app object not being passed to App GUI function, intermittently

In my App code, I am creating a timer object with a callback function.

The timer looks something like this:

app.tenMinuteTimer = timer('ExecutionMode', 'fixedRate', 'Period', 600, 'TimerFcn', @(~,~)app.createNewLogFile())
start(app.tenMinuteTimer);

The callback looks something like this:

function createNewLogFile(app)
    try
        % Explicit null checks
        if isempty(app) || ~isvalid(app)
            error('WARNING: App object is invalid in createNewLogFile context');
        end
    ...

I am seeing the error message intermittently with this code, but only intermittently (about ~5% of the time over long durations of running the App). I would really like to understand what could be causing this to happen intermittently, how to confirm a root cause, and/or how to do this in a more robust way altogether.

I believe that memory issues can be ruled out, or any other issues relating to long-duration testing. I can observe this problem within a relatively short test. I suspect it may have to do with task scheduling on the host running the App, since the App is doing multiple things.

Heavy apologies for editing this post multiple times.. I will not change it again from now.

[dpb]

Looks like your addition of new info removed some prior background? I think need more context for anybody who didn't see the original to be able to get on the moving train...

If this is something that can be run without requiring some specific piece of hardware, I'd suggest creating a minimum example that can reproduce the problem and submit this to Mathworks as an official support request at <Product Support Page>

[Corey]

Yes sorry, I did have to edit it once because I learned new information. I will edit it one last time to make it coherent and have all relevant information.

Thank you for your suggestion - wise idea.

[Steven Lord]

I am seeing the error message

Please show us all the text displayed in red in the Command Window (and if there are any warning messages displayed in orange, please show us those too.) The exact text may be useful and/or necessary to determine what's going on and how to avoid the warning and/or error.

[Corey]

Hi Steven, when I said that I see the error message, I meant the message that I created in the try/catch code that I wrote and you see above in the post. "error('WARNING: App object is invalid in createNewLogFile context');"

The code does not fail otherwise - the app continues running and this timer callback fails silently unless I put in that null check myself.

Answer 1

[Vedant Shah]

Hi @Corey,

An intermittent error occurs in MATLAB App Designer app because a timer is set to execute a callback “createNewLogFile” every 10 minutes. Intermittently, when the timer fires, the app object is either in the process of closing or has already been deleted, causing the callback to detect that the “app” object is invalid and it throws an error.

This happens because timers in MATLAB run independently from the main app thread and can attempt to execute their callbacks even as the app is shutting down. This issue is a classic race condition between the timer's execution and the app's lifecycle.

The solution is to ensure that you always stop and delete your timer when the app is closing. To achieve this, add the below code to your existing app designer code snippet:

function UIFigureCloseRequest(app, event) 
try 
    if isprop(app, 'tenMinuteTimer') && isa(app.tenMinuteTimer, 'timer') && isvalid(app.tenMinuteTimer) 
        stop(app.tenMinuteTimer); 
        delete(app.tenMinuteTimer); 
    end 
catch 
    disp("Error Occurred!!"); 
end  
delete(app) 
end 

By doing so, the timer is prevented from running after the app has been closed, which eliminates the possibility of the callback trying to access an invalid app object.

For more information, refer to the following documentation links:

• https://www.mathworks.com/help/releases/R2024a/matlab/ref/timer.html
• https://www.mathworks.com/help/releases/R2024a/matlab/ref/delete.html

[Corey]

Hi, thank you for your response but this is not the correct answer. I have tested this thoroughly and run the app timer with a fresh MATLAB instance, fresh app instance, not closing the app, etc. And the callback still fails.

Answer 2

[dpb]

app.tenMinuteTimer = timer('ExecutionMode', 'fixedRate', 'Period', 600, 'TimerFcn', @(~,~)app.createNewLogFile())

function createNewLogFile(app)
    try
        % Explicit null checks
        if isempty(app) || ~isvalid(app)
            error('WARNING: App object is invalid in createNewLogFile context');
        end
    ...

Your prototype function has no arguments in the line that assigns the callback function, but the function itself expects an argument. Those two are out of whack with each other.

I suspect if you change to

app.tenMinuteTimer=timer('ExecutionMode','fixedRate','Period', 600,'TimerFcn', @app.createNewLogFile);

function createNewLogFile(app,~)
  ...

the symptoms will disappear. I've never tried to use the tilde syntax in the assignment, so not sure about the syntax there for just the one argument, maybe

app.tenMinuteTimer=timer('ExecutionMode','fixedRate','Period', 600,'TimerFcn', @(x,~)app.createNewLogFile);

maybe?