Function not returning updated struct

Hello,

My code passes a struct to a function that builds a gui to allow the user to modify the default values of variables stored in the struct. The struct should then be passed back to the main code. This last stage is not working correctly - the struct in the main code retains the original values. I've tested the code by writing a simpler version without the gui and this seems to work OK. The simple version is:

clear
clc

X.a = 1
X.b = 2

X_out = testfunc(X)

X_out.a
X_out.b

function X_new=testfunc(X_in)
X_new = X_in

X_new.a = 3
X_new.b = 5

ok_fun

    function ok_fun
        X_new.a = 7
        X_new.b = 9
    end

end

The nested function is also there just to replicate the content of my real code. My real code is below, although I've stripped a great deal of it out to shorten it. Changing the value of a in the gui, then pressing OK, should pass the updated variable back to the calling code.

clear
clc

S.a = 1.134

% Open dialogue to get inputs
[not_OK, S_out]=UserInput(S);
S_out.a

function [not_OK, S_new]=UserInput(S_in)
    S_new = S_in;
    % Get the screen size
    screenSize = get(0, 'ScreenSize');
    screenWidth = screenSize(3);
    screenHeight = screenSize(4);
    topOffset = 50;
    rightOffset = 50;
    
    not_OK = 1;
    
    % Set figure size
    figWidth = 700;  % Default MATLAB figure width is 560
    figHeight = 650; % Default MATLAB figure height is 420
    
    hfig = figure('Menu','none',...
        'Position',[screenWidth-figWidth-rightOffset, screenHeight-figHeight-topOffset, figWidth, figHeight]);
    test_list = {'STP','SIN','LFR'};
    fcnList = {@calc_axlepercentage, @calc_rollaxis, @calc_cg2raheight};
    filePath = pwd; %mfilename('fullpath');
    
    % Set control positions and sizes
    tLeft = 20;
    eLeft = 260;
    tWidth = eLeft - tLeft;
    eWidth = 50;
    teHeight = 20;
    tLeft2 = tLeft + tWidth + eWidth + 5 + eWidth + 10; %360;
    eLeft2 = tLeft2 + eLeft - tLeft;
    topHeight = figHeight - 60;
    
    % Create GUI
    uicontrol('Style', 'text', 'String', 'CG to Fr Axle [m]', 'Parent',hfig, ...
        'Position', [tLeft, topHeight-(9.*25), tWidth, teHeight],'HorizontalAlignment','left');
    edit_a = uicontrol('Style', 'edit', 'String', S_new.a, 'Parent',hfig, ...
        'Position', [eLeft, topHeight-(9.*25), eWidth, teHeight]);
    
    uicontrol('Style', 'pushbutton', 'String', 'OK', ...
        'Parent',hfig, ...
        'Position', [4*figWidth/6, topHeight-(23.*25), figWidth/6, 2*teHeight],...
        'Callback',@ok_fun);
    
    
    function ok_fun(~,~)
      not_OK = 0;
      S_new.a = str2double(edit_a.String)
      delete(hfig)
    end

end

Can you help with why S_out is not receiving the changed value of a?

Thanks.

[Stephen23]

"Can you help with why S_out is not receiving the changed value of a?"

GUIs are fundamentally asynchronous event-based code, but you have not really taken that into account. As Walter Roberson recommended, adding WAITFOR after setting up the UICONTROL might give the behavior that you expect:

https://www.mathworks.com/help/matlab/ref/waitfor.html

But that depends on what you expect: do you want the user to be able to update the structure exactly one time, or repeatedly?

[Simon Aldworth]

Thanks. Adding WaitFor was the solution for me. I only want the structure updated the one time.

Answer 1

[Walter Roberson]

When you create uicontrol, they are created and displayed (if appropriate), and then flow immediately continues onward without waiting. So you call UserInput which creates uicontrols and displays them, but then function UserInput ends, leaving the uicontrol on the screen and returning appropriate output.

The user is not given an opportunity to interact with the uicontrol() until one of the following happens:

• pause() is called
• drawnow() is called
• waitfor() or uiwait() is called
• figure() is called
• keyboard() is called, or control returns to the keyboard

In your situation, flow passes through the UserInput() call and UserInput() returns, the S_out.a happens, and then the flow of execution ends, returning MATLAB to the keyboard -- and finally user interaction is permitted.

Thus, the S_out.a happens before the uicontrol() pushbutton Callback is executed. The update to S_new.a has not happened yet at the time the S_out.a occurs.

When the user presses the pushbutton, tthe nested S_new is updated, affectting the variable stored inside UserInput... but UserInput has already returned.

If you want the pushbutton to have a chance to execute, you should probably call waitfor after creating the uicontrol pushbotton.

Remember to delete or deactivate the pushbotton after it has been clicked, as otherwise it will hang around and uselessly accumulate changes to S_new with no way to get at those changed versions...