Listener being called twice

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Victor Lim
Victor Lim 2015-2-18
编辑： per isakson 2015-2-19
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)

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回答（2 个）

per isakson
per isakson 2015-2-18
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;
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per isakson
per isakson 2015-2-18
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
per isakson 2015-2-19
编辑：per isakson 2015-2-19
Did you read the documentation on
>> fileObj.NotifyFilter
ans =
FileName, DirectoryName, LastWrite

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Guillaume
Guillaume 2015-2-19
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.

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