Clearing undo manager's history while keeping document marked as edited

gistfile1.m

- (void)clearUndoHistoryOfDoc:(NSDocument *)doc
{
    NSUndoManager *undoManager = [doc undoManager];
    
    // Post a checkpoint first so any pending registrations
    // (e.g. from Core Data) are committed
    [[NSNotificationCenter defaultCenter]
       postNotificationName:NSUndoManagerCheckpointNotification
       object:undoManager];
    
    // Now it's safe to clear the stack
    [undoManager removeAllActions];
    
    // Autosave-in-place sees the clearing of the undo stack as
    // an indication that the doc is saved. Force it to see the
    // doc as edited instead, with no way to undo the change
    [doc updateChangeCount:NSChangeDone];
}