Add proposal for TreeView SelectionChanged event

active/TreeView/SelectionChangedEvent.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. -->
+
+<!-- There are two audiences for the spec. The first are people
+that want to evaluate and give feedback on the API, as part of
+the submission process.  When it's complete
+it will be incorporated into the public documentation at
+docs.microsoft.com (http://docs.microsoft.com/uwp/toolkits/winui/).
+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. -->
+
+<!-- If you're modifying an existing API, included a link here to the
+existing page(s) -->
+
+<!-- For example, this section is a place to explain why you're adding this API rather than
+modifying an existing API. -->
+
+<!-- For example, this is a place to provide a brief explanation of some dependent
+area, just explanation enough to understand this new API, rather than telling
+the reader "go read 100 pages of background information posted at ...". -->
+The `TreeView` control is lacking an event to track when selection has changed. While other list based controls (`ListView`, `ComboBox`, ...) have such API the `TreeView` control does not. 
+
+The original issue in the WinUI repo for this is here: https://github.com/microsoft/microsoft-ui-xaml/issues/322
+
+# 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). -->
+This API is adding an event whenever the selected item of a `TreeView` control has changed.
+
+# 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.-->
+```
+private void Initialize()
+{
+    var treeView = new TreeView();
+    treeView.SelectionChanged += OnSelectionChanged;
+}
+
+private void OnSelectionChanged(object sender, TreeViewSelectionChangedEventArgs e)
+{
+    foreach (var addedItem in e.AddedItems)
+    {
+        // ...
+    }
+
+    foreach (var removedItems in e.RemovedItems)
+    {
+        // ...
+    }
+}
+```
+
+
+<!-- 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). -->
+
+
+# Remarks
+<!-- Explanation and guidance that doesn't fit into the Examples section. -->
+
+<!-- APIs should only throw exceptions in exceptional conditions; basically,
+only when there's a bug in the caller, such as argument exception.  But if for some
+reason it's necessary for a caller to catch an exception from an API, call that
+out with an explanation either here or in the Examples -->
+The API is available as preview already in WinUI and was merged in this PR: https://github.com/microsoft/microsoft-ui-xaml/pull/2063
+
+# API Notes
+
+N/A
+<!-- 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.) -->
+
+<!-- Option 2: Put these descriptions in the below API Details section,
+with a "///" comment above the member or type. -->
+
+# API Details
+<!-- The exact API, in MIDL3 format (https://docs.microsoft.com/en-us/uwp/midl-3/) -->
+```
+runtimeclass TreeViewSelectionChangedEventArgs
+{
+    Windows.Foundation.Collections.IVector<Object> AddedItems{ get; };
+    Windows.Foundation.Collections.IVector<Object> RemovedItems{ get; };
+}
+
+runtimeclass TreeView : Windows.UI.Xaml.Controls.Control {
+    event Windows.Foundation.TypedEventHandler<TreeView, TreeViewSelectionChangedEventArgs> SelectionChanged;
+}
+```
+
+# 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. -->
+The underlying implementation of this event is using two different approaches depending on multiselection status. 
+
+When multiselection **is not** enabled on the treeview control, the implementation is just using the `SelectionChanged` event of the `ListView` control used internally.
+
+When multiselection **is** enabled on the treeview control, selection changed is tracked differently.
+
+While there are two code paths, this does not imply any user facing difference in how the API can be consumed.