GUI callback interrupts timer callback causing deadlock

Problem Context:

I am working on a project that has a GUI which is used to communicate with external hardware via a serial interface.

There are two types of communication with the external hardware in this project:

• logging: (example: ask the hardware for its current status) done in regular intervals using timer
• user-initiated: (example: make the hardware do something the user requests) done using a button callback function

It is essential that either type of communication does not get interrupted.

Problem:

1. When a logging function (which was started by a timer) is running, the button callback function is able to interrupt the logging function's execution.
2. In the button callback function, there is no way to pause execution of the button callback until the timer callback has finished its execution.

Things I have already tried:

• a) waitfor in the button callback: causes the button callback to freeze up
• b) pause in the button callback until the timer callback is done: does never goes back to the timer callback

Code example showcasing the issue:

classdef deadlock_gui < matlab.apps.AppBase

    % Properties that correspond to app components
    properties (Access = public)
        UIFigure  matlab.ui.Figure
        Button    matlab.ui.control.Button
        running
        t
    end

    % Callbacks that handle component events
    methods (Access = private)

        % Button pushed function: Button
        function ButtonPushed(app, event)
            disp('Button has been clicked');
            if app.running
                disp('Timer function is currently running');
                % waiting for the timer to stop running
                % attempts to solve the problem
                % a) 
                % waitfor(app, 'running', false);
                % b
                while app.running
                    pause(0.2);
                end
            end
            if app.running
                disp('Timer function is still running');
            end
        end
    end

    % Component initialization
    methods (Access = private)

        % Create UIFigure and components
        function createComponents(app)

            % Create UIFigure and hide until all components are created
            app.UIFigure = uifigure('Visible', 'off');
            app.UIFigure.Position = [100 100 411 268];
            app.UIFigure.Name = 'MATLAB App';

            % Create Button
            app.Button = uibutton(app.UIFigure, 'push');
            app.Button.ButtonPushedFcn = createCallbackFcn(app, @ButtonPushed, true);
            app.Button.Position = [158 124 100 22];

            % Show the figure after all components are created
            app.UIFigure.Visible = 'on';
        end
    end

    % App creation and deletion
    methods (Access = public)

        % Construct app
        function app = deadlock_gui

            % Create UIFigure and components
            createComponents(app)

            % Register the app with App Designer
            registerApp(app, app.UIFigure)

            if nargout == 0
                clear app
            end

            app.running = false;

            app.t = timer('TimerFcn', @(~, ~)app.timer_callback(), 'Period', 0.5, 'BusyMode','drop','ExecutionMode','fixedRate');
            
            start(app.t);
        end

        function timer_callback(app)
            app.running = true;
            % communicate with external device that takes a long time
            disp('start communicating with external device');
            pause(4);
            disp('stop communicating with external device');
            app.running = false;
        end

        % Code that executes before app deletion
        function delete(app)

            % Delete UIFigure when app is deleted
            delete(app.UIFigure);
            stop(app.t);
            delete(app.t)
        end
    end
end

To reproduce the error:

1. save the above code in a file called "deadlock_gui.m"
2. in the command window, run: "gui = deadlock_gui()"
3. hit the button in the GUI that pops up
4. To stop: hit CTRL-C, then run: "delete(gui)"

Callback trace will look something like this:

>> gui = deadlock_gui()
start communicating with external device
stop communicating with external device
start communicating with external device
Button has been clicked
Timer function is currently running

[chrisw23]

Since you drop your timer event (per setup) in case of still busy state, you could also stop the timer before you start the interactive (button) communication. Be sure the serial port is ready to receive your request, because this has to be handled seperately.

I think the simulated 'pause(4)' isn't really what's happening during your serial communication, which should be a series of send and receive, giving Matlab the chance to update other code parts. The pause cmd will block all.

I would also use appdesigner instead of writing the code as a m File, which gives you the full editor support. If it's about code difference review use Matlab's VisualDiff tool for mlapp files.

Answer 1

[Bruno Luong]

Checkout parfeval that can be used run both tasks in parallel (I would put the log in background)