How to understand gui handles ?

Hello,

I'm programming a GUI. I need help by understanding how handles are working. This is simple code :

Starting creating a Structure S. Storing my figure into it.

S.f = figure('Position', [400,400,1000,400], ...
             'Name', 'Store data');

Storing an edit box and a pushbutton.

S.eT_name      = uicontrol( 'Style', 'edit', ...
                          'Position', [400,100,200,50], ...
                          'FontSize', 15, ...
                          'BackgroundColor', 'white');

S.pB_save       = uicontrol( 'Style', 'pushbutton', ...
                           'String', 'Save', ...
                           'Position', [700,75,200,50], ...
                           'FontSize', 20, ...
                           'BackgroundColor', 'red');

Setting callbacks.

set(S.pB_save, 'Callback', {@pB_save_callback, S});
set(S.eT_name, 'Callback', {@eT_name_callback, S});

Here are the 2 Callback functions.

function [] = eT_name_callback(hObject, eventdata, S)
    s_Name = get(hObject, 'String');
    set(S.data, 's_Name', s_Name);
    guidata(S.f, S.data);

function [] = pB_save_callback(hObject, eventdata, S)
    display(S.data);

My questions are :

- How can i save the string, that I'm getting by the edit box, into my GUI so that I can display it by clicking on the pushbutton ?

- I supposed S is the guihandle, isn't it ? Is there a solution so that i can create for example an array into the GUI to save data ?

Thank you in advance.

Answer 1

[Geoff Hayes]

Adrien - you will want to use the guidata function to set and retrieve user-defined data to the GUI. If you were developing your GUI with GUIDE, this would typically be a structure called handles which is usually an input parameter to all callbacks. Here, you are doing almost the same thing but with a structure called S. (You may want to change your S to handles just to make it clear what this structure contains.)

You have assigned your callbacks as

 set(S.pB_save, 'Callback', {@pB_save_callback, S});
 set(S.eT_name, 'Callback', {@eT_name_callback, S});

which is fine, but you have to realize that the S that gets passed as the third input parameter to these callbacks is a copy of the S at the time that these callbacks were assigned. So if one callback updates some data in this structure, another callback won't see these changes in S because it receives an out-of-date copy of S. So you can remove the S from each of the above to just have

 set(S.pB_save, 'Callback', {@pB_save_callback});
 set(S.eT_name, 'Callback', {@eT_name_callback});

We will use guidata to get the most up-to-date handle and user-defined data.

Note that you should add the following line once all handles have been set to S

 guidata(S.f,S);

This will save all handles to the GUI so that you can access them at will whenever you call guidata(hObject) from within any of your callbacks.

Your eT_name callback then becomes

 function [] = eT_name_callback(hObject, event data)
    % get the handles and user-defined data 
    handles = guidata(hObject);

    % set the name field within the (local) handles structure
    handles.s_Name = get(hObject, 'String');

    % save the data to the (global) handles structure
    guidata(hObject, handles);

Note that I have just replaced S with handles and am using hObject to retrieve and set the handles structure.

Your pB_save callback then becomes

 function [] = pB_save_callback(hObject, eventdata)
    % get the handles and user-defined data 
    handles = guidata(hObject);

    if isfield(handles,'s_Name')
        % display the name
        display(handles.s_Name);
    end

Note how we check to make sure that s_Name is a field within the handles structure before we try to access it.

Of course, the alternative to the accessing handles for s_Name is to just access the widget directly using its handle

 display(get(handles.eT_name,'String'));

--------------

The above describes two ways on how you can access the string data from your callback.

S is the structure of GUI widget handles and user-defined data, where the latter is set using the guidata function.

To save an array to the GUI, just do the same thing as you did with the string

 handles.arrayData = [1 2 3 4 5];
 guidata(hObject,handles);

The above saves an array with five elements, that you can then access in any other callback.

Try the above and see what happens!

[Adrien]

Thank you Geoff, this helped me a lot !!

But do I have to place also

guidata(hObject,handles);

when I'm defining my array ? I mean I'm writting :

handles.arrayData = [1 2 3 4 5];

in the function GUI. At the same place where I'm defining the pushbutton or ... Is it not saving it into the handles Automatically ?

[Adrien]

One more comprehension question.

guidata(hObject,handles);

By writting this. hObject just means that matlab is saving my data in the figure in wich the Object from the handle hobject is ?

[Geoff Hayes]

Yes, Adrian - at some point, before "exiting" your function (or GUI body) or wherever you have set

 handles.arrayData = [1 2 3 4 5];

you must follow this with a

guidata(hObject,handles);

Just think of handles as a copy of the structure with all widget handles and user-defined data. You must "save" it to the "master" copy using guidata.

From the documentation, guidata(object_handle,data) stores the variable data as GUI data. If object_handle is not a figure handle, then the object's parent figure is used. data can be any MATLAB® variable, but is typically a structure, which enables you to add new fields as required. So yes, MATLAB is saving the data to the parent figure of hObject.

Answer 2

[Adrien]

Geoff, i have an other Problem ... I explain it to you : I have a function called GUI_1 :

function [] = GUI_1()

      S.f  = figure(...);
      S.pB_Start = uicontrol('Style', 'pushbutton', ...);
      set(S.pB_Start, 'Callback', {@pB_Start_callback});

function [] = pB_Start_callback(varargin)
    S = guidata(varargin{1});
    GUI_2(S);       % Or GUI_2(guidata(varargin{1})), no difference, right ?

So this was my first GUI/function, which contains only a figure and a pushbutton. Here comes the second :

function [] = GUI(S)
    S.pB_1 = uicontrol('Style', 'pushbutton', ...);
    S.pB_2 = uicontrol('Style', 'pushbutton', ...);
guidata(S.f, S);

But it is not working. I would like to save the pushbuttons into the guidata of the figure f, so that I can acces it into the GUI_1(). I would like that my S structure also in GUI_1() contains the pushbuttons handles, certainly after having executed GUI_2(S). Is that possible, or do I have to set S as an output of the function GUI_2(S) ?

[Geoff Hayes]

Adrien - please create a new question for this problem as it does not relate to the first one. As well, describe in more detail what you are trying to accomplish.