Proposal: WinUI Expander Control

A spec has been opened with a PR for this issue. Feel free to continue general discussion in this proposal!

Summary

Throughout Windows, different expander controls are used by Windows Security, Settings, Photos, Paint 3D, the Notification Center, 3D Viewer, Toasts and the UWP Onedrive app. There’s currently no consistent "Windows" way of addressing this common UX pattern. An Expander control is motivated by both its use in many app scenarios and achieving parity with WPF.

Rationale

• There should be a built-in and consistent Fluent way to expand and collapse content.
• Expander scenarios exist across many surfaces that are not aligned in behavior and style, including being Narrator-friendly, keyboard controls, animations, designs, and high contrast changes.

Scope

*The v1 Expander could be scoped to only expand in the downwards direction, with default down and ExpandDirection added later.

Important Notes

Proposed API

Public enum ExpandDirection 
{ 
Down = 0 
Up = 1 
Left = 2 
Right = 3 
} 

public class Expander : ContentControl 
{ 
Expander(); 

public object Header {get;set;} 
public DataTemplate HeaderTemplate { get;set; } 
public DataTemplate HeaderTemplateSelector {get;set;} 

public static readonly DependencyProperty HeaderProperty; 
public static readonly DependencyProperty HeaderTemplateProperty; 
public static readonly DependencyProperty HeaderTemplateSelectorProperty {get;} 

public bool IsExpanded { get;set} 
public ExpandDirection ExpandDirection { get;set;} 

protected virtual void OnExpanded(); 
protected virtual void OnCollapsed();

public event Expanded; 
public event Collapsed; 

public static readonly DependencyProperty IsExpandedProperty; 
public static readonly DependencyProperty ExpandDirectionProperty; 
} 

Visual States:  
ExpandStates: Expanded/Collapsed 

Accessibility: 
Support Expand/Collapse pattern (IExpandCollapseProvider) 

Expander Controls Elsewhere

• WPF
• Windows Community Toolkit
• Telerik UI
• Xamarin.Forms
  
• Syncfusion.XForms.Expander
• Avalonia UI

Examples of Expander Scenarios

Open Questions

• In discussion with the WinUI team, it was brought up that a V1 scoped to expand downward would shorten development time and make Expander available for developers sooner (as the scenarios for Expander thus far have all been downward), and a planned V2 could add ExpandDirection soon after. In your apps, are there any scenarios where Expander would need to expand in directions other than downward?

• How should Expander work with InfoBar? Adding expanding functionality to InfoBar? Putting InfoBar as the content of an Expander? The reverse?