dotnet
[Android] Fix app crash caused by dynamic template switching in ListView
Exception I – Object reference not set to an instance of an object at GraphicsExtensions.AsColor :
During the execution of the device test case - ChangingTemplateTypeDoesNotCrash, a fatal exception was thrown, causing a crash.
Root Cause
The root cause of the NullReferenceException was identified as a failure to check for a valid context or color before calling the AsColor method. Specifically, the line of code responsible for this was:
var color = Current?.Windows?.Count > 0
? Current.Windows[0].MauiContext.Context?.GetAccentColor()
: null;
In the UpdateSeparatorColor method, when returning null for Application.AccentColor , the method requests:
bline.SetBackgroundColor(separatorColor.ToPlatform(Application.AccentColor)); Since no default separator color was set, this led to the issue. The failure was particularly observed in our test case that enabled grouping, which is why only the ChangingTemplateTypeDoesNotCrash test case was failing.
Description of Change
To resolve this issue, the following modifications were implemented in the relevant code segment:
Default Color Implementation: A fallback mechanism was implemented to provide a default color (e.g., Color.FromRgba(50, 79, 133, 255)) when the window.count resource is unavailable. This approach prevents the application from crashing and ensures that the UI continues to function smoothly even in command line device test cases.
Exception II – The specified child already has a parent. you must call removeView() on the child’s parent first at ViewGroup.addViewInner.
Root cause
In the ViewCellRenderer class, the platform view was being recreated for each cell without properly removing the existing parent view (i.e., the ViewCellContainer ) before attempting to add the new view. This caused an exception:'The specified child already has a parent. You must call removeView() on the child's parent first. 'The issue arose due to improper handling of the platform view's parent when switching between different DataTemplates in a ListView using the RetainElement caching strategy in .NET MAUI.
Description of Issue Fix
The fix involved checking if the platformView already had a parent (i.e., ViewGroup) and ensuring that the parent was removed before adding the platformView to the new container. By introducing this conditional check, the system prevents the platform view from being associated with multiple parents, which caused the "The specified child already has a parent" exception. This change ensures that dynamic template changes in the ListView are handled correctly, improving view recycling and maintaining stable rendering without conflicts.
Tested the behavior in the following platforms.
- [x] Android
- [x] Windows
- [x] iOS
- [x] Mac
Issues Fixed
Fixes #24701
Output
Before fix:
After fix:
Testcase:
The Testcase is already available for this fix. Please refer to the PR link below for reference.
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Can you re-enable this device test for android?
https://github.com/dotnet/maui/pull/24700/files#diff-9ef75bbb1d7dbaf152e43bba50d7dfc6ed46b9db1b351e0ff5da3272718f811aR36-R41
@PureWeen, I have modified changes. Could you please validate it once and let me know if there are any further concerns.
Looks like the test is failing
09-23 03:31:56.106 4471 4471 E AndroidRuntime: FATAL EXCEPTION: main 09-23 03:31:56.106 4471 4471 E AndroidRuntime: Process: com.microsoft.maui.controls.devicetests, PID: 4471 09-23 03:31:56.106 4471 4471 E AndroidRuntime: android.runtime.JavaProxyThrowable: [System.NullReferenceException]: Object reference not set to an instance of an object. 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Graphics.Platform.GraphicsExtensions.AsColor(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Platform.ColorExtensions.ToPlatform(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Platform.ColorExtensions.ToPlatform(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Controls.Handlers.Compatibility.ListViewAdapter.UpdateSeparatorColor(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Controls.Handlers.Compatibility.ListViewAdapter.GetView(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Controls.Handlers.Compatibility.ListViewRenderer.GetDesiredSize(Unknown Source:0) 09-23 03:31:56.106 4471 4471 E AndroidRuntime: at Microsoft.Maui.Controls.Handlers.Compatibility.VisualElementRenderer`1[[Microsoft.Maui.Controls.ListView, Microsoft.Maui.Controls, Version=1.0.0.0, Culture=neutral,
@PureWeen, The test failure is caused by the UpdateSeparatorColor method, which calls the default separator color through
Application.AccentColor. We have addressed this issue by implementing a fix at this line of code, where a default color is now provided to prevent a NullReferenceException.
