Expander Control

active/Expander/Expander.md

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+<!-- The purpose of this spec is to describe a new feature and
+its APIs that make up a new feature in WinUI. -->
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+that want to evaluate and give feedback on the API, as part of
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+Hopefully we'll be able to copy it mostly verbatim.
+So the second audience is everyone that reads there to learn how
+and why to use this API. -->
+
+# Background
+<!-- Use this section to provide background context for the new API(s) 
+in this spec. -->
+
+<!-- This section and the appendix are the only sections that likely
+do not get copied to docs.microsoft.com; they're just an aid to reading this spec. -->
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+<!-- If you're modifying an existing API, included a link here to the
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+ > This spec corresponds to [this issue](https://github.com/microsoft/microsoft-ui-xaml/issues/3279) on the WinUI repo. 
+
+Throughout Windows, different expander controls are used by various apps and surfaces. There’s currently no consistent way to address this common UX pattern. This control is needed for situations where expanding (instead of overlaying) content is needed.  An Expander control is motivated by its use in many app scenarios and supporting developers in migrating from [WPF](https://docs.microsoft.com/en-us/dotnet/desktop/wpf/controls/expander-overview?view=netframeworkdesktop-4.8) and [WCT](https://docs.microsoft.com/en-us/windows/communitytoolkit/controls/expander).  
+
+# Description
+<!-- Use this section to provide a brief description of the feature.
+For an example, see the introduction to the PasswordBox control 
+(http://docs.microsoft.com/windows/uwp/design/controls-and-patterns/password-box). -->
+An Expander control is a UI component that provides a standard interaction for showing more content in a container that pushes adjacent content while expanding and collapsing. An Expander cannot be light dismissed and is independent of the contents inside it, including controls. Expander should be used when some content is only relevant some of the time (for example to read more information or access additional options for an item). 
+
+![A collapsed Expander](images/expander_collapsed.PNG)
+![An expanded Expander](images/expander_expanded.PNG)
+# Examples
+<!-- Use this section to explain the features of the API, showing
+example code with each description. The general format is: 
+  feature explanation,
+  example code
+  feature explanation,
+  example code
+  etc.-->
+
+<!-- Code samples should be in C# and/or C++/WinRT -->
+
+<!-- As an example of this section, see the Examples section for the PasswordBox control 
+(https://docs.microsoft.com/windows/uwp/design/controls-and-patterns/password-box#examples). -->
+
+## Create an Expander
+
+![A collapsed Expander](images/expander_collapsed.PNG)
+XAML
+~~~~
+<muxc:Expander x:Name="SimpleExpander" />
+~~~~
+## Put controls inside an Expander
+![An expanded Expander with a ToggleButton in the Header and a Button in the Content](images/expander_example_buttons.PNG)
+XAML
+~~~~
+<muxc:Expander x:Name="Expander2" IsExpanded="False" ExpandDirection ="Down">
+    <muxc:Expander.Header>
+        <ToggleButton>This is a ToggleButton in the header</ToggleButton>
+    </muxc:Expander.Header>
+    <muxc:Expander.Content>
+        <Button>This is a Button in the content</Button>
+    </muxc:Expander.Content>
+</muxc:Expander>
+~~~~
+
+Updated pictures with final visuals to come. 
+
+# API Notes
+<!-- Option 1: Give a one or two line description of each API (type
+and member), or at least the ones that aren't obvious
+from their name.  These descriptions are what show up
+in IntelliSense. For properties, specify the default value of the property if it
+isn't the type's default (for example an int-typed property that doesn't default to zero.) -->
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+<!-- Option 2: Put these descriptions in the below API Details section,
+with a "///" comment above the member or type. -->
+## Notable Properties
+| Name | Description | Default |
+| :---------- | :------- | :------- |
+| ExpandDirection | Sets the direction of expansion | Down = 0 |
+| IsExpanded | Whether or not control is expanded | False |
+
+## Events
+| Name | Description | 
+| :---------- | :------- | 
+| Expanding | Occurs when expanded |
+| Collapsed| Occurs when collapsed |
+
+# API Details
+<!-- The exact API, in MIDL3 format (https://docs.microsoft.com/en-us/uwp/midl-3/) -->
+~~~~
+enum ExpandDirection
+{
+    Down = 0,
+    Up = 1
+};
+
+runtimeclass ExpanderExpandingEventArgs
+{
+}
+
+runtimeclass ExpanderCollapsedEventArgs
+{
+}
+
+unsealed runtimeclass Expander : Windows.UI.Xaml.Controls.ContentControl
+{
+    Expander();
+
+    Object Header{ get; set; };
+    Windows.UI.Xaml.DataTemplate HeaderTemplate{ get; set; };
+    Windows.UI.Xaml.Controls.DataTemplateSelector HeaderTemplateSelector{ get; set; };
+
+    [MUX_PROPERTY_CHANGED_CALLBACK(TRUE)]
+    Boolean IsExpanded{ get; set; };
+
+    [MUX_DEFAULT_VALUE("winrt::ExpandDirection::Down")]
+    [MUX_PROPERTY_CHANGED_CALLBACK(TRUE)]
+    ExpandDirection ExpandDirection{ get; set; };
+
+    event Windows.Foundation.TypedEventHandler<Expander, ExpanderExpandingEventArgs> Expanding;
+    event Windows.Foundation.TypedEventHandler<Expander, ExpanderCollapsedEventArgs> Collapsed;
+
+    static Windows.UI.Xaml.DependencyProperty HeaderProperty{ get; };
+    static Windows.UI.Xaml.DependencyProperty HeaderTemplateProperty{ get; };
+    static Windows.UI.Xaml.DependencyProperty HeaderTemplateSelectorProperty{ get; };
+
+    static Windows.UI.Xaml.DependencyProperty IsExpandedProperty{ get; };
+    static Windows.UI.Xaml.DependencyProperty ExpandDirectionProperty{ get; };
+}
+~~~~
+
+## Theme Resources
+
+| Name| Description | 
+| :---------- | :------- | 
+| ExpanderHeaderBackground | Header background color| 
+| ExpanderChevronBackground | Chevron background color| 
+| ExpanderChevronForeground | Chevron foreground color| 
+| ExpanderChevronMargin | Chevron margin thickness| 
+| ExpanderChevronGlyph | Chevron glyph| 
+| ExpanderChevronWidth | Chevron width|
+| ExpanderPopinVerticalOffset | vertical offset for animation| 
+
+# Inputs and Accessibility
+## UI Automation Patterns
+Expander will use a ExpandCollapsePattern. Expanding/Collapsing the expander will raise [RaisePropertyChanged](https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.automation.peers.automationpeer.raisepropertychangedevent?view=winrt-19041) with the property changed being the [ExpandCollapseProperty](https://docs.microsoft.com/en-us/uwp/api/windows.ui.xaml.automation.expandcollapsepatternidentifiers.expandcollapsestateproperty?view=winrt-19041) property.
+
+## Keyboard
+* Tabbing brings focus to the Header, neither the entire Expander nor the chevron is a tab-stop
+* When focus is on the Header, space key expands and collapse and focus does not move
+* Keyboard navigation inside the Expander's Header/Content is based on the content inside them
+
+## GamePad
+The Expander can be expanded and collapsed with A. Spatial navigation will navigate between the content of the Expander. 
+
+## Narrator
+When focus is on the Header of an Expander, Narrator will announce "Expander collapsed" or "Expander expanded". With focus staying on the header, Narrator will then anounce "expanded" or "collapsed" accordingly. 
+
+# Appendix
+<!-- Anything else that you want to write down for posterity, but 
+that isn't necessary to understand the purpose and usage of the API.
+For example, implementation details. -->