[Bug] [iOS] CollectionView blank when bound ObservableCollection has contents rapidly change

[Silic0nS0ldier]

This bug report is a work in progress. Missing details will be added at soonest opportunity.

Description

When a CollectionView with a bound ObservableCollection (that has been enhanced to allow bulk changes without overwhelming the UI) has the contents of the ObservableCollection change rapidly (e.g. in response to a filter changing as the user types), the CollectionView appears to enter a corrupted state where nothing is shown when there are items in the ObservableCollection.

The set EmptyView continues to function correctly when in the state.

Steps to Reproduce

TODO Pending creation of reproduction repo.

Expected Behavior

CollectionView shows a populated ItemTemplate in the UI for each item in the bound ObservableCollection.

Actual Behavior

CollectionView shows nothing.

Basic Information

• Version with issue: 4.8.0.1687
• Last known good version: NA
• Platform Target Frameworks:
  • iOS: 14.3
• NuGet Packages: NA
• Affected Devices: iPhone Simulator, iPhone SE (2nd generation) iOS 14.3 (not tested on physical device which may respond differently given this appears to involve race conditions)

Environment

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Microsoft Visual Studio Community 2019
Version 16.8.3
VisualStudio.16.Release/16.8.3+30804.86
Microsoft .NET Framework
Version 4.8.04084

Installed Version: Community

Visual C++ 2019   00435-60000-00000-AA682
Microsoft Visual C++ 2019

ASP.NET and Web Tools 2019   16.8.554.20160
ASP.NET and Web Tools 2019

ASP.NET Core Razor Language Services   16.1.0.2052803+84e121f1403378489b842e1797df2f3f5a49ac3c
Provides languages services for ASP.NET Core Razor.

Azure App Service Tools v3.0.0   16.8.554.20160
Azure App Service Tools v3.0.0

C# Tools   3.8.0-5.20604.10+9ed4b774d20940880de8df1ca8b07508aa01c8cd
C# components used in the IDE. Depending on your project type and settings, a different version of the compiler may be used.

Common Azure Tools   1.10
Provides common services for use by Azure Mobile Services and Microsoft Azure Tools.

Extensibility Message Bus   1.2.6 (master@34d6af2)
Provides common messaging-based MEF services for loosely coupled Visual Studio extension components communication and integration.

IntelliCode Extension   1.0
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Microsoft Continuous Delivery Tools for Visual Studio   0.4
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Microsoft MI-Based Debugger   1.0
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Microsoft Visual C++ Wizards   1.0
Microsoft Visual C++ Wizards

Microsoft Visual Studio Tools for Containers   1.1
Develop, run, validate your ASP.NET Core applications in the target environment. F5 your application directly into a container with debugging, or CTRL + F5 to edit & refresh your app without having to rebuild the container.

Microsoft Visual Studio VC Package   1.0
Microsoft Visual Studio VC Package

Mono Debugging for Visual Studio   16.8.43 (00471f8)
Support for debugging Mono processes with Visual Studio.

NuGet Package Manager   5.8.0
NuGet Package Manager in Visual Studio. For more information about NuGet, visit https://docs.nuget.org/

ProjectServicesPackage Extension   1.0
ProjectServicesPackage Visual Studio Extension Detailed Info

SQL Server Data Tools   16.0.62010.06180
Microsoft SQL Server Data Tools

Syntax Visualizer   1.0
An extension for visualizing Roslyn SyntaxTrees.

Test Adapter for Boost.Test   1.0
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Test Adapter for Google Test   1.0
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TypeScript Tools   16.0.21016.2001
TypeScript Tools for Microsoft Visual Studio

Visual Basic Tools   3.8.0-5.20604.10+9ed4b774d20940880de8df1ca8b07508aa01c8cd
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Visual F# Tools   16.8.0-beta.20507.4+da6be68280c89131cdba2045525b80890401defd
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Visual Studio Code Debug Adapter Host Package   1.0
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Visual Studio Container Tools Extensions   1.0
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Visual Studio Tools for CMake   1.0
Visual Studio Tools for CMake

Visual Studio Tools for Containers   1.0
Visual Studio Tools for Containers

VisualStudio.DeviceLog   1.0
Information about my package

VisualStudio.Foo   1.0
Information about my package

VisualStudio.Mac   1.0
Mac Extension for Visual Studio

Xamarin   16.8.000.260 (d16-8@fd405a2)
Visual Studio extension to enable development for Xamarin.iOS and Xamarin.Android.

Xamarin Designer   16.8.0.507 (remotes/origin/d16-8@e87b24884)
Visual Studio extension to enable Xamarin Designer tools in Visual Studio.

Xamarin Templates   16.8.112 (86385a3)
Templates for building iOS, Android, and Windows apps with Xamarin and Xamarin.Forms.

Xamarin.Android SDK   11.1.0.17 (d16-8/c0e2b8e)
Xamarin.Android Reference Assemblies and MSBuild support.
    Mono: be2226b
    Java.Interop: xamarin/java.interop/d16-8@79d9533
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    SQLite: xamarin/sqlite/3.32.1@1a3276b
    Xamarin.Android Tools: xamarin/xamarin-android-tools/d16-8@2fb1cbc


Xamarin.iOS and Xamarin.Mac SDK   14.6.0.15 (87a1b18d8)
Xamarin.iOS and Xamarin.Mac Reference Assemblies and MSBuild support.

Screenshots

TODO Pending creation of reproduction repo.

Reproduction Link

TODO To be created.

Workaround

Reduce rate of changes to ObservableCollection (via a queue with items delayed by a timeframe proportional to the change impact, or a throttle/debounce at a higher level if applicable for use case) so that UI has a chance to apply all changes before the next change. This reduces the chance of the race condition that causes this issue from manifesting by a considerable margin (but does not necessarily eliminate the possibility).

No quick workarounds here unfortunately, aside from going the slow route (add items one-by-one, slowing the UI considerably when many changes are made).

[Tommigun1980]

This is the same issue as #13126 (comment) and #13203.

[Silic0nS0ldier]

I don't think this is the same issue. The view defined on the EmptyView property is showing when it should here. When there is something to show (and the issue is manifesting), nothing is shown. This appears to be different to the issue described on those other tickets.

Once I've got a reproduction together (hoped to have this together by now, should have accounted for the "end of project rush") , the differences (or lack there-of) should be clearer.

[stepkillah]

Experiencing the same issue when collection changes rapidly, basically - data in ItemSource do not correspond actual CollectionView views

[Tommigun1980]

I don't think this is the same issue. The view defined on the EmptyView property is showing when it should here. When there is something to show (and the issue is manifesting), nothing is shown. This appears to be different to the issue described on those other tickets.

Once I've got a reproduction together (hoped to have this together by now, should have accounted for the "end of project rush") , the differences (or lack there-of) should be clearer.

Ah, sorry. I think it’s probably closely related though. There’s another issue reported (severe performance regression) in 5.0.0 unless a batch add is used, #13429, that may also be closely related.

[PureWeen]

Moving this to needsinfo for now until a reproduction is attached

[Silic0nS0ldier]

This issue is proving difficult to recreate (my guess is that it requires bottlenecks in the "right" places to manifest) and I do not have time to dig deeper into the issue. Without the repo, its difficult to articulate specifics of the issue which makes it close to impossible for anyone figure this out (unless they stumble onto this by chance). As such, I'll be closing this issue.

For anyone who might stumble across this later, here is an overview of the issue that is being reported (the manifestation that is).

A search term would be typed in quickly, each input fires an event which alters collection view contents (for filtering results), after some time of input changing results are no longer visible. When in this state the collection view being empty still correctly shows the empty view, however when results should be shown there is just a blank space.