Receive notification of file creation in VBA without polling - vba

Question

I am writing a program that integrates with a ScanSnap scanner. ScanSnap scanners do not support TWAIN. Once a document is scanned it is automatically saved to a PDF.
I want to monitor the directory where the files will be saved and take some action when the file appears (and is done being written to). A simple approach is to use the MS Access form Timer event and check for an existing file at some small interval of time.
Is there a better alternative via Windows Messaging, the FileSystemObject, or some Windows API function that supports callbacks?

Answer 1

This provides a native WinAP + VB/VBA way of doing the job, I expect:
http://books.google.com/books?id=46toCUvklIQC&pg=PA757&lpg=PA757&dq=windows+api+monitor+directory+changes+vba&source=bl&ots=jmMY4sJFK4&sig=KCB6B_soEA9_JzjlhyNZvSC91w4&hl=en&sa=X&ei=cUAMUsDzOe3iyAHu8YGwAg&ved=0CFIQ6AEwBw#v=onepage&q=windows%20api%20monitor%20directory%20changes%20vba&f=false

Answer 2

Nothing inside Excel.
You can create another application that monitors the file system, and executes the Excel macro, opening the workbook if required, opening Excel if required.

Answer 3

#Steve effectively answered the question I asked. What I should have asked is how to monitor file system changes in a thread separate from the MS Access UI thread. And the simple answer to that question is that VBA does not support multi-threading in Office applications.
There are a variety of workarounds that generally involve calling an external COM library or integrating with an external application. I decided none of those was very appealing and instead decided to implement the solution in VB.Net using the FileSystemWatcher class.

Answer 4

Not sure if this really solves your Problem, but here is an approach using Excel VBA that helped me monitor a specific file within a specific Folder and execute certain actions (here: copy the file into another folder) if the file is modified and saved (i.e. when the file's timestamp changes):
Option Explicit
Const SourcePath = "C:\YourFolder\"
Const TargetPath = "C:\YourFolder\YourFolder_Changes\"
Const TargetFile = "YourFileName"
Private m_blnLooping As Boolean
Private Sub CommandButton1_Click()
Dim FSO As Scripting.FileSystemObject
Dim n, msg, dt, inttext As String
Dim file, files As Object
Dim d1, d2 As Date
Dim cnt As Integer
Dim wsshell
Application.ScreenUpdating = False
On Error Resume Next
Set FSO = CreateObject("Scripting.FileSystemObject")
Set files = FSO.GetFolder(SourcePath).files
Set wsshell = CreateObject("WScript.Shell")
msg = "FileWatcher started. Monitoring of " & TargetFile & " in progress."
cnt = 0
'Initialize: Loop through Folder content and get file date
For Each file In files
n = file.name
'Get Initial SaveDate of Target File
If n = TargetFile Then
d1 = file.DateLastModified
End If
Next file
m_blnLooping = True
inttext = wsshell.popup(msg, 2, "FileWatcher Ready", vbInformation)
'Message Box should close after 2 seconds automatically
Shell "C:\WINDOWS\explorer.exe """ & TargetPath & "", vbNormalFocus
'Open Windows Explorer and display Target Directory to see changes
Do While m_blnLooping
For Each file In files
n = file.name
If n = TargetFile Then
d2 = file.DateLastModified
If d2 > d1 Then
dt = Format(CStr(Now), "yyyy-mm-dd_hh-mm-ss")
'FSO.CopyFile (SourcePath & TargetFile), (TargetPath & Left(TargetFile, Len(TargetFile) - 4) & "_" & dt & ".txt"), True 'Option with file name extension
FSO.CopyFile (SourcePath & TargetFile), (TargetPath & TargetFile & "_" & dt), True 'Option without file name extension
cnt = cnt + 1
d1 = d2
End If
End If
Next file
'Application.Wait (Now() + CDate("00:00:02")) 'wait 2 seconds, then loop again
DoEvents
Loop
msg = "File " & TargetFile & " has been updated " & cnt & " times."
inttext = wsshell.popup(msg, 2, "FileWatcher Closed", vbInformation)
'Message Box should close after 2 seconds automatically
Application.ScreenUpdating = True
End Sub
Private Sub CommandButton2_Click()
m_blnLooping = False
End Sub
The procedure is activated via a CommandButton ("START") and loops through the speficied Folder (keeps watching the file) until another CommandButton ("STOP") is pressed. You may, however, need to adjust the code to monitor file creation instead of file changes (file.DateCreated instead of file.DateLastModified). The Code is just meant to provide you hint that might solve your Problem.