RegisterWaitForSingleObject To Watch a Directory using RealBasic

Question

I'm trying to watch a directory for changes using the FindFirstChangeNotification function. This works if I take the handle returned by FindFirstChangeNotification and stuff it into WaitForSingleObject. The problem is that WaitForSingleObject blocks the entire application until it returns.

So, I looked around and it seems that RegisterWaitForSingleObject was the way to go:

Sub monitorDir(dir As FolderItem)
  Declare Function FindFirstChangeNotificationW Lib "Kernel32" (dirPath As WString, watchChildren As Boolean, eventTypeFilter As Integer) As Integer
  Declare Function RegisterWaitForSingleObject Lib "Kernel32" (ByRef waiterHWND As Integer, HWND As Integer, cllbck As Ptr, _
  context As Integer, wait As Integer, flags As Integer) As Integer
  Dim allFilters As Integer = &h00000001 Or &h00000002 Or &h00000004 Or &h00000008 Or &h00000010_
      Or &h00000100
  Dim monitorHandle As Integer = FindFirstChangeNotificationW(dir.AbsolutePath, True, allFilters)
  If monitorHandle <> 0 Then
  Call RegisterWaitForSingleObject(myCallbackHWND, monitorHandle, AddressOf MyCallbackFn, 0, &hFFFFFFFF, 0)
End Sub

This appears to work as the application continues to execute normally. However, as soon as the MyCallbackFn is called (that is, when a change occurs in the directory) things get... weird. Applications start crashing or locking up starting with Process Explorer and Windows Explorer. I have to log out of Windows in order to restore things.

At the moment, all that MyCallbackFn does is this:

Sub MyCallbackFn()
  Declare Function UnregisterWaitEx Lib "Kernel32" (waitHWND As Integer, eventHandle As Integer) As Integer
  Call UnregisterWaitEx(myCallbackHWND, 0)
  MsgBox("Change Detected")
End Sub

Am I barking up the wrong tree by using RegisterWaitForSingleObject, have I used it wrongly, or there some limitation in RealBasic which causes callbacks to implode the system?

Answer 1

The callback function you register in RegisterWaitForSingleObject() is called on another thread (http://msdn.microsoft.com/en-us/library/ms685061.aspx):

The callback routine is executed by a worker thread when the object's state becomes signaled or the time-out interval elapses.

I don't know anything about RealBasic's threading support, but at the very least GUI operations on Windows typically need to occur on a specific thread, not just any old worker thread. So the call to MsgBox() on that worker thread is probably a problem.

A simple thing you can try is to call PostMessage() (or whatever the RealBasic equivalent is) to post a custom message to your window message queue that your application can respond to (for example by calling MsgBox()).

Answer 2

Maybe not related, but where is MyCallBackFn() declared? If it is an instance method you should use WeakAddressOf instead of AddressOf.