Listener being called twice

I created a listener to monitor changes to a file. When I make a change to that file the function that is called from the listener runs twice when it should run once. Here is the code for the listener. The function eventhandlerChanged just displays the word 'changed'.

fileObj = System.IO.FileSystemWatcher('c:\Temp')
fileObj.Filter = 'test.txt'
fileObj.EnableRaisingEvents = true
lh=addlistener(fileObj,'Changed',@eventhandlerChanged)

Answer 1

[per isakson]

Doc on System.IO.FileSystemWatcher says:

    The FileSystemWatcher documentation says that a simple file operation can 
    raise multiple events.

    To turn off the event handler, type: fileObj.EnableRaisingEvents = false;

[Victor Lim]

If I turn off the event handler then it won't find the event anymore, right? I've tried turning it off in the event handler function and turning it back on at the end of the function call but it doesn't work.

[per isakson]

I don't know! However, these words in the documentation indicate that what you see is expected under some circumstances. Furthermore, the documentation refers to FileSystemWatcher Class or similar.

[per isakson]

Did you read the documentation on

    >> fileObj.NotifyFilter
    ans = 
    FileName, DirectoryName, LastWrite

Answer 2

[Guillaume]

As per isakson says, this is a possible behaviour of the FileSystemWatcher. There's nothing you can do to stop the event being raised multiple time on a file modification.

Therefore, what you must do is make your event handler more resilient to this possibility. A fairly simple way would be to store the filename and timestamp of the call to your handler. If the filename is the same and the time elapsed between two calls is too short, simply not process again.

Another option is to implement some sort of queue of files to process, delayed by a few seconds. If in the queue the same file is present several times in a row, just collapse them all into one event.