Object Explorer Service (#311)

* moving OE service from an old branch

src/Microsoft.SqlTools.ServiceLayer/ObjectExplorer/Nodes/ChildFactory.cs

@@ -0,0 +1,39 @@
+//
+// Copyright (c) Microsoft. All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+//
+
+using System.Collections.Generic;
+
+namespace Microsoft.SqlTools.ServiceLayer.ObjectExplorer.Nodes
+{
+    /// <summary>
+    /// A <see cref="ChildFactory"/> supports creation of <see cref="TreeNode"/> children
+    /// for a class of objects in the tree. The 
+    /// </summary>
+    public abstract class ChildFactory
+    {
+        /// <summary>
+        /// The set of applicable parents for which the factory can create children.
+        /// </summary>
+        /// <returns>
+        /// the string names for each <see cref="TreeNode.NodeType"/> that 
+        /// this factory can create children for
+        /// </returns>
+        public abstract IEnumerable<string> ApplicableParents();
+
+        /// <summary>
+        /// Expands an element in the 
+        /// </summary>
+        /// <param name="parent"></param>
+        /// <returns></returns>
+        public abstract IEnumerable<TreeNode> Expand(TreeNode parent);
+
+        public abstract bool CanCreateChild(TreeNode parent, object context);
+        public abstract TreeNode CreateChild(TreeNode parent, object context);
+
+        // TODO Consider whether Remove operations need to be supported
+        //public abstract bool CanRemoveChild(TreeNode parent, object context);
+        //public abstract int GetChildIndexToRemove(TreeNode parent, object context);
+    }
+}