Fix #31939: CommandParameter TemplateBinding lost during reparenting

[StephaneDelcroix]

Note

Are you waiting for the changes in this PR to be merged?
It would be very helpful if you could test the resulting artifacts from this PR and let us know in a comment if this change resolves your issue. Thank you!

Description

Fixes #31939

When a Button inside a ControlTemplate has both Command and CommandParameter with TemplateBinding, the async binding application path can cause Command to be evaluated before CommandParameter resolves, resulting in CanExecute being called with a null parameter.

Solution

Add a BindableProperty dependency mechanism via DependsOn() method. When CommandProperty.DependsOn(CommandParameterProperty) is registered, the CommandElement.GetCanExecute() method checks if CommandParameter has a pending binding with null value. If so, it returns true (assumes command can execute) instead of calling CanExecute(null). When the binding resolves, OnCommandParameterChanged fires and CanExecute is properly re-evaluated.

Changes

• BindableProperty: Add Dependencies property and DependsOn() method
• BindableObject: Add HasPendingBindingWithNullValue() helper
• CommandElement: Check dependencies before calling CanExecute
• ButtonElement, CheckBox, SearchBar: Register Command → CommandParameter dependency
• Button, ImageButton, CheckBox, SearchBar: Pass CommandProperty to GetCanExecute()

Testing

Added unit tests that verify:

1. Initial template binding works correctly
2. CommandParameter is preserved after reparenting (the bug scenario)

Verified: Tests fail without fix, pass with fix.

[Copilot]

Pull request overview

This PR fixes issue #31939 where CommandParameter TemplateBinding is lost during control template reparenting, causing CanExecute to be called with a null parameter before CommandParameter resolves. The fix introduces a BindableProperty dependency mechanism that delays CanExecute evaluation until dependent bindings have resolved.

Key changes:

• Added a dependency mechanism via BindableProperty.DependsOn() to establish property dependencies
• Modified CommandElement.GetCanExecute() to check for pending bindings on dependencies before calling CanExecute
• Registered Command → CommandParameter dependencies in Button, ImageButton, CheckBox, and SearchBar controls
• Added comprehensive unit tests to verify the fix works for both initial binding and reparenting scenarios

Reviewed changes

Copilot reviewed 12 out of 12 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Controls/src/Core/BindableProperty.cs	Adds Dependencies property and DependsOn() method to establish property dependencies
src/Controls/src/Core/BindableObject.cs	Adds HasPendingBindingWithNullValue() helper to detect unresolved bindings
src/Controls/src/Core/CommandElement.cs	Updates GetCanExecute() to check dependencies for pending bindings before calling CanExecute
src/Controls/src/Core/Button/ButtonElement.cs	Registers Command → CommandParameter dependency in static constructor and restructures property initialization
src/Controls/src/Core/Button/Button.cs	Passes CommandProperty to GetCanExecute() for dependency checking
src/Controls/src/Core/ImageButton/ImageButton.cs	Passes CommandProperty to GetCanExecute() for dependency checking
src/Controls/src/Core/CheckBox/CheckBox.cs	Passes CommandProperty to GetCanExecute() for dependency checking
src/Controls/src/Core/CheckBox/CheckBox.Mapper.cs	Registers Command → CommandParameter dependency in static constructor
src/Controls/src/Core/SearchBar/SearchBar.cs	Passes SearchCommandProperty to GetCanExecute() for dependency checking
src/Controls/src/Core/SearchBar/SearchBar.Mapper.cs	Registers SearchCommand → SearchCommandParameter dependency in static constructor
src/Controls/tests/Xaml.UnitTests/Issues/Maui31939.xaml	XAML test page with custom control and ControlTemplate demonstrating the bug scenario
src/Controls/tests/Xaml.UnitTests/Issues/Maui31939.xaml.cs	Comprehensive unit tests verifying initial binding and reparenting scenarios, including custom control and command implementations

[Copilot]

@rmarinho I've opened a new pull request, #33022, to work on those changes. Once the pull request is ready, I'll request review from you.