How to Return value from Callback function to the main function?

function PID_Calc
f=figure('position',[10,10,1000,1000]);
hget1 = uicontrol(...
'style','pushbutton',....
'string','G(s)',.....
'position',[200,500,100,50],....
'callback',{@getgsbutton_callback});
f.visible='on';
char x;
uiwait();
display(y);
end
    function x = getgsbutton_callback(source,eventdata)   
    answer = inputdlg('What is the Numerator equation'); 
    display(answer);
    x = str2double(answer{:});
    display(x);
    %display(y);
    return
    end

This is a code i have written to get value from a user using GUI. Please guide me on how to get the user input from the call back function to the main function.

Answer 1

[Walter Roberson]

It is not possible for a uicontrol callback to return a value. See though http://matlab.wikia.com/wiki/FAQ#How_can_I_share_data_between_callback_functions_in_my_GUI.28s.29.3F

[Áron Molnár]

But as a workaround, - I'm not sure how elegant and fault tolerant this solution is - you can use a graphical object's UserData.

In your example: The callback function has a default input, the source. In your case this is an uibutton, which has a UserData property. This can be any MATLAB data type or format (like even class object too).

https://uk.mathworks.com/help/daq/ref/userdata.html

Let's se in case of your program:

function PID_Calc
    f=figure('position',[10,10,1000,1000]);
    hget1 = uicontrol(...
    'style','pushbutton',....
    'string','G(s)',.....
    'position',[200,500,100,50],....
    'callback',{@getgsbutton_callback});
    f.Visible='on';
    
    %Wait until you set the x
    waitfor(~isempty(hget1.UserData))
    
    %Evaluate the "return value" - REMEMBER! This is evaluated just once, as
    %the program runs, so even if you change the x in the G(s) later, the
    %returnValue variable will not evaluated again
    returnValue=hget1.UserData;
    
    %This is miscal. It is here just tor prove, we have the "return value"
    show=uicontrol('style','pushbutton','String',...
    "Show return value","Position",[400 500 100 50],...
    'callback',{@show,returnValue});%Hence it is not evaluated again, 
    %the first x will be passed always to it


end
    function getgsbutton_callback(source,eventdata)   
        answer = inputdlg('What is the Numerator equation'); 
        display(answer);
        x = str2double(answer{:});
        
        %Use the source object's user data
        source.UserData=x;   
        return;
    end

    function show(src,event,returnValue)
        disp(returnValue)
    end

But let's say you want to be able to display the "return value" every time. You can do that if you change the show function:

    function show(src,event,returnValue)
        %The returnValue is not needed here, but if you remove, remove from
        %the {@show,returnValue} part too
        disp(src.Parent.Children(2).UserData);
    end
    

In this second method, you can reach from the show callback to the first button's user data and use that.

Answer 2

[Benjamin Blacklock]

It's bad practice but you can use global variables to communicate between the functions.