Add ExpanderAnimationBehavior, IExpansionController

[stephenquan]

…EventArgs

Description of Change

Enables animated expand/collapse behavior for the Expander by wrapping the user’s content Expander.Content in a Expander.ContentHost (ContentView) and animating changes to Expander.ContentHost.HeightRequest.

• Add Expander.ContentHost property (read-only).
• Add IExpansionController interface.
• Add Expander.ExpansionController property.
• Add Expander.ExpandedChanging event with ExpandedChangingEventArgs.
• Add ExpansionAnimationBehavior behavior.
• Update the sample app to demonstrate ExpandedAnimationBehavior.

Opt in to animations by assigning ExpansionAnimationBehavior to either (1) Expander.Behaviors, or (2) Expander.ExpansionController. Alternatively, custom animations can be created by implementing IExpansionController.

Linked Issues

• Fixes [Proposal] Expander animation #2521
• Discussion Expander animation #1628
• Replace old PR Refactor Expander.ContentHeight and ExpanderAnimationBehavior #2723
• Replace old PR Change parts of Expander implementation to support animation #2522

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard
• Documentation created or updated: TODO: https://github.com/MicrosoftDocs/CommunityToolkit/pulls

Additional information

This is a reboot of the work that began in PR #2723 and PR #2522.

[Copilot]

Pull request overview

Adds a pluggable expansion/collapse pipeline to Expander to enable animated transitions via an IExpansionController, plus a default animation behavior and updated samples demonstrating the new functionality.

Changes:

• Introduces IExpansionController and wires Expander.ExpansionController into the expand/collapse flow.
• Adds ExpandedChanging + ExpandedChangingEventArgs, and refactors Expander content handling to use a ContentHost container suitable for animation.
• Adds ExpanderAnimationBehavior implementing IExpansionController, and updates the sample page to demonstrate animation timing.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 9 comments.

Show a summary per file

File	Description
src/CommunityToolkit.Maui/Views/Expander/Expander.shared.cs	Adds controller-based expand/collapse pipeline, new event, and ContentHost wrapper used for animation.
src/CommunityToolkit.Maui/Interfaces/IExpansionController.shared.cs	New interface to plug in animated (or instant) expand/collapse implementations.
src/CommunityToolkit.Maui/Behaviors/ExpanderAnimationBehavior.shared.cs	New behavior implementing IExpansionController using Animation to animate expansion/collapse.
src/CommunityToolkit.Maui.Core/Primitives/ExpandedChangingEventArgs.shared.cs	New event args for “about to change expanded state”.
src/CommunityToolkit.Maui.Core/Primitives/Defaults/ExpanderAnimationBehaviorDefaults.shared.cs	Defaults for easing/duration used by ExpanderAnimationBehavior.
samples/CommunityToolkit.Maui.Sample/Pages/Views/Expander/ExpanderPage.xaml.cs	Measures elapsed time between ExpandedChanging and ExpandedChanged and shows it in a toast.
samples/CommunityToolkit.Maui.Sample/Pages/Views/Expander/ExpanderPage.xaml	Applies ExpanderAnimationBehavior in samples and hooks ExpandedChanging/ExpandedChanged handlers.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 7 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 3 comments.

[Copilot]

The GridItemsLayout sample sets a hard-coded HeightRequest="180" on the Expander, which prevents the control from naturally resizing based on expanded/collapsed state and introduces an unexplained magic number. If this is required as a workaround for GridItemsLayout item sizing, consider documenting why in a comment (or remove the fixed height and apply the animation behavior here instead) so the sample demonstrates expected expander behavior.

[Copilot]

ResizeExpanderInItemsView awaits expansionGate.Task, but expansionGate is a mutable field that gets replaced on each IsExpanded change. With rapid toggles, this dispatched lambda can end up awaiting a different gate instance than the one completed by the corresponding transition, causing hangs/missed size updates. Capture the current gate/task into a local before dispatch/await, and ensure the same captured gate instance is the one completed by the matching expand/collapse transition.

[Copilot]

ExpandedChangedAsync signals completion via expansionGate.TrySetResult() on the mutable field. If expansionGate has been replaced by a newer transition while this async method was awaiting the controller, it will complete the wrong gate and leave the previous one uncompleted (which can block awaiting resize logic). Capture the current TaskCompletionSource into a local at the start of the transition and complete that captured instance in finally.