Visual Basic File API Routines
FindFirstChangeNotification: Create a 'Watched' Folder
     
Posted:   Saturday November 22, 1997
Updated:   Monday December 26, 2011
     
Applies to:   VB4-32, VB5, VB6
Developed with:   VB4-32, Windows 95
OS restrictions:   None
Author:   VBnet - Randy Birch
     

Related:  

SHChangeNotifyRegister: Receive Shell Change Notifications
     
 Prerequisites
A folder you can add and delete files from.

Some applications need to know when the contents of a folder have changed. While polling the folder using a timer can provide the desired results, the Windows API exposes three functions to deal with this directly.

I was first directly exposed to 'watched directories' with PageMaker 6, which included a 'lite' version of the Adobe Distiller used to turn PageMaker documents into Adobe PDF files. The premise was simple ... the distiller ran as a background task, watching for files added to a specified folder. When the folder changed, the Distiller app became active and created the PDF file. This was the premise on which this code sample was designed.

The code presented here is coded to react only to the first file in the watched folder (via the WatchChangeAction sub. I leave it to the developer to add the full functionality that their app may require.
 BAS Module Code
None.

 Form Code
Start a new project, and to the form add three command buttons (Command1, Command2 and Command3), a Listbox control (List1), and two labels (Label1 and Label2). Label1 is just the header above the listbox and can be entered as indicated in the illustration. Label2 is set during program run. Add the following code to the form:

Option Explicit
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Copyright 1996-2011 VBnet/Randy Birch, All Rights Reserved.
' Some pages may also contain other copyrights by the author.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
' Distribution: You can freely use this code in your own
'               applications, but you may not reproduce 
'               or publish this code on any web site,
'               online service, or distribute as source 
'               on any media without express permission.
''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''''
Dim hChangeHandle As Long
Dim hWatched As Long
Dim terminateFlag As Long
         
Private Const INFINITE As Long = &HFFFFFFFF
Private Const FILE_NOTIFY_CHANGE_FILE_NAME As Long = &H1
Private Const FILE_NOTIFY_CHANGE_DIR_NAME As Long = &H2
Private Const FILE_NOTIFY_CHANGE_ATTRIBUTES As Long = &H4
Private Const FILE_NOTIFY_CHANGE_SIZE As Long = &H8
Private Const FILE_NOTIFY_CHANGE_LAST_WRITE As Long = &H10
Private Const FILE_NOTIFY_CHANGE_LAST_ACCESS As Long = &H20
Private Const FILE_NOTIFY_CHANGE_CREATION As Long = &H40
Private Const FILE_NOTIFY_CHANGE_SECURITY As Long = &H100
Private Const FILE_NOTIFY_FLAGS = FILE_NOTIFY_CHANGE_ATTRIBUTES Or _
                                 FILE_NOTIFY_CHANGE_FILE_NAME Or _
                                 FILE_NOTIFY_CHANGE_LAST_WRITE

Private Declare Function FindFirstChangeNotification Lib "kernel32" _
    Alias "FindFirstChangeNotificationA" _
   (ByVal lpPathName As String, _
    ByVal bWatchSubtree As Long, _
    ByVal dwNotifyFilter As Long) As Long

Private Declare Function FindCloseChangeNotification Lib "kernel32" _
   (ByVal hChangeHandle As Long) As Long

Private Declare Function FindNextChangeNotification Lib "kernel32" _
   (ByVal hChangeHandle As Long) As Long

Private Declare Function WaitForSingleObject Lib "kernel32" _
   (ByVal hHandle As Long, ByVal dwMilliseconds As Long) As Long

Private Const WAIT_OBJECT_0 As Long = &H0
Private Const WAIT_ABANDONED As Long = &H80
Private Const WAIT_IO_COMPLETION As Long = &HC0
Private Const WAIT_TIMEOUT As Long = &H102
Private Const STATUS_PENDING As Long = &H103



Private Sub Form_Load()

   Label2.Caption = "Press 'Begin Watch'"
   
End Sub


Private Sub Command1_Click()

   Dim watchPath As String
   Dim watchStatus As Long
   
   watchPath = "d:\dummy"
   terminateFlag = False
   Command1.Enabled = False
   
   Label2.Caption = "Using Explorer and Notepad, create, modify, rename, delete or " _
                    "change the attributes of a text file in the watched directory.""

  'get the first file text attributes to the listbox (if any)
   WatchChangeAction watchPath
   
  'show a msgbox to indicate the watch is starting
   MsgBox "Beginning watching of folder " & watchPath & " .. press OK"
   
  'create a watched directory
   hWatched = WatchCreate(watchPath, FILE_NOTIFY_FLAGS)
   
  'poll the watched folder
   watchStatus = WatchDirectory(hWatched, 100)
   
  'if WatchDirectory exited with watchStatus = 0,
  'then there was a change in the folder.
   If watchStatus = 0 Then
   
      'update the listbox for the first file found in the
      'folder and indicate a change took place.
       WatchChangeAction watchPath
       
       MsgBox "The watched directory has been changed.  Resuming watch..."
       
      '(perform actions)
      'this is where you'd actually put code to perform a
      'task based on the folder changing.
       
      'now go into a second loop, this time calling the
      'FindNextChangeNotification API, again exiting if
      'watchStatus indicates the terminate flag was set
       Do
         watchStatus = WatchResume(hWatched, 100)
         
         If watchStatus = -1 Then
         
           'watchStatus must have exited with the terminate flag
            MsgBox "Watching has been terminated for " & watchPath
         
         Else
          
            WatchChangeAction watchPath
            MsgBox "The watched directory has been changed again."
              
              '(perform actions)
              'this is where you'd actually put code to perform a
              'task based on the folder changing.
               
         End If
         
       Loop While watchStatus = 0
   
   
   Else
     'watchStatus must have exited with the terminate flag
      MsgBox "Watching has been terminated for " & watchPath
   
   End If
   
End Sub


Private Sub Command2_Click()

  'clean up by deleting the handle to the watched directory
   Call WatchDelete(hWatched)
   hWatched = 0
      
   Command1.Enabled = True
   Label2.Caption = "Press 'Begin Watch'"

End Sub


Private Sub Command3_Click()

   If hWatched > 0 Then Call WatchDelete(hWatched)
   Unload Me

End Sub


Private Function WatchCreate(lpPathName As String, flags As Long) As Long

  'FindFirstChangeNotification members:
  '
  '  lpPathName: folder to watch
  '  bWatchSubtree:
  '     True = watch specified folder and its sub folders
  '     False = watch the specified folder only
  '  flags: OR'd combination of the FILE_NOTIFY_ flags to apply
  
   WatchCreate = FindFirstChangeNotification(lpPathName, False, flags)

End Function


Private Sub WatchDelete(hWatched As Long)
    
   terminateFlag = True
   DoEvents

   Call FindCloseChangeNotification(hWatched)
 
End Sub


Private Function WatchDirectory(hWatched As Long, interval As Long) As Long

  'Poll the watched folder.
  'The Do..Loop will exit when:
  '   r = 0, indicating a change has occurred
  '   terminateFlag = True, set by the WatchDelete routine
  
   Dim r As Long
   
   Do
   
      r = WaitForSingleObject(hWatched, interval)
      DoEvents
   
   Loop While r <> 0 And terminateFlag = False
   
   WatchDirectory = r
   
End Function


Private Function WatchResume(hWatched As Long, interval) As Boolean

   Dim r As Long
   
   r = FindNextChangeNotification(hWatched)
   
   Do
      
      r = WaitForSingleObject(hWatched, interval)
      DoEvents
   
   Loop While r <> 0 And terminateFlag = False
   
   WatchResume = r
   
End Function


Private Sub WatchChangeAction(fPath As String)

   Dim fName As String
   
   With List1
   
      .Clear

      fName = Dir(fPath & "\" & "*.txt")
   
      If Len(fName) > 0 Then
   
         .AddItem "path: " & vbTab & fPath
         .AddItem "file: " & vbTab & fName
         .AddItem "size: " & vbTab & FileLen(fPath & "\" & fName)
         .AddItem "attr: " & vbTab & GetAttr(fPath & "\" & fName)
   
      End If
   End With

End Sub
 Comments
Run the app, and press 'Begin'. After closing the MsgBox, launch notepad and create a file in the folder you specified as the watched directory. As soon as the file is saved, the 'changed' MsgBox should appear, and the file specs form the WatchChangeAction sub will populate the listbox. Make modifications to the file or its attributes, and save again. Each change will fire the API.

As mentioned above, this demo is coded only to take the first file returned by the Dir() function in WatchChangeAction .. you will need to customize this routine to achieve the effect you desire.

 
 

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