Stop an app designer callback thread like the debugger does?

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Ken 2024-4-21

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评论： Walter Roberson 2024-4-23

It is evident that whenever the user clicks a button in an app designer app, the callback is called in a new thread. The debugger takes advantage of this, the red "stop" square button obviously terminates the currently running callback thread. Very clean, very effective.

Now, is there any way that I as a programmer can do the same thing? Here's my problem. I'm building a scientific analysis program, and the users have a lot of options. The wrong options combined with the wrong data can produce a VERY long-running computation, and the user has no good way of getting out of it. Indeed, my users are tending to click extra buttons, setting yet more computations in motion, and it ends up in a meltdown.

So, can I kill the thread, or do I just have to put checkpoints in all my long-running code and check for a stop variable?

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Answer 1

Walter Roberson 2024-4-21

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"It is evident that whenever the user clicks a button in an app designer app, the callback is called in a new thread."

No, the current thread is interrupted, pushing the location onto the stack, and the callback is run in the same thread. At the end of execution of the callback, the old information is popped and execution is resumed.

The entire execution is on a single thread, unless background pool is used or unless one of the facilities of the Parallel Computing Toolbox are used. (Well, graphics runs on a seperate thread.)

(The single thread used is distinct from threads that manage the user interface.)

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Ken 2024-4-22

编辑：Ken 2024-4-22

在 MATLAB Online 中打开

Yes, but our observation has been that if the user presses a second button before the first one has completed its long running execution, things happen concurrently.

Furthermore, the following happens:I press a button, the callback starts executing, and lands in the debugger at a breakpoint. Without resolving that, I press a second button, its callback starts executing, and lands in the debugger at a breakpoint. I find that I have to press the red square Stop button twice to clear it.Without multiple threads, how could this be?

Here is the core of a very simple app which I just tested. It just contains two buttons. The calculate button displays a series of numbers. It takes a few seconds. If, while that is going, I press the hello button, the word "hello" appears in the middle of the output. This can be repeated multiple times. If this is not a demonstration of multi-threaded architecture, please tell me what is actually going on behind the scenes! It sure seems like it!

methods (Access = public)
    function calculate(app, p)  
        sum = 0;
        for i = 1:1000000000
            sum = sum + i^p;
            test = mod(i,10000000);
            if test == 0
                disp(i/10000000)
                drawnow;
            end
        end
    end
end
% Callbacks that handle component events
methods (Access = private)
    % Button pushed function: calculateButton
    function calculateButtonPushed(app, event)
        app.calculate(2)
    end
    
    % Button pushed function: helloButton
    function helloButtonPushed(app, event)
        disp("hello")
    end
end