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Using AppDesigner with FileSystemWatcher

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Darrell
Darrell on 29 Jun 2020
Answered: Geoff Hayes on 30 Jun 2020
I am trying to create a MATLAB app that uses FileSystemWatcher to call an event handler each time a specific file has changed--using version 2018b. I have created the following two functions in Code View using the appdesigner; however the app never calls 'eventhandlerChanged' function. I've spent a lot of time trying to get this to work, so any help would be greatly appreciated.
% Button pushed function: TestFSWButton
function AddFSW(app, event)
fileObj = System.IO.FileSystemWatcher('D:\Matlab\Work');
fileObj.Filter = 'test.csv';
fileObj.EnableRaisingEvents = true;
addlistener(fileObj,'Changed',@app.eventhandlerChanged);
end
function eventhandlerChanged(app,evt)
tic;
toc;
end

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Geoff Hayes
Geoff Hayes on 29 Jun 2020
Darrell - from Create event listener bound to event source it shows that the call to addlistener returns an object. I wonder if you need to make that object a member of your app? (i.e. assign the result of this call to a member within your App class.) Because if you don't do this, then the local object - created within the AddFSW method - would be destroyed as the method completes and you would no longer have an object listening for changes to your file. (?)
Darrell
Darrell on 29 Jun 2020
Geoff,
That worked! Below is the final code.
properties (Access = private)
myFSW % FileSystemWatcher object
end
function eventhandlerChanged(app,~,~)
% do stuff
disp('Successful!');
end
% Button pushed function: TestButton
function AddFSW(app, event)
app.myFSW = System.IO.FileSystemWatcher('D:\Matlab\Work');
app.myFSW.Filter = 'test.csv';
app.myFSW.EnableRaisingEvents = true;
addlistener(app.myFSW,'Changed',@app.eventhandlerChanged);
end
Note. If you modify the test file using Notepad the method will fire twice; but if you are watching for an actual file it all works correctly. Thank you!

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Accepted Answer

Geoff Hayes
Geoff Hayes on 30 Jun 2020
Summary of issue: from Create event listener bound to event source it shows that the call to addlistener returns an object. In the above code, this object is local to the AddFSW method and so would be destroyed when the method "completes" which would mean that we no longer have an object listening for changes to the file. The fix is to make the listener object a member of the app so that it exists for the lifetime of the app (or until removed or deleted if needed).

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