derive DbUpdater from external event and replace Idling

diff --git a/RoomEditorApp/App.cs b/RoomEditorApp/App.cs
--- a/RoomEditorApp/App.cs
+++ b/RoomEditorApp/App.cs
@@ -36,7 +36,8 @@ namespace RoomEditorApp
     /// <summary>
     /// Store the Idling event handler when subscribed.
     /// </summary>
-    static EventHandler<IdlingEventArgs> _handler = null;
+    //static EventHandler<IdlingEventArgs> _handler = null;
+    static ExternalEvent _event = null;

@@ -211,14 +212,15 @@ namespace RoomEditorApp
     /// Toggle on and off subscription to
     /// automatic cloud updates.
     /// </summary>
-    public static void ToggleSubscription(
-      EventHandler<IdlingEventArgs> handler )
+    public static ExternalEvent ToggleSubscription(
+      IExternalEventHandler handler ) // EventHandler<IdlingEventArgs>
     {
       if( Subscribed )
       {
         Debug.Print( "Unsubscribing..." );
-        _uiapp.Idling -= _handler;
-        _handler = null;
+        //_uiapp.Idling -= _handler; _handler = null;
+        _event.Dispose();
+        _event = null;
         _buttons[3].ItemText = _subscribe;
         _timer.Stop();
         _timer.Report( "Subscription timing" );
@@ -228,12 +230,14 @@ namespace RoomEditorApp
       else
       {
         Debug.Print( "Subscribing..." );
-        _uiapp.Idling += handler;
-        _handler = handler;
+        //_uiapp.Idling += handler;
+        //_handler = handler;
+        _event = ExternalEvent.Create( handler );
         _buttons[3].ItemText = _unsubscribe;
         _timer = new JtTimer( "Subscription" );
         Debug.Print( "Subscribed." );
       }
+      return _event;
     }

     public Result OnStartup(
@@ -251,7 +255,8 @@ namespace RoomEditorApp
     {
       if( Subscribed )
       {
-        _uiapp.Idling -= _handler;
+        //_uiapp.Idling -= _handler;
+        _event.Dispose();
       }
       return Result.Succeeded;
     }

diff --git a/RoomEditorApp/CmdSubscribe.cs b/RoomEditorApp/CmdSubscribe.cs
--- a/RoomEditorApp/CmdSubscribe.cs
+++ b/RoomEditorApp/CmdSubscribe.cs

-      App.ToggleSubscription( OnIdling );
+
+      DbUpdater.ToggleSubscription(
+        commandData.Application );

diff --git a/RoomEditorApp/CmdUpdate.cs b/RoomEditorApp/CmdUpdate.cs
--- a/RoomEditorApp/CmdUpdate.cs
+++ b/RoomEditorApp/CmdUpdate.cs
@@ -96,7 +96,9 @@ namespace RoomEditorApp
-      DbUpdater updater = new DbUpdater( doc );
+
+      DbUpdater updater = new DbUpdater( uiapp );

diff --git a/RoomEditorApp/DbUpdater.cs b/RoomEditorApp/DbUpdater.cs
--- a/RoomEditorApp/DbUpdater.cs
+++ b/RoomEditorApp/DbUpdater.cs
@@ -3,14 +3,17 @@ using System;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Linq;
+using System.Threading;
+using Autodesk.Revit.ApplicationServices;
 using Autodesk.Revit.DB;
+using Autodesk.Revit.UI;
 using DreamSeat;
 #endregion

 namespace RoomEditorApp
 {

-  class DbUpdater
+  class DbUpdater : IExternalEventHandler

@@ -38,7 +41,12 @@ namespace RoomEditorApp
+    /// <summary>
+    /// Revit UI application.
+    /// </summary>
+    UIApplication _uiapp = null;

@@ -47,6 +55,23 @@ namespace RoomEditorApp
+    /// <summary>
+    /// External event to raise event
+    /// for pending database changes.
+    /// </summary>
+    static ExternalEvent _event = null;
+
+    /// <summary>
+    /// Separate thread running loop to
+    /// check for pending database changes.
+    /// </summary>
+    static Thread _thread = null;

@@ -54,12 +79,13 @@ namespace RoomEditorApp

-    public DbUpdater( Document doc )
+    public DbUpdater( UIApplication uiapp )
     {
       using( JtTimer pt = new JtTimer( "DbUpdater ctor" ) )
       {
-        _doc = doc;
-        _creapp = doc.Application.Create;
+        _uiapp = uiapp;
+        _creapp = _uiapp.Application.Create;
       }
     }

@@ -67,9 +93,11 @@ namespace RoomEditorApp
     /// Update a piece of furniture.
     /// Return true if anything was changed.
     /// </summary>
-    bool UpdateBimFurniture(
+    bool UpdateBimFurniture(
       DbFurniture f )
     {
+      Document doc = _uiapp.ActiveUIDocument.Document;
+
@@ -82,7 +110,7 @@ namespace RoomEditorApp

-      Element e = _doc.GetElement( f.Id );
+      Element e = doc.GetElement( f.Id );

@@ -131,24 +159,24 @@ namespace RoomEditorApp
-        using( Transaction tx = new Transaction(
-          _doc ) )
+        using( Transaction tx = new Transaction(
+          doc ) )

-            ElementTransformUtils.MoveElement(
-              _doc, e.Id, translation );
+            ElementTransformUtils.MoveElement(
+              doc, e.Id, translation );

-            ElementTransformUtils.RotateElement(
-              _doc, e.Id, axis, rotation );
+            ElementTransformUtils.RotateElement(
+              doc, e.Id, axis, rotation );

@@ -165,10 +193,12 @@ namespace RoomEditorApp
     {
       using( JtTimer pt = new JtTimer( "UpdateBim" ) )
       {
+        Document doc = _uiapp.ActiveUIDocument.Document;
+
         // Retrieve all room unique ids in model:

         FilteredElementCollector rooms
-          = new FilteredElementCollector( _doc )
+          = new FilteredElementCollector( doc )
             .OfClass( typeof( SpatialElement ) )
             .OfCategory( BuiltInCategory.OST_Rooms );

@@ -225,5 +255,133 @@ namespace RoomEditorApp

+    public void Execute( UIApplication a )
+    {
+      UpdateBim();
+    }
+
+    public string GetName()
+    {
+      return string.Format(
+        "{0} DbUpdater", App.Caption );
+    }
+
+    /// <summary>
+    /// Count total number of checks for
+    /// database updates made so far.
+    /// </summary>
+    static int _nLoopCount = 0;
+
+    /// <summary>
+    /// Count total number of checks for
+    /// database updates made so far.
+    /// </summary>
+    static int _nCheckCount = 0;
+
+    /// <summary>
+    /// Count total number of database
+    /// updates requested so far.
+    /// </summary>
+    static int _nUpdatesRequested = 0;
+
+    /// <summary>
+    /// Wait far a moment before requerying database.
+    /// </summary>
+    //static Stopwatch _stopwatch = null;
+
+    /// <summary>
+    /// Number of milliseconds to wait and relinquish
+    /// CPU control before next check for pending
+    /// database updates.
+    /// </summary>
+    const int _timeout = 100;
+
+    /// <summary>
+    /// Repeatedly check database status and raise
+    /// external event when updates are pending.
+    /// Relinquish control and wait for timeout
+    /// period between each attempt. Run in a
+    /// separate thread.
+    /// </summary>
+    static void CheckForPendingDatabaseChanges()
+    {
+      while( true )
+      {
+        ++_nLoopCount;
+
+        Debug.Assert( null != _event,
+        "expected non-null external event" );
+
+        if( _event.IsPending )
+        {
+          Util.Log( string.Format(
+            "CheckForPendingDatabaseChanges loop {0} - "
+            + "database update event is pending",
+            _nLoopCount ) );
+        }
+        else
+        {
+          using( JtTimer pt = new JtTimer(
+            "CheckForPendingDatabaseChanges" ) )
+          {
+            ++_nCheckCount;
+
+            Util.Log( string.Format(
+              "CheckForPendingDatabaseChanges loop {0} - "
+              + "check for changes {1}",
+              _nLoopCount, _nCheckCount ) );
+
+            RoomEditorDb rdb = new RoomEditorDb();
+
+            if( rdb.LastSequenceNumberChanged(
+              DbUpdater.LastSequence ) )
+            {
+              _event.Raise();
+
+              ++_nUpdatesRequested;
+
+              Util.Log( string.Format(
+                "database update pending event raised {0} times",
+                _nUpdatesRequested ) );
+            }
+          }
+        }
+
+        // Wait a moment and relinquish control before
+        // next check for pending database updates.
+
+        Thread.Sleep( _timeout );
+      }
+    }
+
+    public static void ToggleSubscription(
+      UIApplication uiapp )
+    {
+      _event = App.ToggleSubscription(
+        new DbUpdater( uiapp ) );
+
+      if( null == _event )
+      {
+        _thread.Abort();
+        _thread = null;
+      }
+      else
+      {
+        // Start new thread to regularly check
+        // database status and raise external event
+        // when updates are pending.
+
+        _thread = new Thread(
+          CheckForPendingDatabaseChanges );
+
+        _thread.Start();
+      }
+    }