Fix CodeSnip crash after resume from hiberanation.

It seems that the problem is that, sometimes (not always) Windows recreates the tree view displayed in the overview pane and all its nodes. The tree view nodes are custom classes that have a property that reference an `IView` instance relating to the displayed items. Unfortunately when Windows recreates the nodes the `IView` property is set to `nil`.

This explains the nil `IView` references that have been causing the access violation.

The solution used in the fix is to handle the Windows messages sent when the computer hibernates and resumes. On hibernation the state of the tree view is recorded. On restoration we assume that the tree view is corrupted and so forcibly rebuild it and restore the saved state.

There is a problem thought. The message we handle is issued twice after resuming from hibernation. There is no easy way to tell which message has been issued. Therefore the tree view is rebuilt twice. There is not much performance penalty to this, so we can let it go. The potential problem is that if the tree view is recreated it happens after the 1st message and before the 2nd. Should, for example, the message only be triggered once then the bug will be back!

Even after all this it is possible that the program will redraw the tree view before the `IView` instances are restored. I've added code to the tree node custom drawing code to test if a node's `IView` instance is nil. This leads to some nodes not being drawn correctly. However, this doesn't matter because the tree view is forcibly redrawn again after the `IView` instances are restored.

All in all, I've not totally happy with this solution, which is more of a work around than a fix, but it's the best I can come up with without completely revising the overview frame code.

Fixes #70

Author: delphidabbler

Src/FmMain.pas

@@ -522,6 +522,11 @@   TMainForm = class(THelpAwareForm)
       ///  <summary>Object that manages favourites.</summary>
       fFavouritesMgr: TFavouritesManager;
 
+    ///  <summary>Handles the <c>WM_POWERBROADCAST</c> messages to detect and
+    ///  respond to hibernation messages.</summary>
+    ///  <remarks>This is necessary as part of the fix for an obscure bug. See
+    ///  https://github.com/delphidabbler/codesnip/issues/70</remarks>
+    procedure WMPowerBroadcast(var Msg: TMessage); message WM_POWERBROADCAST;
     ///  <summary>Displays view item given by TViewItemAction instance
     ///  referenced by Sender and adds to history list.</summary>
     procedure ActViewItemExecute(Sender: TObject);
@@ -1324,7 +1329,6 @@ procedure TMainForm.FormDestroy(Sender: TObject);
   // fStatusBarMgr MUST be nilled: otherwise it can be called after status bar
   // control has been freed and so cause AV when trying to use the control
   FreeAndNil(fStatusBarMgr);
-
 end;
 
 procedure TMainForm.FormResize(Sender: TObject);
@@ -1582,5 +1586,22 @@ procedure TMainForm.splitVertCanResize(Sender: TObject;
     Accept := False;
 end;
 
+procedure TMainForm.WMPowerBroadcast(var Msg: TMessage);
+begin
+  // Sometimes when the computer is resumed from hibernation the tree view in
+  // the overview frame is destroyed and recreated by Windows. Unfortunately the
+  // IView instances associated with the recreated tree nodes are lost.
+  // Attempting to read those (now nil) IView instances was resulting in an
+  // access violation.
+  case Msg.WParam of
+    PBT_APMSUSPEND:
+      // Get ready for isolation
+      fMainDisplayMgr.PrepareForHibernate;
+    PBT_APMPOWERSTATUSCHANGE:
+      // Restore from hibernation: ensure the IView instances are recreeated
+      fMainDisplayMgr.RestoreFromHibernation;
+  end;
+end;
+
 end.
 

Src/FrOverview.pas

@@ -86,6 +86,7 @@       TTVDraw = class(TSnippetsTVDraw)
             @return True if node is a section header, False if not.
           }
       end;
+
     var
       fTVDraw: TTVDraw;             // Object that renders tree view nodes
       fNotifier: INotifier;         // Notifies app of user initiated events
@@ -966,7 +967,12 @@ function TOverviewFrame.TTVDraw.IsSectionHeadNode(
   ViewItem: IView;  // view item represented by node
 begin
   ViewItem := (Node as TViewItemTreeNode).ViewItem;
-  Result := ViewItem.IsGrouping;
+  // Workaround for possibility that ViewItem might be nil when restarting after
+  // hibernation.
+  if Assigned(ViewItem) then
+    Result := ViewItem.IsGrouping
+  else
+    Result := False;
 end;
 
 function TOverviewFrame.TTVDraw.IsUserDefinedNode(
@@ -979,8 +985,12 @@ function TOverviewFrame.TTVDraw.IsUserDefinedNode(
   ViewItem: IView;  // view item represented by node
 begin
   ViewItem := (Node as TViewItemTreeNode).ViewItem;
-  // TODO -cBug: Exception reported as issue #70 seems to be triggered here
-  Result := ViewItem.IsUserDefined;
+  // Workaround for possibility that ViewItem might be nil when restarting after
+  // hibernation.
+  if Assigned(ViewItem) then
+    Result := ViewItem.IsUserDefined
+  else
+    Result := False;
 end;
 
 end.

Src/UMainDisplayMgr.pas

@@ -291,6 +291,23 @@   TMainDisplayMgr = class(TObject)
 
     /// <summary>Prepares display ready for database to be reloaded.</summary>
     procedure PrepareForDBReload;
+
+    ///  <summary>Gets the overview frame prepared for program hibernation.
+    ///  </summary>
+    ///  <remarks>Saves the overview tree view state ready for restoring after
+    ///  hibernation.</remarks>
+    procedure PrepareForHibernate;
+
+    ///  <summary>Restores the overview's tree view to have the correct IView
+    ///  instances after hibernation restores the previously saved state.
+    ///  </summary>
+    ///  <remarks>Sometimes, Windows quietly recreates the node of the tree view
+    ///  after resuming from hibernation, without restoring the associated IView
+    ///  instances, leading to access violations. This method should be called
+    ///  after resuming from hibernation to recreate the tree view with the
+    ///  correct IView instances.</remarks>
+    procedure RestoreFromHibernation;
+
   end;
 
 
@@ -566,6 +583,12 @@ procedure TMainDisplayMgr.PrepareForDBViewChange(View: IView);
   fPendingViewChange := True;
 end;
 
+procedure TMainDisplayMgr.PrepareForHibernate;
+begin
+  // simply save the state of the overview tree view ready for later restoration
+  (fOverviewMgr as IOverviewDisplayMgr).SaveTreeState;
+end;
+
 procedure TMainDisplayMgr.RedisplayOverview;
 begin
   (fOverviewMgr as IOverviewDisplayMgr).Display(Query.Selection, True);
@@ -593,6 +616,12 @@ procedure TMainDisplayMgr.ReStart;
   (fOverviewMgr as IOverviewDisplayMgr).Display(Query.Selection, True);
 end;
 
+procedure TMainDisplayMgr.RestoreFromHibernation;
+begin
+  (fOverviewMgr as IOverviewDisplayMgr).Display(Query.Selection, True);
+  (fOverviewMgr as IOverviewDisplayMgr).RestoreTreeState;
+end;
+
 procedure TMainDisplayMgr.SelectAll;
 begin
   // Only details pane supports text selection