Managing GUI Data with the Handles Structure :: Programming GUIs (Creating Graphical User Interfaces)

Creating Graphical User Interfaces

Managing GUI Data with the Handles Structure

Note GUI data is application-defined data that is saved and retrieved using the guidata function. GUIDE uses guidata to manage the handles structure.

GUIDE provides a mechanism, called the handles structure, for storing and retrieving shared data using the same structure that contains the GUI component handles.

This structure, which initially contains only the handles of the components in the GUI, is passed to the handles argument of each callback in the GUI M-file. Because this structure is passed to all callbacks for the GUI, any data you add to it become available to all the callbacks. The following example illustrates this technique.

If you are not familiar with MATLAB structures, see Structures for more information.

Example: Passing Data Between Callbacks

This example demonstrates how to use the handles structure to pass data between callbacks. The example passes data between a slider and an editable text box in the following way:

When a user moves the slider, the text box displays the slider's current value.
When a user types a value into the text box, the slider updates to this value.
If a user enters a value in the text box that is out of range for the slider -- that is, a value that is not between 0 and 1 -- the application returns a message indicating how many times the user has entered an erroneous value.

The following figure shows the GUI with a static text field above the edit text box.

GUI with static text field: Enter a value or click the slider

Defining the Data Fields in the Opening Function

The GUI records the number of times a user enters an erroneous value in the text box and stores this number in a field of the handles structure. You can define this field, called number_errors, in the opening function as follows:

```
handles.number_errors = 0;
```

Type this command before the following line, which GUIDE automatically inserts into the opening function.

guidata(hObject, handles); % Save the updated structure

The guidata command saves the handles structure so that it can be retrieved in the callbacks.

Note To save any changes that you make to the handles structure, you must add the command guidata(hObject, handles) following the code that implements the changes.

Setting the Edit Text Value from the Slider Callback

The following command in the slider callback updates the value displayed in the edit text when a user moves the slider and releases the mouse button.

set(handles.edit1,'String',...
    num2str(get(handles.slider1,'Value')));

The code combines three commands:

The get command obtains the current value of the slider.
The num2str command converts the value to a string.
The set command resets the String property of the edit text to the updated value.

Setting the Slider Value from the Edit Text Callback

The edit text callback sets the slider's value to the number the user types in, after checking to see if it is a single numeric value between 0 and 1. If the value is out of range, then the error count is incremented and the edit text displays a message telling the user how many times they have entered an invalid number.

val = str2double(get(handles.edit1,'String'));
% Determine whether val is a number between 0 and 1
if isnumeric(val) & length(val)==1 & ...
    val >= get(handles.slider1,'Min') & ...
    val <= get(handles.slider1,'Max')
    set(handles.slider1,'Value',val);
else
% Increment the error count, and display it
    handles.number_errors = handles.number_errors+1;
    guidata(hObject,handles); % store the changes
    set(handles.edit1,'String',...
    ['You have entered an invalid entry ',...
num2str(handles.number_errors),' times.']);
end

If the user types a number between 0 and 1 in the edit box and then clicks outside the edit box, the callback sets handles.slider1 to the new value and the slider moves to the corresponding position.

If the entry is invalid -- for example, 2.5 -- the GUI increments the value of handles.number_errors and displays a message like the following:

Message: You have entered an invalid entry 3 times

Figures Managing Application Data