Matlab GUI – Callback function

>> guide 

Choose the default option (blank GUI).

Add (drag) two ‘edit text’ boxes (which will be your inputs), two ‘static text’ boxes (one will be just an equal sign and the other will be the output), and add a ‘push button’ (which will be the ‘+’ sign, as in a calculator).

Resize your elements, figure and window as necessary (by dragging their anchors on the corners). You must end-up having something similar to this:

Now, double-click on each element, look and modify their properties as indicated on this table:

You must now have something similar to this (save it with any name, for example: adder.fig):

Before we develop the code for this interface, we must mention that there are three very important instructions when working with GUIs: ‘get’, ‘guidata’, and ‘set’.

 
The ‘get’ instruction, gets the string value from an input component. For example, if we want to get the number that the user inputs in ‘edit1’, we can do it like this (preceed the identifier tag with ‘handles.’):  

get(handles.edit1, 'String')

However, this instruction gets a string, not a number; thus, if we need to numerically manipulate that value, we have to transform it into a number first. For example, something typical is: 

num = str2double(get(handles.edit1,'String')); 

Now, our variable ‘num’ does contain a number (double), and we can manipulate it. 

Finally, we must update the value of the component and keep that variable in our memory-workspace (using the ‘guidata’ instruction) to be used in other callback functions. We can achieve it by doing this:

handles.edit1 = num;
guidata(hObject,handles) 

Generally speaking, we need to finish every callback function with 

guidata(hObject,handles) 

in order to maintain in memory the values of the variables.
 

The ‘set’ instruction sets the properties of the element that you indicate. The property ‘Tag’ is the identifier to use for this purpose. Do you remember that you have one ‘static text’ with the tag (identifier) ‘result’? We are going to modify it when the user pushes the button with the string ‘+’. This is to be accomplished with the instruction 

set(handles.output_line,'String',value) 

where ‘value’ contains the addition edit1 + edit2
 

We have to take care about two more things:

• What if the user doesn’t enter numbers in the edit boxes?
• Do we need an initialization procedure to make sure everything is in order before the user uses our GUI?

To modify the Matlab code for the components displayed in your interface, right-click on any of them and choose ‘View Callbacks’ -> ‘Callback’. You will be taken to the corresponding m-file (there are many automatically written lines, just add the new ones). 

We can test for the input value from the user. Let’s check the code for the callback function related to ‘edit1’:
 

function edit1_Callback(hObject, eventdata, handles)
% hObject    handle to edit1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MLB
% handles    structure with handles and user data (see GUIDATA)
% Hints: get(hObject,'String') returns contents of edit1 as text
%        str2double(get(hObject,'String')) returns contents of 
%        edit1 as a double
num = str2double(get(hObject,'String'));
if isnan(num)
    num = 0;
    set(hObject,'String',num);
    errordlg('Input must be a number', 'Error')
end
handles.edit1 = num;
guidata(hObject,handles)
 

In this callback function, we first read the input from the user and transform it to a number (since we are within the edit1_Callback, using get(handles.edit1,’String’) is the same as using get(hObject,'String')).

Then, we use the ‘isnan’ (for IS Not A Number) to launch an error message if the value is not a number, and set the value to 0, too. Finally, we keep the corresponding numerical value in the variable ‘edit1’ (officially recognized by Matlab as ‘handles.edit1’).
 

We can do the same thing for the 'edit2' callback function (you only need to add the last 8 lines):
 

function edit2_Callback(hObject, eventdata, handles)
% hObject    handle to edit2 (see GCBO)
% eventdata  reserved - to be defined in a future version of MLB
% handles    structure with handles and user data (see GUIDATA)
% Hints: get(hObject,'String') returns contents of edit2 as text
%        str2double(get(hObject,'String')) returns contents of 
%        edit2 as a double
num = str2double(get(hObject,'String'));
if isnan(num)
    num = 0;
    set(hObject,'String',num);
    errordlg('Input must be a number', 'Error')
end
handles.edit2 = num;
guidata(hObject,handles) 

And, since the values are kept in memory due to the 'guidata' instruction used in both callback functions (in variables handles.edit1 and handles.edit2), we can easily code the section for the push-button, like this (you have to add only the last two lines):

% --- Executes on button press in pushbutton1.
function pushbutton1_Callback(hObject, eventdata, handles)
% hObject    handle to pushbutton1 (see GCBO)
% eventdata  reserved - to be defined in a future version of MLB
% handles    structure with handles and user data (see GUIDATA)
addition = handles.edit1 + handles.edit2;
set(handles.result, 'String', addition);
 

Bingo! We’re almost done!

We need to consider the initialization part. What if the user press the ‘+’ button without entering any data? Will everything work fine? 

Unfortunately not. ‘edit1’ and ‘edit2’ don’t have zero values, they display a ‘0’ string (not a number). So, if the user press the ‘+’ button without entering any data, your GUI will display a number, but nothing to do with the expected numerical 0 value. 

To tackle this down, we just add some lines in the section code that runs just before ‘adder’ (your m-file, remember?) is made visible (most of the lines are written by Matlab automatically, you just add three lines), like this:
 

% --- Executes just before adder is made visible.
function adder_OpeningFcn(hObject, eventdata, handles, varargin)
% This function has no output args, see OutputFcn.
% hObject    handle to figure
% eventdata  reserved - to be defined in a future version of MLB
% handles    structure with handles and user data (see GUIDATA)
% varargin   command line arguments to adder (see VARARGIN) 

% Choose default command line output for adder
handles.output = hObject;
num = 0;
handles.edit1 = num;
handles.edit2 = num;

% Update handles structure
guidata(hObject, handles); 

% UIWAIT makes adder wait for user response (see UIRESUME)
% uiwait(handles.figure1);

Done! Run your GUI by clicking on the ‘run’ icon at the top of your editor window...