MATLAB 2013a GUI saving handles fails in checkbox callback

Question

I'm making a GUI that reads out data from a measurement board via a serial interface. It consists of 8 channels and I want to be able to enable and disable the channels in the figure plot, but the state of my variable handles.channelsEnable isn't saved.

When opening the GUI: handles.channelEnable = [0;0;1;1;0;0;0;0]; While running the GUI I want to modify a handles.channelEnable. Inside the callback function of a checkbox it is changed, but not outside the function. I'm using guidata(hObject,handles) to save the changes made. Why aren't the changes made to handles.channelEnable saved?

    % --- Executes on button press in checkbox2.
    function checkbox2_Callback(hObject, eventdata, handles)
    % hObject    handle to checkbox2 (see GCBO)
    % eventdata  reserved - to be defined in a future version of MATLAB
    % handles    structure with handles and user data (see GUIDATA)

    % Hint: get(hObject,'Value') returns toggle state of checkbox2
        handles.channelEnable(2) = get(hObject,'Value');
        guidata(hObject, handles);
    end

Answer 1

You can use guidata to include a new handle in the handles variable, but whenever you want to use handles you must call guidata to obtain them. The variable handles as input variable of the callback won't have it included.

Answer 2

@YisasL's answer is the correct one, but I wanted to clarify it a bit.

When providing an input variable to a callback function, the variable that is passed when you invoke the callback is that variable as it exists when the callback is defined. You can see this with a simple example:

function testcode
handles.mainwindow = figure();

handles.button1 = uicontrol( ...
    'Style','pushbutton', ...
    'Units','normalized', ...
    'Position',[0.05 0.05 .30 .90], ...
    'String','Button1', ...
    'Callback',{@button1,handles} ...
    );
handles.button2 = uicontrol( ...
    'Style','pushbutton', ...
    'Units','normalized', ...
    'Position',[0.35 0.05 .30 .90], ...
    'String','Button2', ...
    'Callback',{@button2,handles} ...
    );
handles.button3 = uicontrol( ...
    'Style','pushbutton', ...
    'Units','normalized', ...
    'Position',[0.65 0.05 .30 .90], ...
    'String','Button3', ...
    'Callback',{@button3,handles} ...
    );
end

function button1(~,~,handles)
fieldnames(handles)
end

function button2(~,~,handles)
fieldnames(handles)
end

function button3(~,~,handles)
fieldnames(handles)
end

Push each button and look at the displayed output from fieldnames. You'll notice that button 1 only has mainwindow, button 2 has mainwindow and button1, and button3 has mainwindow, button1, and button2. As you've now noticed, no matter what changes you make elsewhere in your code, this result will remain constant.

It's an interesting quirk I noticed when I moved to programmatic GUIs rather than utilizing GUIDE. You generally don't notice with a GUIDE GUI because all of the initialization is taken care of in the background and the users don't tend to modify the handles structure. With a programmatic GUI you need to be conscious of the order in which you define your callbacks (define them after your handles structure is built).

The alternative is to utilize guidata like @YisasL said. Then you don't need to worry about passing variables to your callbacks.

Answer 3

I don't think I fully understand you.

Do you mean that I have to add guidata(hObject) to get the handles? So the code looks like this:

% --- Executes on button press in checkbox2.
function checkbox2_Callback(hObject, eventdata, handles)
% hObject    handle to checkbox2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MATLAB
% handles    structure with handles and user data (see GUIDATA)

% Hint: get(hObject,'Value') returns toggle state of checkbox2
    handles
    guidata(hObject);
    handles
    handles.channelEnable(2) = get(hObject,'Value');
    guidata(hObject, handles);
end

It does not make any difference with or without. In both cases all handles are known (also channelEnable).