MATLAB: How to update handles structure from a helper function in GUIDE

guideMATLABmatlab gui

I wrote an image browser using GUIDE (Matlab 2014a). Originally, one of the function callback looks like this, and it worked.
    function btnLineProfileXY_Callback(hObject, eventdata, handles)
    figure(4);hold off;
    imshow(squeeze(handles.FRET(:,:,getappdata(handles.figure1,'Zind'),:)));
    hold on;
    handles.hL1 = line(getappdata(handles.figure1,'Xind')*[1 1],[1 handles.Ynum]);
    handles.hL2 = line([1 handles.Xnum], getappdata(handles.figure1,'Yind'));
    lineBtnDownFcn(handles);
    guidata(hObject, handles);
But then because many other callbacks use the same codes, so I decided to write a helper function for these repetitive codes.
    function btnLineProfileXY_Callback(hObject, eventdata, handles)
    figure(4);hold off;
    imshow(squeeze(handles.FRET(:,:,getappdata(handles.figure1,'Zind'),:)));
    hold on;
    lineRoutine(handles);
    lineBtnDownFcn(handles);
    guidata(hObject, handles);
    function lineRoutine(handles) % Helper function
    figure(4);hold off;
    imshow(squeeze(handles.FRET(:,:,getappdata(handles.figure1,'Zind'),:)));
    hold on;
    handles.hL1 = line(getappdata(handles.figure1,'Xind')*[1 1],[1 handles.Ynum]);
    handles.hL2 = line([1 handles.Xnum], getappdata(handles.figure1,'Yind'));
    guidata(handles.figure1, handles);
When structured like this, it doesn't work anymore. The line object handles are not save to the handles structure. The function call was successful and there was no error message generated. But when I examine the handles structure after the function call, there was no handles.hL1 and handles.hL2. What did I do wrong?

Best Answer

The debugger reveals the reasons for such problems. Set a breakpoint in the code and check, what's going on by stepping through the code line by line.
In lineRoutine the variable "handles" is modified and stored in the figure. The same happens in lineBtnDownFcn. But then the version of "handles" in the function btnLineProfileXY_Callback is stored finally, such that the modifications vanish.
Omit the guidata(hObject, handles) in btnLineProfileXY_Callback to avoid overwriting the changes.
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