
+       _scroller?.RegisterAnchorCandidate(element);
    }

    public void OnElementCleared(Control element)
    {
        // Unlike OnElementPrepared, we can't make the change and add the virt info
        // as the caller of this (ItemsRepeater.ClearElementImpl) doesn't have a ref
        // and that method has multiple refs which don't have a the virt info either
        ItemsRepeater.GetVirtualizationInfo(element).CanBeScrollAnchor = false;
-       _scroller.UnregisterAnchorCandidate(element);
+       _scroller?.UnregisterAnchorCandidate(element);
    }

[amwx]

Ok I've added in the null checks and stopped this crash. Everything is actually running ok, but in Top pane mode there isn't a ScrollViewer in the tree and so it was never set. This code is slightly different from WinUI because Avalonia handles scroll anchor candidates slightly differently. So adding the null check won't hurt anything since if there's no ScrollViewer, there's no need to register anchor candidates.

2.1 branch has been updated with this fix along with a couple other crashes I found. If you find any other issues, just tack them on here.