TreeView selection API spec

active/TreeView/TreeViewSelectionAPI.md

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+# Background
+
+The Xaml [TreeView](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView) control shows a list of hierarchical data, similar to a ListView. 
+
+When TreeView was first released, you populated it using the [RootNode property](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.RootNodes), giving it a tree of nodes, each node referencing a data item. There was negative feedback about this nodes approach, though, from developers preferring a more data binding friendly pattern along the lines of [ListView.ItemsSource](https://docs.microsoft.com/uwp/api/Windows.UI.Xaml.Controls.ItemsControl.ItemsSource). 
+
+So a [TreeView.ItemsSource](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.ItemsSource) property was added, and now you can use either the nodes approach or the ItemsSource approach. But unlike ListView, TreeView doesn't have the corresponding SelectedItem and SelectedItems properties for single- and multi-select. Those are being added in this spec.
+
+Also being added in this spec, for the case where the RootNode is being used instead of ItemsSource, is an API to select a single node. That is, there is a [SelectedNodes](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.SelectedNodes) property today, and this spec is adding a singular SelectedNode property.
+
+Related GitHub issues:  
+https://github.com/microsoft/microsoft-ui-xaml/issues/197  
+https://github.com/microsoft/microsoft-ui-xaml/issues/124  
+https://github.com/microsoft/microsoft-ui-xaml/issues/386
+
+# Description
+
+Extend TreeView selection API sets to make selections easier.
+
+- Add `SelectedNode`: get/set the selected TreeViewNode for single selection
+- Add `SelectedItem`: get/set the selected item for single selection
+- Add `SelectedItems`: get the SelectedItems collection.
+- Fix existing `SelectedNodes` property: Currently this only works in multi-select mode ([SelectionMode](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.SelectionMode) set to `Multiple`). It will now also work in single selection mode (consistent with ListView).
+
+# Examples
+
+## TreeView.SelectedNode
+Ouput the depth of the selected tree view node.
+```C#
+TreeViewNode selectedNode = treeView.SelectedNode;
+if (selectedNode != null)
+{
+    Debug.WriteLine($"Selected node depth: {selectedNode.Depth}");
+}
+```
+
+## TreeView.SelectedItem
+Bind the TreeView control to a View Model with the data source and the currently selected item.
+```Xaml
+<TreeView ItemsSource="{x:Bind Descendants}"
+            SelectedItem="{x:Bind CurrentDescendant, Mode=TwoWay}" />
+```
+
+## TreeView.SelectedItems
+```C#
+void AddToSelection(TreeView treeView, object item)
+{
+    IList<Object> selectedItems = treeView.SelectedItems;
+    selectedItems.Add(item);
+}
+```
+
+# API Details
+
+```
+namespace Microsoft.UI.Xaml.Controls
+{
+    [webhosthidden]
+    unsealed runtimeclass TreeView : Windows.UI.Xaml.Controls.Control
+    {
+        ...
+
+        TreeViewNode SelectedNode{ get; set; };
+        Object SelectedItem{ get; set; };
+        Windows.Foundation.Collections.IVector<Object> SelectedItems{ get; };
+
+        ...
+    }
+}
+```
+
+# Remarks
+
+SelectedItem(s) returns Object(collection of objects), it will be the ItemsSource type object when using databinding, or TreeViewNode when not using databinding.
+
+Prior to WinUI 2.2, the [TreeView.SelectedNodes](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.SelectedNodes) property can only be used when the [SelectionMode](https://docs.microsoft.com/uwp/api/Microsoft.UI.Xaml.Controls.TreeView.SelectionMode) is set to `Multiple`.
+
+# API Notes
+
+## Class: TreeView
+| Member Name | Description |
+|:- |:--|
+| SelectedNode | Gets or sets the selected TreeViewNode. |
+| SelectedNodes | The collection of nodes that are selected in the tree. The default is an empty collection. |
+| SelectedItem | Gets or sets the selected item. |
+| SelectedItems | The collection of items that are selected in the tree. The default is an empty collection. |