Sharing Data Among a GUI's Callbacks :: Managing Application-Defined Data (MATLAB®)

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Sharing Data Among a GUI's Callbacks

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Nested Functions GUI Data Application Data UserData Property

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See Mechanisms for Managing Data for general information about these methods.

Nested Functions

You can use GUI data, application data, and the UserData property to share data among a GUI's callbacks. In many cases nested functions enables you to share data among callbacks without using the other data forms.

Nested Functions Example: Passing Data Between Components

This example uses a GUI that contains a slider and an edit text component as shown in the following figure. A static text component instructs the user to enter a value in the edit text or click the slider. The example initializes and maintains an error counter as well as the old and new values of the slider in a nested functions environment.

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

The GUI behavior is as follows:

When a user moves the slider, the edit text component displays the slider's current value and prints a message to the command line, similar to the following, indicating how many units the slider moved.
```
You moved the slider 25 units.
```
When a user types a value into the edit text component and then presses Enter or clicks outside the component, the slider updates to this value an d the edit text component prints a message to the command line indicating how many units the slider moved.
If a user enters a value in the edit text component that is out of range for the slider—that is, a value that is not between the slider's Min and Max properties—the application returns a message in the edit text indicating how many times the user has entered an erroneous value.

The following code constructs the components, initializes the error counter and the previous and new slider values in the initialization section of the function, and uses two callbacks to implement the interchange between the slider and the edit text component. Copy this code into an M-file and save it in your current directory or on your path as slider_gui.m. Run the script by typing slider_gui at the command line.

function slider_gui
fh = figure('Position',[250 250 350 350]);
sh = uicontrol(fh,'Style','slider',...
               'Max',100,'Min',0,'Value',25,...
               'SliderStep',[0.05 0.2],...
               'Position',[300 25 20 300],...
               'Callback',@slider_callback);
eth = uicontrol(fh,'Style','edit',...
               'String',num2str(get(sh,'Value')),...
               'Position',[30 175 240 20],...
               'Callback',@edittext_callback);
sth = uicontrol(fh,'Style','text',...
               'String','Enter a value or click the slider.',...
               'Position',[30 215 240 20]);
number_errors = 0;
previous_val = 0;
val = 0;
% ----------------------------------------------------
% Set the value of the edit text component String property
% to the value of the slider.
   function slider_callback(hObject,eventdata)
      previous_val = val;
      val = get(hObject,'Value');
      set(eth,'String',num2str(val));
      sprintf('You moved the slider %d units.',abs(val - previous_val))
   end
% ----------------------------------------------------
% Set the slider value to the number the user types in 
% the edit text or display an error message.
   function edittext_callback(hObject,eventdata)
      previous_val = val;
      val = str2double(get(hObject,'String'));
      % Determine whether val is a number between the 
      % slider's Min and Max. If it is, set the slider Value.
      if isnumeric(val) && length(val) == 1 && ...
         val >= get(sh,'Min') && ...
         val <= get(sh,'Max')
         set(sh,'Value',val);
         sprintf('You moved the slider %d units.',abs(val - previous_val))
      else
      % Increment the error count, and display it.
         number_errors = number_errors+1;
         set(hObject,'String',...
             ['You have entered an invalid entry ',...
             num2str(number_errors),' times.']);
         val = previous_val;
      end
   end
end

Because the components are constructed at the top level, their handles are immediately available to the callbacks that are nested at a lower level of the routine. The same is true of the error counter number_errors, the previous slider value previous_val, and the new slider value val. There is no need to pass these variables as arguments.

Both callbacks use the input argument hObject to get and set properties of the component that triggered execution of the callback. This argument is available to the callbacks because the components' Callback properties are specified as function handles. See Associating Callbacks with Components for more information.

Slider Callback. The slider callback, slider_callback, uses the edit text component handle, eth, to set the edit text 'String' property to the value the user typed.

The slider Callback saves the previous value, val, of the slider in previous_val before assigning the new value to val. These variables are known to both callbacks because they are initialized at a higher level. They can be retrieved and set by either callback.

previous_val = val;
val = get(hObject,'Value');

The following statements in the slider Callback update the value displayed in the edit text component when a user moves the slider and releases the mouse button.

val = get(hObject,'Value');
set(eth,'String',num2str(val));

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 sets the String property of the edit text component to the updated value.

Edit Text Callback. The edit text Callback, edittext_callback, uses the slider handle, sh, to determine the slider's Max and Min properties and to set the slider Value property, which determine's the position of the slider thumb.

The edit text Callback uses the following code to set the slider's value to the number the user types in, after checking to see if it is a single numeric value within the allowed range.

if isnumeric(val) && length(val) == 1 && ...
   val >= get(sh,'Min') && ...
   val <= get(sh,'Max')
   set(sh,'Value',val);

If the value is out of range, the if statement continues by incrementing the error counter, number_errors, and displaying a message telling the user how many times they have entered an invalid number.

else
   number_errors = number_errors+1;
   set(hObject,'String',...
   ['You have entered an invalid entry ',...
   num2str(number_errors),' times.']);
end

GUI Data

GUI data, which you manage with the guidata function, is accessible to all callbacks of the GUI. A callback for one component can set a value in GUI data, which can then be read by a callback for another component. See GUI Data for more information.

GUI Data Example: Passing Data Between Components

The previous topic, Nested Functions Example: Passing Data Between Components, uses nested function capabilities to initialize and maintain an error counter as well as the old and new values of the slider. This example shows you how to initialize and maintain the old and new values of the slider using GUI data and make them available to the both callbacks. Refer to the previous topic for details of the example.

The following code is similar to the previous topic but uses GUI data to initialize and maintain the old and new slider values in the edit text and slider Callbacks. Copy this code into an M-file and save it in your current directory or on your path as slider_gui.m. Run the script by typing slider_gui at the command line.

function slider_gui
fh = figure('Position',[250 250 350 350]);
sh = uicontrol(fh,'Style','slider',...
               'Max',100,'Min',0,'Value',25,...
               'SliderStep',[0.05 0.2],...
               'Position',[300 25 20 300],...
               'Callback',@slider_callback);
eth = uicontrol(fh,'Style','edit',...
               'String',num2str(get(sh,'Value')),...
               'Position',[30 175 240 20],...
               'Callback',@edittext_callback);
sth = uicontrol(fh,'Style','text',...
               'String','Enter a value or click the slider.',...
               'Position',[30 215 240 20]);
number_errors = 0;
slider.val = 25;
guidata(fh,slider);
% ----------------------------------------------------
% Set the value of the edit text component String property
% to the value of the slider.
   function slider_callback(hObject,eventdata)
        slider = guidata(fh);  % Get GUI data.
        slider.previous_val = slider.val;
        slider.val = get(hObject,'Value');
        set(eth,'String',num2str(slider.val));
        sprintf('You moved the slider %d units.',...
                 abs(slider.val - slider.previous_val))
        guidata(fh,slider) % Save GUI data before returning.
   end
% ----------------------------------------------------
% Set the slider value to the number the user types in 
% the edit text or display an error message.
   function edittext_callback(hObject,eventdata)
        slider = guidata(fh);   % Get GUI data.
        slider.previous_val = slider.val;
        slider.val = str2double(get(hObject,'String'));
      % Determine whether slider.val is a number between the 
      % slider's Min and Max. If it is, set the slider Value.
      if isnumeric(slider.val) && length(slider.val) == 1 && ...
         slider.val >= get(sh,'Min') && ...
         slider.val <= get(sh,'Max')
         set(sh,'Value',slider.val);
         sprintf('You moved the slider %d units.',...
                 abs(slider.val - slider.previous_val))
      else
      % Increment the error count, and display it.
         number_errors = number_errors+1;
         set(hObject,'String',...
             ['You have entered an invalid entry ',...
             num2str(number_errors),' times.']);
         slider.val = slider.previous_val;
      end
      guidata(fh,slider); % Save the changes as GUI data.
   end
end

Slider Values. In this example, both the slider callbackslider_callback and the edit text callback edittext_callback retrieve the GUI data structure slider which hold previous and current values of the slider. They then save the value, slider.val to slider.previous_val before retrieving the new value and assigning it to slider.val. Before returning, each callback saves the slider structure to GUI data.

slider = guidata(fh);   % Get GUI data.
slider.previous_val = slider.val;
slider.val = ...;
...

guidata(fh,slider)  % Save GUI data before returning.

Both callbacks use the guidata function to retrieve and save the slider structure as GUI data.

Application Data

Application data can be associated with any object—a component, menu, or the figure itself. To access application data, a callback must know the name of the data and the handle of the component with which it is associated. Use the functions setappdata, getappdata, isappdata, and rmappdata to manage application data.

See Application Data for more information about application data.

Application Data Example: Passing Data Between Components

The earlier topic, Nested Functions Example: Passing Data Between Components, uses nested function capabilities to initialize and maintain an error counter as well as the old and new values of the slider. This example shows you how to initialize and maintain the old and new values of the slider using application data (appdata) and make them available to the both callbacks. Refer to the earlier topic for details of the example.

The following code is similar to the earlier topic but uses application data to initialize and maintain the old and new slider values in the edit text and slider Callbacks. Copy this code into an M-file and save it in your current directory or on your path as slider_gui.m. Run the script by typing slider_gui at the command line.

function slider_gui
fh = figure('Position',[250 250 350 350]);
sh = uicontrol(fh,'Style','slider',...
               'Max',100,'Min',0,'Value',25,...
               'SliderStep',[0.05 0.2],...
               'Position',[300 25 20 300],...
               'Callback',@slider_callback);
eth = uicontrol(fh,'Style','edit',...
               'String',num2str(get(sh,'Value')),...
               'Position',[30 175 240 20],...
               'Callback',@edittext_callback);
sth = uicontrol(fh,'Style','text',...
               'String','Enter a value or click the slider.',...
               'Position',[30 215 240 20]);
number_errors = 0;
slider_data.val = 25;
% Create appdata with name 'slider'.
setappdata(fh,'slider',slider_data); 
% ----------------------------------------------------
% Set the value of the edit text component String property
% to the value of the slider.
   function slider_callback(hObject,eventdata)
      % Get 'slider' appdata.
      slider_data = getappdata(fh,'slider'); 
      slider_data.previous_val = slider_data.val;
      slider_data.val = get(hObject,'Value');
      set(eth,'String',num2str(get(slider_data.val)));
      sprintf('You moved the slider %d units.',...
              abs(slider_data.val - slider_data.previous_val))
      % Save 'slider' appdata before returning.
      setappdata(fh,'slider',slider_data) 
end
% ----------------------------------------------------
% Set the slider value to the number the user types in 
% the edit text or display an error message.
   function edittext_callback(hObject,eventdata)
      % Get 'slider' appdata.
      slider_data = getappdata(fh,'slider');
      slider_data.previous_val = slider_data.val;
      slider_data.val = str2double(get(hObject,'String'));
      % Determine whether val is a number between the 
      % slider's Min and Max. If it is, set the slider Value.
      if isnumeric(slider_data.val) && ...
            length(slider_data.val) == 1 && ...
         slider_data.val >= get(sh,'Min') && ...
         slider_data.val <= get(sh,'Max')
         set(sh,'Value',slider_data.val);
      else
      % Increment the error count, and display it.
         number_errors = number_errors+1;
         set(hObject,'String',...
             ['You have entered an invalid entry ',...
             num2str(number_errors),' times.']);
         slider_data.val = slider_data.previous_val;
      end
      % Save appdata before returning.
      setappdata(fh,'slider',slider_data); 
   end
end

Slider Values. In this example, both the slider callbackslider_callback and the edit text callback edittext_callback retrieve the application data structure slider_data which holds previous and current values of the slider. They then save the value, slider_data.val to slider_data.previous_val before retrieving the new value and assigning it to slider_data.val. Before returning, each callback saves the slider_data structure in the slider application data.

% Get 'slider' appdata.
slider_data = getappdata(fh,'slider'); 
slider_data.previous_val = slider_data.val;
slider_data.val = ...;
...
% Save 'slider' appdata before returning.
setappdata(fh,'slider',slider_data)

Both callbacks use the getappdata and setappdata functions to retrieve and save the slider_data structure as slider application data.

UserData Property

Every GUI component, and the figure itself, has a UserData property that you can use to store application-defined data. To access UserData, a callback must know the handle of the component with which a specific UserData property is associated.

Use the get function to retrieve UserData, and the set function to set it.

UserData Property Example: Passing Data Between Components

The previous topic, Nested Functions Example: Passing Data Between Components, uses nested function capabilities to initialize and maintain an error counter. This example shows you how to do the same thing using the edit text component's UserData property to store the error count. Refer to the earlier example for example details.

The following code is the same as in the earlier topic but uses the UserData property to initialize and increment the error counter.

function slider_gui
fh = figure('Position',[250 250 350 350]);
sh = uicontrol(fh,'Style','slider',...
               'Max',100,'Min',0,'Value',25,...
               'SliderStep',[0.05 0.2],...
               'Position',[300 25 20 300],...
               'Callback',@slider_callback);
eth = uicontrol(fh,'Style','edit',...
               'String',num2str(get(sh,'Value')),...
               'Position',[30 175 240 20],...
               'Callback',@edittext_callback);
sth = uicontrol(fh,'Style','text',...
               'String','Enter a value or click the slider.',...
               'Position',[30 215 240 20]);
number_errors = 0;
slider.val = 25;
% Set edit text UserData property to slider structure.
set(eth,'UserData',slider)
% ----------------------------------------------------
% Set the value of the edit text component String property
% to the value of the slider.
   function slider_callback(hObject,eventdata)
      % Get slider from edit text UserData.
      slider = get(eth,'UserData');  
      slider.previous_val = slider.val;
      slider.val = get(hObject,'Value');
      set(eth,'String',num2str(slider.val));
      sprintf('You moved the slider %d units.',...
              abs(slider.val - slider.previous_val))
      % Save slider in UserData before returning.
      set(eth,'UserData',slider) 
   end
% ----------------------------------------------------
% Set the slider value to the number the user types in 
% the edit text or display an error message.
   function edittext_callback(hObject,eventdata)
      % Get slider from edit text UserData.
      slider = get(eth,'UserData');
      slider.previous_val = slider.val;
      slider.val = str2double(get(hObject,'String'));
      % Determine whether slider.val is a number between the 
      % slider's Min and Max. If it is, set the slider Value.
      if isnumeric(slider.val) && ...
            length(slider.val) == 1 && ...
         slider.val >= get(sh,'Min') && ...
         slider.val <= get(sh,'Max')
         set(sh,'Value',slider.val);
         sprintf('You moved the slider %d units.',...
                 abs(slider.val - slider.previous_val))
      else
      % Increment the error count, and display it.
         data = get(hObject,'UserData');
         data.number_errors = data.number_errors+1;
         set(hObject,'UserData',data);     % Save the changes.
         set(hObject,'String',...
            ['You have entered an invalid entry ',...
            num2str(number_errors),' times.']);
         slider.val = slider.previous_val;
      end
      % Save slider structure in UserData before returning.
      set(eth,'UserData',slider)
   end
end

Slider Values. In this example, both the slider callbackslider_callback and the edit text callback edittext_callback retrieve the structure slider from the edit text UserData property. The slider structure holds previous and current values of the slider. The callbacks then save the value slider.val to slider.previous_val before retrieving the new value and assigning it to slider.val. Before returning, each callback saves the slider structure in the edit textUserData property.

% Get slider structure from edit text UserData.
slider = get(eth,'UserData',slider); 
slider.previous_val = slider.val;
slider.val = ...;
...
% Save slider structure in UserData before returning.
set(eth,'UserData',slider)

Both callbacks use the get and set functions to retrieve and save the slider structure in the edit text UserData property.

Mechanisms for Managing Data

Managing Callback Execution