Error "Invalid or deleted object." when close a GUI in App Designer

Hi, I tried to create an app simply to display capture from a webcam. The main functions are like below.

      properties (Access = private)
           KeepRunning = 1;
           cam = webcam('USB Video Device');  
      end

      methods (Access = private)

      % Code that executes after component creation
            function startupFcn(app)
                app.cam.Resolution = '640x480';
                while 1
                    rgbImage = snapshot(app.cam);
                    imshow(rgbImage,'Parent',app.UIAxes);
                    drawnow;
                end          
            end

            % Close request function: UIFigure
            function UIFigureCloseRequest(app, event)
                 delete(app.cam)
                 delete(app)
            end
        end

Everything works fine; however, as I close the GUI (using X button), this error popped up. Any ideas?

Invalid or deleted object.

Error in app_test/startupFcn (line 23)
                rgbImage = snapshot(app.cam);

Error in app_test (line 84)
            runStartupFcn(app, @startupFcn)

Answer 1

[Guillaume]

I'm not convinced of the wisdom of having a never ending loop in your startupFcn.

Anyway, the problem is simple when you close the app, your close request callback deletes app.cam making it an invalid handle. Your infinite loop in startupFcn is still running and tries to access the now deleted app.cam, hence the error.

The naive fix would be to change the

while 1

to

while isvalid(app.cam)

99% of the time, it'll fix the error. However, were the user to close the app just after the while has executed but before the snapshot line has executed, you'll still get the error. A foolproof way would be:

while true
  try
     rgbImage = snapshot(app.cam);
  catch
     %error occured, probably app.cam was deleted. Quit loop
      break;
  end
  imshow(rgbImage,'Parent',app.UIAxes);
  drawnow;    
end

[Trinh Nguyen]

@Guillaume

"I'm not convinced of the wisdom of having a never ending loop in your startupFcn.": Yeah, I'm aware of this but don't know how to fix it :D

Your solution works, and there are no errors in App Designer anymore. But there is error in Command Window:

Error using app_test/addlistener
Invalid or deleted object.

Error in appdesigner.internal.model.AppModel/onCleanupRunningAppReference (line 641)
                addlistener(obj.RunningApp, 'ObjectBeingDestroyed', @(src,
                e)cleanRunningAppReference(obj));
Error in appdesigner.internal.model.AppModel.runAppHelper (line 738)
            currentAppModel.onCleanupRunningAppReference();
Error in
appdesigner.internal.model.AppModel>@()appdesigner.internal.model.AppModel.runAppHelper(obj,appArguments)
(line 499)
            funcHandle = @()appdesigner.internal.model.AppModel.runAppHelper(obj, appArguments);

I assume that this error is from the 'catch', right?

[Guillaume]

I assume that this error is from the 'catch', right?

I don't think so. I don't know the App Designer enough to say for sure but I suspect that it's because you have that infinite loop in the startup function that you get this error. It's certainly nothing to do with the catch. The code that is causing the error looks like code that should run when the App window is created. However, since you only return from the startup function when the app is closed, that code only runs after the app is destroyed, by which point most objects no longer exist.

Yeah, I'm aware of this but don't know how to fix it

First thing that comes to mind is to use a timer at the same or slightly faster rate than the frame rate of your camera.

[Trinh Nguyen]

I fixed it. I created a public function which includes the view-capture code above. This would avoid the infinite loop in the startup function. Thank you for your help :D