For example, setting the numerical precision of the labels, or adding some short descriptive text (for example, the units)? If you have, then I bet that you have encountered the following dilemma: Once we modify the tick labels (for discussion sake, let’s assume the Y axis, so this is done by updating the YTickLabel property), then the corresponding YTickLabelMode property changes from ‘auto’ to ‘manual’ and loses its relationship to the tick values (YTick). So, if we now zoom or pan the plot, our new labels remain unchanged although the tick values have changed, causing much panic and frustration… If we also set the tick values manually, this solves that problem but leaves us with another: now, when we zoom or pan, we no longer see any ticks or tick labels at all!

Original plot (left)
Manual labels, auto ticks (center)
Manual labels, manual ticks (right)

Of course, we can always trap the zoom and pan callback functions to update the tick labels dynamically while keeping the tick values automatically. This will work for these cases, but we need to do it separately for zoom and pan. Also, if we modify the axes limits explicitly (via the corresponding YLim property) or indirectly (by modifying the displayed plot data), then the callbacks are not called and the labels are not updated.

The solution – using a property change listener

A better way to solve this problem is to simply trap changes to the displayed tick values, and whenever these occur to call our dedicated function to update the labels according to the new tick values. This can be done by using UDD, or more precisely the ability to trap update events on any property (in our case, YTick). Such a mechanism was already demonstrated here in 2010, as one way to achieve continuous slider feedback. The idea is to use the built-in handle.listener function with the PropertyPostSet event, as follows:

hhAxes = handle(hAxes);  % hAxes is the Matlab handle of our axes
hProp = findprop(hhAxes,'YTick');  % a schema.prop object
hListener = handle.listener(hhAxes, hProp, 'PropertyPostSet', @myCallbackFunction);
setappdata(hAxes, 'YTickListener', hListener);

Note that we have used setappdata to store the hListener handle in the axes. This ensures that the listener exists for exactly as long as the axes does. If we had not stored this listener handle somewhere, then Matlab would have immediately deleted the listener hook and our callback function would not have been called upon tick value updates. Forgetting to store listener handles is a common pitfall when using them. If you take a look at the addlistener function’s code, you will see that it also uses setappdata after creating the listener, for exactly this reason. Unfortunately, addlistsner cannot always be used, and I keep forgetting under which circumstances, so I generally use handle.listener directly as above: It’s simple enough to use that I find I never really need to use the simple addlistener wrapper, but you are welcome to try it yourself.

That’s all there is to it: Whenever YTick changes its value(s), our callback function (myCallbackFunction) will automatically be called. It is quite simple to set up. While we cannot use TeX in tick labels yet (this will change in the upcoming HG2), using sprintf formatting does enable quite a bit of flexibility in formatting the labels. For example, let’s say I want my tick labels to have the format ‘%.1fV’ (i.e., always one decimal, plus the Volts units):

function myCallbackFunction(hProp,eventData)    %#ok - hProp is unused
   hAxes = eventData.AffectedObject;
   tickValues = get(hAxes,'YTick');
   newLabels = arrayfun(@(value)(sprintf('%.1fV',value)), tickValues, 'UniformOutput',false);
   set(hAxes, 'YTickLabel', newLabels);
end  % myCallbackFunction

Manual labels, automatically updated

Handling duplicate tick labels

Of course, ‘%.1fV’ may not be a good format when we zoom in to such a degree that the values differ by less than 0.1 – in this case all the labels will be the same. So let’s modify our callback function to add extra decimals until the labels become distinct:

function myCallbackFunction(hProp,eventData)    %#ok - hProp is unused
   hAxes = eventData.AffectedObject;
   tickValues = get(hAxes,'YTick');
 
   %newLabels = arrayfun(@(value)(sprintf('%.1fV',value)), tickValues, 'UniformOutput',false);
   digits = 0;
   labelsOverlap = true;
   while labelsOverlap
      % Add another decimal digit to the format until the labels become distinct
      digits = digits + 1;
      format = sprintf('%%.%dfV',digits);
      newLabels = arrayfun(@(value)(sprintf(format,value)), tickValues, 'UniformOutput',false);
      labelsOverlap = (length(newLabels) > length(unique(newLabels)));
 
      % prevent endless loop if the tick values themselves are non-unique
      if labelsOverlap && max(diff(tickValues))< 16*eps
         break;
      end
   end
 
   set(hAxes, 'YTickLabel', newLabels);
end  % myCallbackFunction

Non-distinct labels                   distinct labels

ticklabelformat

Based on a file that I received from an anonymous reader a few years ago, I have prepared a utility called ticklabelformat that automates much of the set-up above. Feel free to download this utility and modify it for your needs - it's quite simple to read and follow. The usage syntax is as follows:

ticklabelformat(gca,'y','%.6g V')  % sets y axis on current axes to display 6 significant digits
ticklabelformat(gca,'xy','%.2f')   % sets x & y axes on current axes to display 2 decimal digits
ticklabelformat(gca,'z',@myCbFcn)  % sets a function to update the Z tick labels on current axes
ticklabelformat(gca,'z',{@myCbFcn,extraData})  % sets an update function as above, with extra data

1. Axes LooseInset property Matlab plot axes have an undocumented LooseInset property that sets empty margins around the axes, and can be set to provide a tighter fit of the axes to their surroundings....
2. Determining axes zoom state The information of whether or not an axes is zoomed or panned can easily be inferred from an internal undocumented object....
3. FIG files format FIG files are actually MAT files in disguise. This article explains how this can be useful in Matlab applications....
4. Customizing Matlab labels Matlab's text uicontrol is not very customizable, and does not support HTML or Tex formatting. This article shows how to display HTML labels in Matlab and some undocumented customizations...

The basic idea

The basic idea, as reader Mory pointed out in a comment, is to overlay a semi-transparent empty figure window, having just the right size and position, on top of the main (disabled) figure window:

+ =

Main (unblurred) window + semi-transparent window = blurred window effect

In addition to ensuring the correct figure size and position, there are several other things we should care for: We need to synchronize the figure/blurring color, menubar/toolbar, and title. We also need to handle docking, resizing and figure movement. Finally we need to connect the two figures so that whenever one is closed then so is the other.

blurFigure

My blurFigure utility on the Matlab File Exchange attempts to handle all these setups for the user. The utility is quite simple to use:

blurFigure(hFig) blurs figure hFig and prevents interaction with it. The only interaction possible is with user-created controls on the blurring panel (see below).

hFigBlur = blurFigure(hFig) returns the overlaid blurred figure pane. This is useful to present a progress bar or other GUI controls, for user interaction during the blur phase (see the demo below).

blurFigure(hFig,STATE) sets the blur status of figure hFig to STATE, where state is ‘on’,'off’,true or false (default=’on’/true). blurFigure(hFig,’on’) or blurFigure(hFig,true) is the same as: blurFigure(hFig); blurFigure(hFig,’off’) or blurFigure(hFig,false) is the same as: close(hFigBlur).

blurFigure(‘demo’) displays a simple demo of the blurring. In fact, this runs the following simple code:

% Create the main (blurred) figure window
hFigMain = figure;
 
% Display some GUI controls in the main window
try oldWarn = warning('off','MATLAB:uitree:MigratingFunction'); catch, end
hTree = uitree('root','c:\');
drawnow;
try hTree.getTree.expandRow(0); catch, end
try warning(oldWarn); catch, end
uicontrol('string','click me!', 'units','pixel', 'pos',[300,50,100,20]);
axes('parent',gcf, 'units','pixel', 'pos',[230,100,300,300]);
surf(peaks);
set(gcf,'ToolBar','figure');  % restore the toolbar that was removed by the uicontrol() call above
 
% Call blurFigure() to add a semi-transparent overlaid window
hFigBlurTemp = blurFigure(hFigMain);
 
% Add some non-blurred controls on top of the blur
uicontrol('parent',hFigBlurTemp, 'style','text', 'units','pixel', 'pos',[130,85,390,80], ...
          'string','Processing - please wait...', 'FontSize',12, 'FontWeight','bold','Fore','red','Back','yellow');
jProgressBar = javacomponent('javax.swing.JProgressBar', [180,115,310,20], hFigBlurTemp);
jProgressBar.setValue(67);
uicontrol('parent',hFigBlurTemp, 'string','Cancel', 'pos',[280,90,100,20], 'Callback','close(gcbf)');

Non-blurred controls displayed over a blurred figure window

Do you have some other interesting uses for window transparency in Matlab? If so, please share your thought in a comment.

1. Transparent Matlab figure window Matlab figure windows can be made fully or partially transparent/translucent or blurred - this article explains how...
2. Minimize/maximize figure window Matlab figure windows can easily be maximized, minimized and restored using a bit of undocumented magic powder...
3. Enable/disable entire figure window Disabling/enabling an entire figure window is impossible with pure Matlab, but is very simple using the underlying Java. This article explains how....
4. Detecting window focus events Matlab does not have any documented method to detect window focus events (gain/loss). This article describes an undocumented way to detect such events....

Introduction to the UDD-Java relationship

Over the course of this series we have mentioned connections between UDD and Java. In UDD Events and Listeners we described how in Matlab each Java object can have a UDD companion. In Hierarchical Systems with UDD we briefly noted that a UDD hierarchy may be passed to Java. This suggests that there is a two way relationship between between UDD and Java.

In this article we will use some undocumented built-in methods such as java and classhandle to explore the UDD-Java relationship. We have used built-in methods for UDD objects before. We have also mentioned the importance of studying code from The MathWorks. When you come across something that looks like it may be a UDD method you can check with the which command:

>> which java -all
C:\MATLAB\R2010b\toolbox\matlab\general\java.m
java is a built-in method                               % javahandle.com.mathworks.hg.peer.Echo method
java is a built-in method                               % ui.figure method
java is a built-in method                               % hg2utils.HGHandle method
java is a built-in method                               % JavaVisible method
java is a built-in method                               % hg.figure method
java is a built-in method                               % hg.GObject method
java is a built-in method                               % schema.class method
java is a built-in method                               % handle.handle method
java is a built-in method                               % schema.method method
C:\MATLAB\R2010b\toolbox\rptgen\rptgen\@sgmltag\java.m  % sgmltag method

If you find schema.class in the comments for built-in methods, then the method is a general UDD method.

UDD javahandle companions for Java object

Whenever a Java class is instantiated in Matlab, it is possible to create a companion UDD object. The created companion can be in either the javahandle or the javahandle_withcallbacks package. The primary reason for creating the companion object is to avoid memory leaks when attaching a Matlab callback to a callback property. It makes sense in general to use the javahandle_withcallbacks package.

>> % creage a java instance and the companion UDD object
>> javaFrame = javax.swing.JFrame;
 
>> % dot notation
>> javaFrameUDD = javaFrame.handle('CallbackProperties');
 
>> % or Matlab notation
>> javaFrameUDD = handle(javaFrame,'CallbackProperties');

We can use the built-in classhandle method to inspect our UDD companion object. This can be used, for example, to obtain a list of the events that the Java class generates:

>> % use classhandle to list a java classes events
>> jch = javaFrame.handle.classhandle;
 
>> for index = 1:numel(jch.Events), disp(jch.Events(index).Name); end
MouseWheelMoved
MouseClicked
MouseEntered
MouseExited
MousePressed
MouseReleased
WindowGainedFocus
WindowLostFocus
WindowActivated
WindowClosed
WindowClosing
WindowDeactivated
WindowDeiconified
WindowIconified
WindowOpened
ComponentHidden
ComponentMoved
ComponentResized
ComponentShown
MouseDragged
MouseMoved
ComponentAdded
ComponentRemoved
AncestorMoved
AncestorResized
FocusGained
FocusLost
WindowStateChanged
HierarchyChanged
CaretPositionChanged
InputMethodTextChanged
PropertyChange
KeyPressed
KeyReleased
KeyTyped
 
>> % Use one of the object's callbacks
>> set(javaFrameUDD,'WindowGainedFocusCallback',@myCallbackFcn);

If we do not wish to use callback properties, then we can create our UDD companion in the javahandle package and use handle.listener to respond to events.

javaFrame = javax.swing.JFrame;
javaFrameUDD = javaFrame.handle;
lis = handle.listener(javaFrameUDD,'WindowGainedFocus',@myCallbackFcn);

Passing UDD objects to Java code

You can pass any UDD object to your Java classes in Matlab. Matlab will create a Java bean adapter for the UDD object. The bean adapter created is a subclass of com.MathWorks.jmi.bean.UDDObject. UDDObject implements the Java interfaces com.MathWorks.jmi.bean.DynamicProperties, com.MathWorks.jmi.bean.MTObject, com.MathWorks.jmi.bean.TreeObject, and com.mathworks.services.Browseable.

The generated bean adapter will have the methods of the parent class the methods of the UDD class, as well as set and get methods for the class properties. To understand how this works, let’s start with our simple.object and use the java method to inspect the bean adapter:

>> myObj = simple.object('myObj', 2);
 
>> % using dot notation with the java method 
>> myObj.java.getClass
ans =
class objectBeanAdapter0
 
>> myObj.java.methods
 
Methods for class objectBeanAdapter0:
 
acquireReference                    createNullMatlabObjectListener      lastDown                            
addBelow                            createNullPropertyChangeListener    left                                
addBrowseableListener               dialog                              notify                              
addFirstBelow                       disp                                notifyAll                           
addLeft                             dispose                             objectBeanAdapter0                  
addMatlabObjectListener             equals                              releaseReference                    
addObjectPropertyChangeListener     findProperty                        removeBrowseableListener            
addRight                            firstDown                           removeMatlabObjectListener          
browseableCanHaveChildren           getChildAt                          removeObjectPropertyChangeListener  
browseableChild                     getChildCount                       right                               
browseableChildCount                getClass                            setDirtyFlag                        
browseableChildFetchCount           getClassName                        setDynamicPropertyValue             
browseableChildren                  getDynamicProperties                setName                             
browseableDataObject                getDynamicPropertyValue             setPropertyValue                    
browseableDisplayObject             getIndex                            setThreadSafetyCheckLevel           
browseableHasChildren               getIndexOfChild                     setValue                            
browseableNChildren                 getName                             toString                            
browseableNextNSiblings             getNewInstance                      up                                  
browseableNextSibling               getPropertyValue                    updateCache                         
browseableParent                    getValue                            updateChildCount                    
browseablePrevNSiblings             hashCode                            updateIndex                         
browseablePrevSibling               isDirty                             wait                                
checkThreadSafety                   isLeaf                              
clearDirtyFlag                      isObservable                        
compareTo                           isValid

The parent class has added a large number of methods to the bean adapter for our original class. By looking at the list we can see our dialog and disp methods. There are also getName, setName, getValue, and setValue methods for our classes properties. The rest of the methods were inherited from the base UDDObject superclass. We can use any superclass method directly with the bean adapter object. For example:

>> myObj.java.getPropertyValue('Name')
ans =
myObj

Java interface class

To be able to use our UDD object in user-written Java code, we need a Java interface class for it. While we could manually write an interface file, UDD provides a very handy convenience method to automatically create the interface file. For this, we use the classhandle method again. The schema.class object obtained using classhandle has a method called createJavaInterface that takes two string arguments: the Java interface classname, and the folder in which to place the interface file. The steps to create and use this interface file are:

1. Create and test your UDD class
2. Create a Java interface file using schema.class‘s CreateJavaInterface
3. Modify your UDD class definition file (schema.m) to reference the Java interface file
4. Create the Java code that uses your class

For example, to create a Java interface file for the simple object we created above, use the following commands in Matlab:

classH = classhandle(myObj);
classH.createJavaInterface('simpleObjectInterface',pwd);

This will create the following simpleObjectInterface.java file in the current working directory:

public interface simpleObjectInterface 
       extends com.mathworks.jmi.bean.TreeObject
{
    /* Properties */
    public java.lang.String getName();
    public void setName(java.lang.String value);
 
    public double getValue();
    public void setValue(double value);
 
    /* Methods */
    public void dialog();
    public void disp();
}

The interface file contains set and get accessor methods for our UDD object properties, and Java prototypes for the UDD methods (in our case, dialog and disp).

The next step is to modify our class definition file (schema.m) to reference the Java interface file we have created. This modification provides the information that Matlab needs to create the bean adapter that implements the Java interface:

simpleClass = schema.class(simplePackage, 'object');
simpleClass.JavaInterfaces = { 'simpleObjectInterface' };

We can verify that the generated bean adapter implements the interface using Java Reflection techniques. As always when we have made changes to the class definition file, we need to use the clear classes command, and then recreate our objects:

>> myObj = simple.object('myObj', pi)
myObj =
  Name: myObj
 Value: 3.141593
 
>> myObjBean = java(myObj) 
myObjBean =
  Name: myObj
 Value: 3.141593
 
>> interfaces = myObjBean.getClass.getInterfaces 
interfaces =
java.lang.Class[]:
    [java.lang.Class]
 
>> interfaces(1) 
ans =
interface simpleObjectInterface

Using UDD in Java

Let’s create a simple Java class that illustrates passing a UDD object to Java. Here we will just have two methods: The first gets the Value property from a class instance and doubles it; the second launches the class instance dialog:

public class accessUDDClass 
{
  double localValue;
 
  public void accessUDDClass() { 
  }
 
  public void doubleValue(simpleObjectInterface UDDObj) {
    localValue = UDDObj.getValue();
    UDDObj.setValue(2*localValue);
  }
 
  public void launchDialog(simpleObjectInterface UDDObj) {
    UDDObj.dialog();
  }
}

If we have set up the Java compiler and environment variables correctly, we can compile our interface and Java class files from inside Matlab using the system command (alternately, we can compile using any external Java compiler or IDE):

>> system('javac accessUDDClass.java simpleObjectInterface.java')
ans =
     0

Now test our simple Java class with the UDD object created earlier:

>> javaObj = accessUDDClass
javaObj =
accessUDDClass@eb9b73
 
>> javaObj.doubleValue(myObj)  % pi => 2*pi
 
>> myObj
myObj =
  Name: myObj
 Value: 6.283185

This concludes the UDD series. I would like to thank Yair for his help in preparing and presenting this information.

Editor’s note

I would like to thank Donn for his enourmously detailed work on UDD, and for preparing it in easy-to-follow articles. I can personally attest to the huge time investment it has taken him. I trully believe he deserves a warm “thank you” from the Matlab community. Please visit Donn’s website, or add a short comment below.

In the following weeks, I return to the regular stuff that made this website famous: solving day-to-day Matlab problems using simple undocumented built-in Matlab gems.
- Yair

1. Matlab callbacks for Java events Events raised in Java code can be caught and handled in Matlab callback functions - this article explains how...
2. JMI – Java-to-Matlab Interface JMI enables calling Matlab functions from within Java. This article explains JMI's core functionality....
3. Extending a Java class with UDD Java classes can easily be extended in Matlab, using pure Matlab code. ...
4. FindJObj – find a Matlab component’s underlying Java object The FindJObj utility can be used to access and display the internal components of Matlab controls and containers. This article explains its uses and inner mechanism....

The UDD event model

The UDD event model is very similar to the MCOS event model. There is an excellent discussion of the MCOS event model in Matlab’s official documentation. Most of the MCOS information also applies to UDD if you make the following substitutions:

Event handler functions

To begin the UDD event model discussion we will start at the end, with the event handler. The event handler function requires at least two input arguments: the source object which triggered the event, and an object of type handle.EventData or a subclass of handle.EventData.

To demonstrate how this works, let’s write a simple event handler function. This event handler will display the class of the source event and the class of the event data:

function displayEventInfo(source, eventData)
%DISPLAYEVENTINFO display the classes of source, data objects
%
%   DISPLAYEVENTINFO(SOURCE, EVENTDATA) returns the classes
%   of the source object and the event data object
%
%   INPUTS:
%       SOURCE    : the event source
%       EVENTDATA : the event data
  if ~isempty(source)
    fprintf(1, 'The source object class is: %s',class(source));
  end
  if ~isempty(eventData)
    fprintf(1, 'The event data class is: %s',class(eventData));
  end
end

Creating a listener

In the section on Creating a Simple UDD Class we used schema.event in our simple.object class definition file to create a simpleEvent event. We now create an instance of simple.object, then use handle.listener to wait (“listen”) for the simpleEvent event to occur and call the displayEventInfo event handler function:

a = simple.object('a', 1);
hListener = handle.listener(a,'simpleEvent',@displayEventInfo);
setappdata(a, 'listeners', hListener);

Important: The hListener handle must remain stored somewhere in Matlab memory, or the listener will not be used. For this reason, it is good practice to attach the listener handle to the listened object, using the setappdata function, as was done above. The listener will then be alive for exactly as long as its target object is alive.

Creating an EventData object

Next, create the handle.EventData object. The handle.EventData object constructor requires two arguments: an instance of the events source object, and the name of the event:

evtData = handle.EventData(a, 'simpleEvent')

Generating an event

The last step is actually triggering an event. This is done by issuing the send command for the specified object, event name and event data:

>> a.send('simpleEvent', evtData)
The source object class is: simple.object
The event data class is: handle.EventData

If there is other information that you wish to pass to the callback function you can create a subclass of the handle.EventData. Add properties to hold your additional information and use your subclass as the second argument of the send method.

Builtin UDD events

The builtin handle package has six event data classes which are subclasses of the base handle.EventData class. Each of these classes is paired with specific UDD events that Matlab generates. Actions that trigger these events include creating/destroying an object, adding/removing objects from a hierarchy, and getting/setting property values. The following table lists the event names and handle.*EventData data types returned for these events:

As an example of some of these events let’s look at a question recently asked on the CSSM newsgroup. The basic idea is that we want to monitor an axis, automatically make any added lines to be green in color, and prevent patches from being added.

The solution is to monitor the ObjectChildAdded event for an axis. We will write an event handler which checks the handle.ChildEventData to see what type of child was added. In the case of lines we will set their color to green; patch objects will be deleted from the axis. Here is our event handler function:

function modifyAxesChildren(~, eventData)
%MODIFYAXESCHILDREN monitor and axis and modify added children
%
%   MODIFYAXESCHILDREN(SOURCE,EVENTDATA) is an event handler to
%   change newly-added lines to green and remove added patches
%
%   INPUTS:
%       EVENTDATA : handle.ChildEventData object
   switch eventData.Child.classhandle.Name
      case 'line'
         eventData.Child.set('Color', 'green');
         disp('Color changed to green.')
      case 'patch'
         eventData.Child.delete;
         disp('Patch removed.')
   end
end

Next create an axis, and a listener which is triggered when children are added:

% create a new axes and get its handle
a = hg.axes;
 
% create the listener
listen = handle.listener(a, 'ObjectChildAdded', @modifyAxesChildren);
 
% add a line
>> hg.line;
Color changed to green.
 
% try to add a patch
>> hg.patch;
Patch removed.

Removing a child with either the delete or the disconnect method generates an ObjectChildRemoved event. The delete method also generates the ObjectBeingDestroyed event. Changing a child’s parent with the up method generates an ObjectParentChanged event.

Reading an object’s properties with either dot notation or with the get method generates PropertyPreGet and PropertyPostGet events.

Changing the value of a property generates the PropertyPreSet and PropertyPostSet events. As we saw in the section on UDD properties, when the AbortSet access flag is ‘on’, property set events are only generated when a set operation actually changes the value of the property (as opposed to leaving it unchanged).

Note that the handle.listener syntax is slightly different for property events:

hProp = findprop(a, 'Value');
hListener = handle.listener(a,hProp,'PropertyPreGet',@displayEventInfo);

Java events

The final specialized event data object in the handle package is handle.JavaEventData. In Matlab, Java classes are not UDD classes, but each Java instance can have a UDD peer. The peer is created using the handle function. The Java peers are created in either UDD’s javahandle package or the javahandle_withcallbacks package. As their names imply, the latter enables listening to Java-triggered events using a Matlab callback.

To illustrate how this works we will create a Java Swing JFrame and listen for MouseClicked events:

% Create the Java Frame
javaFrame = javax.swing.JFrame;
javaFrame.setSize(200, 200);
javaFrame.show;
 
% Create a UDD peer for the new JFrame (two alternatives)
javaFramePeer = javaFrame.handle('CallbackProperties');  % alternative #1
javaFramePeer = handle(javaFrame, 'CallbackProperties');  % alternative #2
 
% Create the a listener for the Java MouseClicked event
listen = handle.listener(javaFramePeer, 'MouseClicked', @displayEventInfo);

a simple Java Swing JFrame

When we click on the JFrame, our UDD peer triggers the callback:

The source object class is: javahandle_withcallbacks.javax.swing.JFrame
The event data class is: handle.JavaEventData

Since we created our peer in the javahandle_withcallbacks package, it is not necessary to create a listener using handle.listener. If we place our callback function handle in the MouseClickedCallback property it will be executed whenever the MouseClicked event is triggered. Such *Callback properties are automatically generated by Matlab when it creates the UDD peer (details).

clear listen
javaFramePeer.MouseClickedCallback = @displayEventInfo

This will work the same as before without the need to create and maintain a handle.listener object. If we had created our UDD peer in the javahandle package rather than javahandle_withcallbacks, we would not have the convenience of the MouseClickedCallback property, but we could still use the handle.listener mechanism to monitor events.

Creating callback properties for custom UDD classes

It is easy to add callback properties to user created UDD objects. The technique involves embedding a handle.listener object in the UDD object. To illustrate this, we add a SimpleEventCallback property to our simple.object, then use a SimpleEventListener property to hold our embedded handle.listener. Add the following to simple.object‘s schema.m definition file:

   % Property to hold our callback handle
   prop = schema.prop(simpleClass, 'SimpleEventCallback', 'MATLAB callback');
   prop.setFunction = @setValue;
 
   % hidden property to hold the listener for our callback
   prop = schema.prop(simpleClass, 'SimpleEventListener', 'handle');
   prop.Visible = 'off';
end
 
function propVal = setValue(self, value)
   %SETVALUE function to transfer function handle from callback property to listener
   self.SimpleEventListener.Callback = value;
   propVal = value;
end

Next we add the following to our simple.object constructor file:

% set the hidden listener property to a handle.listener
simpleObject.SimpleEventListener = handle.listener(simpleObject, 'simpleEvent', []);

Now if we set the SimpleObjectCallback property to a function handle, the handle is transferred to the embedded handle.listener Callback property. When a simpleEvent event is generated, our SimpleEventCallback function will be executed.

This series will conclude next week with a look at the special relationship between UDD and Java.

1. Matlab callbacks for Java events Events raised in Java code can be caught and handled in Matlab callback functions - this article explains how...
2. Detecting window focus events Matlab does not have any documented method to detect window focus events (gain/loss). This article describes an undocumented way to detect such events....
3. Introduction to UDD UDD classes underlie many of Matlab's handle-graphics objects and functionality. This article introduces these classes....
4. Creating a simple UDD class This article explains how to create and test custom UDD packages, classes and objects...

Background on UDD

Matlab has used objects for a long time. In R8 (Matlab 5.0), their first user accessible class system was introduced. Andy Register wrote a detailed reference on using this system. Although that original system is obsolete, it is still available in R24 (R2010b).

UDD objects (also referred to as schema objects) were introduced with R12 (Matlab 6.0). UDD has been a foundation platform for a number of core Matlab technologies. MathWorks have consistently maintained that UDD is only meant for internal development and not for Matlab users. So, while UDD has no formal documentation, there are plenty of examples and tools to help us learn about it.

It is somewhat odd that despite Matlab’s new object-oriented system (MCOS)’s introduction 3 years ago, and the ongoing concurrent development of HG2 classes, the older-technology UDD is still being actively developed, as evidenced by the increasing number of UDD classes in recent releases. More background on the differences between these different sets of classes can be found here.

Why should we bother learning UDD?

There are some things to consider before deciding if you want to spend the time to learn about the UDD class system:

The case against studying UDD classes

• There is no documentation from The MathWorks for these classes
• You will not get any help from The MathWorks in applying these classes
• The UDD system is now more than a decade old and may be phased out in future Matlab releases (perhaps in HG2?)

The case for studying UDD classes

• UDD is currently the foundation of handle graphics, Java integration, COM, and Simulink
• The m code versions of UDD may be considered a forerunner of the newer MCOS class system
• To avoid memory leaks when using Callbacks in GUI applications you currently need to use UDD
• UDD techniques facilitate Matlab interaction with Java GUIs
• UDD directly supports the Matlab style method invocation as well as dot notation for methods without the need to write subsasgn and subsref routines

Tools for Learning about UDD

We start by describing some undocumented Matlab tools that will help us investigate and understand UDD classes.

• findpackage – All UDD Classes are defined as members of a package. findpackage takes the package name as an input argument and returns a schema.package object which provides information about the package
• findclass – This method of the schema.package object returns a schema.class object of the named class if the class exists in the package
• classhandle – For a given UDD object classhandle returns a schema.class object with information about the class. classhandle and findclass are two ways of getting the same information about a UDD class. findclass works with a schema.package object and a class name and does not require an instance of the class. classhandle works with an instance of a class
• findprop – This method of the schema.class object returns a schema.prop object which contains information about the named property
• findevent – This method of the schema.class object returns a schema.prop object which contains information about the named event
• handle – handle is a multifaceted and unique term for The MathWorks. There are both UDD and MCOS handle classes. There is a UDD handle package. In terms of the tools we need, handle is also an undocumented function which converts a numeric handle into a UDD handle object. Depending on your background you may want to think of handle as a cast operator which casts a numeric handle into a UDD object.
• methods – This is used to display the methods of an object
• methodsview – Provides a graphic display of an objects methods
• uiinspect – Yair Altman’s object inspection tool, which can be used for COM, Java and Matlab classes (uiinspect will be described in a separate article in the near future).

Before we apply these tools we need to discuss the basic structure of UDD classes. Let’s compare them with the newer, well documented MCOS classes:

MCOS classes can be defined simply as a standalone class or scoped by placing the class in a package or a hierarchy of packages. With UDD, all classes must be defined in a package. UDD Packages are not hierarchical so a UDD package may not contain other packages. UDD classes can always be instantiated with syntax of packageName.className. By default MCOS classes are value classes. With MCOS you can subclass the handle class to create handle classes. UDD classes are handle classes by default, but it is possible to create UDD value classes.

Exploring some important built-in UDD Classes

The current versions of Matlab include a number of built-in UDD packages. We will use our new tools to see what we can learn about these packages. Let us begin by inspecting the two packages that form the basis of the UDD class system.

The schema package

The built-in schema package contains the classes for creating user written UDD classes. It also is used to provide meta information about UDD classes. Using findpackage we will obtain a schema.package object for the schema package and then use it obtain information about the classes it contains:

>> pkg = findpackage('schema')
pkg =
        schema.package
 
>> pkg.get
               Name: 'schema'
    DefaultDatabase: [1x1 handle.Database]
            Classes: [9x1 schema.class]
          Functions: [0x1 handle]
        JavaPackage: ''
         Documented: 'on'

Note that here we have used the dot-notation pkg.get – we could also have used the Matlab notation get(pkg) instead.

We have now learned that that there are nine classes in the schema package. The information about them in a schema package’s Classes property. To see the information about individual classes we inspect this property:

>> pkg.Classes(1).get
               Name: 'class'
            Package: [1x1 schema.package]
        Description: ''
        AccessFlags: {0x1 cell}
             Global: 'off'
             Handle: 'on'
       Superclasses: [0x1 handle]
    SuperiorClasses: {0x1 cell}
    InferiorClasses: {0x1 cell}
            Methods: [4x1 schema.method]
         Properties: [13x1 schema.prop]
             Events: []
     JavaInterfaces: {0x1 cell}

Not surprisingly, the first class in the schema.package is ‘class’ itself. Here we can see that schema.class has 4 methods and 13 properties. We can also see that the schema.class objects have a Name property. Let’s use that information to list all the classes in the schema package:

>> names = cell(numel(pkg.Classes), 1);
>> for index = 1:numel(names), names{index} = pkg.Classes(index).Name; end;
>> names
names =
    'class'
    'method'
    'signature'
    'package'
    'event'
    'prop'
    'type'
    'EnumType'
    'UserType'

These are the base classes for the UDD package schema. To illustrate a different way to get information, let’s use the findclass method of schema.package to get information about the schema.prop class:

>> p = findclass(pkg, 'prop')
p =
        schema.class
 
>> get(p)
               Name: 'prop'
            Package: [1x1 schema.package]
        Description: ''
        AccessFlags: {0x1 cell}
             Global: 'off'
             Handle: 'on'
       Superclasses: [0x1 handle]
    SuperiorClasses: {0x1 cell}
    InferiorClasses: {0x1 cell}
            Methods: [2x1 schema.method]
         Properties: [9x1 schema.prop]
             Events: []
     JavaInterfaces: {0x1 cell}

The handle package

The second basic UDD package is the handle package. Handle holds a special place in Matlab and has multiple meanings: Handle is a type of Matlab object that is passed by reference; handle is a function which converts a numeric handle to an object; handle is an abstract object in the new MCOS class system and handle is also a UDD package as well as the default type for UDD objects.

There is an interesting connection between UDD and MCOS that involves handle. In Matlab releases R12 through R2007b, the UDD handle package had up to 12 classes and did not have any package functions (package functions are functions which are scoped to a package; their calling syntax is [outputs] = packageName.functionName(inputs)).

Beginning with the formal introduction of MCOS in R2008a, the abstract MCOS class handle was introduced. The MCOS handle class has 12 methods. It also turns out that beginning with R2008a, the UDD handle package has 12 package functions which are the MCOS handle methods.

The 12 UDD classes in the handle package fall into two groups: The database and transaction classes work with the schema.package to provide a UDD stack mechanism; the listener and family of EventData classes work with schema.event to provide the UDD event mechanism:

>> pkg = findpackage('handle')
pkg =
        schema.package
 
>> pkg.get
               Name: 'handle'
    DefaultDatabase: [1x1 handle.Database]
            Classes: [12x1 schema.class]
          Functions: [12x1 schema.method]
        JavaPackage: ''
         Documented: 'on'
 
>> names = cell(numel(pkg.Classes), 1);
>> for index = 1:numel(names), names{index} = pkg.Classes(index).Name; end
>> names
names =
    'Operation'
    'transaction'
    'Database'
    'EventData'
    'ClassEventData'
    'ChildEventData'
    'ParentEventData'
    'PropertyEventData'
    'PropertySetEventData'
    'listener'
    'JavaEventData'
    'subreference__'

The hg package

Arguably the most important UDD package in Matlab is the handle graphics package hg. Among the built-in UDD packages, hg is unique in several respects. As Matlab has evolved from R12 through R2011a, the number of default classes in the hg package has nearly doubled going from 17 classes to 30 (UDD has a mechanism for automatically defining additional classes as needed during run-time).

The hg package contains a mixture of Global and non Global classes. These classes return a numeric handle, unless they have been created using package scope. The uitools m-file package provides a great example of extending built-in UDD classes with user written m-file UDD classes.

The UDD class for a Handle-Graphics object can be obtained either by explicitly creating it with the hg package, or using the handle function on the numeric handle obtained from normal hg object creation. Using figure as an example, you can either use figh = hg.figure or fig = figure followed by figh = handle(fig):

>> pkg = findpackage('hg')
pkg =
        schema.package
 
>> pkg.get
               Name: 'hg'
    DefaultDatabase: [1x1 handle.Database]
            Classes: [30x1 schema.class]
          Functions: [0x1 handle]
        JavaPackage: 'com.mathworks.hg'
         Documented: 'on'
 
>> for index = 1:numel(names), names{index} = pkg.Classes(index).Name; end
>> names
names =
    'GObject'
    'root'
    'LegendEntry'
    'Annotation'
    'figure'
    'uimenu'
    'uicontextmenu'
    'uicontrol'
    'uitable'
    'uicontainer'
    'hgjavacomponent'
    'uipanel'
    'uiflowcontainer'
    'uigridcontainer'
    'uitoolbar'
    'uipushtool'
    'uisplittool'
    'uitogglesplittool'
    'uitoggletool'
    'axes'
    'hggroup'
    'text'
    'line'
    'patch'
    'surface'
    'rectangle'
    'light'
    'image'
    'hgtransform'
    'uimcosadapter'

So far we have just explored the very basic concepts of UDD. You may well be wondering what the big fuss is about, since the information presented so far does not have any immediately-apparent benefits.

The following set of articles will describe more advanced topics in UDD usage and customizations, using the building blocks presented today. Hopefully you will quickly understand how using UDD can help achieve some very interesting stuff with Matlab.

1. Creating a simple UDD class This article explains how to create and test custom UDD packages, classes and objects...
2. UDD Events and Listeners UDD event listeners can be used to listen to property value changes and other important events of Matlab objects...
3. Hierarchical Systems with UDD UDD objects can be grouped in structured hierarchies - this article explains how...
4. New information on HG2 More information on Matlab's new HG2 object-oriented handle-graphics system...

Legend Plotting

Whilst I enjoy using the many time saving features of Matlab, one area where I feel it suffers is the technical plotting and annotation. This tale relates the development of a legend plotting routine, gridLegend, in an effort to improve the presentation.

In my day job we have a requirement to condense a large quantity of data onto summary charts. However, there is only so much data consolidation possible before you start losing the information required. We often need to plot 40 or 50 lines of test data to visualize trends or outliers, using the legend to identify the number of test hours against each test specimen.

Using the standard Matlab legend function resulted in a long legend over twice the size of the associated plot:

Standard Matlab legend

I wanted some way of generating a more compact legend format.

Fortunately earlier in the year, an entry on Matlab Central allowing a multi-column legend to be generated was posted, columnlegend. Although lacking some features, columnlegend gave me a good start on developing what I wanted for a multi column legend, culminating in gridLegend:

Multi-column legend

Delving into the Undocumented Matlab

So where is the link with undocumented Matlab?

As mentioned in the original columnlegend entry, it was relatively simple to redraw the legend as required on the screen. However, as soon as the figure was printed or exported to an image file, internal Matlab workings would redraw the figure, including the legend, thereby undoing my careful legend crafting, squeezing it back into one column (Yuck!):

Matlab-reverted multi-column legend

As we wanted to automatically output images files, I had to delve into the hidden areas of Matlab to try to solve this problem.

My initial thought was to find out where the figure got redrawn for printing or export and override the standard legend call with a call to my new function. I couldn’t find the obvious culprit, stepping as far as I could through the print function there didn’t appear to be any call to the legend function.

In my search for information on how the legend worked I found the undocumented Matlab article about generating dynamic legends. This dynamic-legend post covered details about attaching a listener to a child of the legend axes, in Matlab the legend function creates its own set of axes on the figure to display the legend.

Armed with the information that legend axes objects could have listeners attached, I considered that these might be the source of redrawing the legend for printing. So with the legend I had generated I took a look at what listeners were attached, using the undocumented hidden axes property ScribeLegendListeners:

>> legendListener = get(gca,'ScribeLegendListeners')
legendListener = 
        fontname: [1x1 handle.listener]
        fontsize: [1x1 handle.listener]
      fontweight: [1x1 handle.listener]
       fontangle: [1x1 handle.listener]
       linewidth: [1x1 handle.listener]
         deleted: [1x1 handle.listener]
    proxydeleted: [1x1 handle.listener]

The font size and line positioning were all being redrawn for printing so this was potentially the source of my problem. However I’d not looked at a handle.listener before, so a little further digging was required:

K>> get(legendListener.fontname)
      SourceObject: [1x1 schema.prop]
         Container: [1x1 axes]
         EventType: 'PropertyPostSet'
          Callback: {2x1 cell}
    CallbackTarget: []
           Enabled: 'on'

The option Enabled immediately drew my attention, and so the following lines were added to my gridLegend function to switch off these listeners and apply it back to the legend:

LL = get(gca,'ScribeLegendListeners');
set(LL.fontname,'enabled','off');
set(LL.fontsize,'enabled','off');
set(LL.fontweight,'enabled','off');
set(LL.fontangle,'enabled','off');
set(LL.linewidth,'enabled','off');
set(gca,'ScribeLegendListeners',LL);

Finally allowing me to output the image files with a multi-column legend:

Printed multi-column legend

So my thanks to the contributors on Matlab Central who enabled me to get started on gridLegend and to Yair for collating the many nuggets of information on Undocumented Matlab which allowed me to complete the function and get it posted on Matlab Central.

Multi-column legend in action

1. Legend ‘-DynamicLegend’ semi-documented feature The built-in Matlab legend function has a very useful semi-documented feature for automatic dynamic update, which is explained here....
2. Multi-line tooltips Multi-line tooltips are very easy to set up, once you know your way around a few undocumented hiccups....
3. Inactive Control Tooltips & Event Chaining Inactive Matlab uicontrols cannot normally display their tooltips. This article shows how to do this with a combination of undocumented Matlab and Java hacks....
4. Introduction to UDD UDD classes underlie many of Matlab's handle-graphics objects and functionality. This article introduces these classes....

It so happens that only a few weeks ago I completed a consulting project which required exactly this. The project was to integrate a Matlab computational engine with a Java interface to Interactive Brokers (IB) – a well-known online brokerage firm. The idea was to use the Java interface to fetch real-time data about securities (stocks, bonds, options etc.), use a Matlab processing utility, then use the Java interface again to send Buy or Sell orders back to IB.

If you are interested in the final result (i.e., a complete and field-tested Matlab-IB interface), look here.

The challenge

A big challenge in this project (aside from handling quite a few IB interface quirks), was to propagate events from the Java interface to the Matlab application. This had to be done asynchronously, since events such as order execution can occur at any time following the order placement. Moreover, even simple requests such as retrieving security information (bid/ask prices for example) is handled by IB via Java events, not as simple function return values.

Handling Java-based events in Matlab is not a trivial task. Not only merely undocumented, but it is also not intuitive. I have spent quite a few hours trying to crack this issue. In fact, I believe it was one of my more challenging tasks in figuring out the undocumented aspects of the Matlab-Java interface. Few other challenges were as difficult, yet with a happy ending (Drag & Drop is a similar issue – I will describe it in another article sometime).

The solution

Fast-forward all the fruitless attempted variations, here is the bottom line. Refer to the following simple Java class example:

public class EventTest
{
    private java.util.Vector data = new java.util.Vector();
    public synchronized void addMyTestListener(MyTestListener lis) {
        data.addElement(lis);
    }
    public synchronized void removeMyTestListener(MyTestListener lis) {
        data.removeElement(lis);
    }
    public interface MyTestListener extends java.util.EventListener {
        void testEvent(MyTestEvent event);
    }
    public class MyTestEvent extends java.util.EventObject {
        private static final long serialVersionUID = 1L;
        public float oldValue,newValue;        
        MyTestEvent(Object obj, float oldValue, float newValue) {
            super(obj);
            this.oldValue = oldValue;
            this.newValue = newValue;
        }
    }
    public void notifyMyTest() {
        java.util.Vector dataCopy;
        synchronized(this) {
            dataCopy = (java.util.Vector)data.clone();
        }
        for (int i=0; i<dataCopy.size(); i++) {
            MyTestEvent event = new MyTestEvent(this, 0, 1);
            ((MyTestListener)dataCopy.elementAt(i)).testEvent(event);
        }
    }
}

When compiling EventTest.java, three class files are created: EventTest.class, EventTest$MyTestEvent.class and EventTest$MyTestListener.class. Place them on Matlab’s Java static classpath, using edit(‘classpath.txt’) (using the dynamic classpath causes many problems that using the static classpath solves). They can now be accessed as follows:

>> which EventTest
EventTest is a Java method  % EventTest constructor
 
>> evt = EventTest
evt =
EventTest@16166fc
 
>> evt.get
	Class = [ (1 by 1) java.lang.Class array]
	TestEventCallback = 
	TestEventCallbackData = []
 
	BeingDeleted = off
	ButtonDownFcn = 
	Children = []
	Clipping = on
	CreateFcn = 
	DeleteFcn = 
	BusyAction = queue
	HandleVisibility = on
	HitTest = on
	Interruptible = on
	Parent = []
	Selected = off
	SelectionHighlight = on
	Tag = 
	Type = EventTest
	UIContextMenu = []
	UserData = []
	Visible = on
 
>> set(evt)
	Class
	TestEventCallback: string -or- function handle -or- cell array
	...
 
>> set(evt,'TestEventCallback',@(h,e)disp(h))
 
>> get(evt)
	Class = [ (1 by 1) java.lang.Class array]
	TestEventCallback = [ (1 by 1) function_handle array]   <= ok
	TestEventCallbackData = []
	...
 
>> evt.notifyMyTest   % invoke Java event
              0.0009765625   % <= Matlab callback

Note how Matlab automatically converted the Java event testEvent, declared in interface MyTestListener, into a Matlab callback TestEventCallback (the first character is always capitalized). All Java events are automatically converted in this fashion, by appending a ‘Callback’ suffix. Here is a code snippet from R2008a’s \toolbox\matlab\uitools\@opaque\addlistener.m that shows this (slightly edited):

hSrc = handle(jobj,'callbackproperties');
allfields = sortrows(fields(set(hSrc)));
for i = 1:length(allfields)
   fn = allfields{i};
   if ~isempty(findstr('Callback',fn))
      disp(strrep(fn,'Callback',''));
   end
end
 
callback = @(o,e) cbBridge(o,e,response);
hdl = handle.listener(handle(jobj), eventName, callback);
function cbBridge(o,e,response)
   hgfeval(response, java(o), e.JavaEvent)
end

Note that hgfeval, which is used within the cbBridge callback function, is a semi-documented pure-Matlab built-in function, which I described a few weeks ago.

If several events have the same case-insensitive name, then the additional callbacks will have an appended underscore character (e.g., ‘TestEventCallback_’):

// In the Java class:
public interface MyTestListener extends java.util.EventListener
{
    void testEvent(MyTestEvent e);
    void testevent(TestEvent2 e);
}

% …and back in Matlab:
>> evt=EventTest; evt.get
	Class = [ (1 by 1) java.lang.Class array]
	TestEventCallback = 
	TestEventCallbackData = []
	TestEventCallback_ = 
	TestEventCallback_Data = [] 
	...

To complete this discussion, it should be noted that Matlab also automatically defines corresponding Events in the Java object’s classhandle. Unfortunately, classhandle events are not differentiated in a similar manner – in this case only a single event is created, named Testevent. classhandle events, and their relationship to the preceding discussion, will be described in Donn Scull’s upcoming series on UDD.

An alternative to using callbacks on Java events, as shown above, is to use undocumented handle.listeners:

hListener = handle.listener(handle(evt),'TestEvent',callback);

There are several other odds and ends, but this article should be sufficient for implementing a fully-functional Java event handling mechanism in Matlab. Good luck!

1. UDD Events and Listeners UDD event listeners can be used to listen to property value changes and other important events of Matlab objects...
2. uisplittool & uitogglesplittool callbacks Matlab's undocumented uisplittool and uitogglesplittool are powerful toolbar controls - this article explains how to customize their behavior...
3. UDD and Java UDD provides built-in convenience methods to facilitate the integration of Matlab UDD objects with Java code - this article explains how...
4. Uicontrol callbacks This post details undocumented callbacks exposed by the underlying Java object of Matlab uicontrols, that can be used to modify the control's behavior in a multitude of different events...

In my last post I explored some tips on tooltips. One of these tips involved displaying tooltips on disabled uicontrols. I explained that displaying tooltips on inactive controls is problematic since Matlab appears to intercept mouse events to these inactive controls, so even setting the tooltip on the underlying Java object will not work: The java object appears not to receive the mouse-hover event and therefore does not “know” that it’s time to display the tooltip.

When Yair and I deliberated this issue, he pointed me to his comment on a previous article showing an undocumented Java technique (Java also has some…) for forcing a tooltip to appear using the ActionMap of the uicontrol’s underlying Java object to get at a postTip action. We discussed using a WindowButtonMotionFcn callback to see if the mouse was above the inactive control, then triggering the forced tooltip display. Yair then went on to remind me and I quote: “you’ll need to chain existing WindowButtonMotionFcn callbacks and take into account ModeManagers that override them.”

Frankly, having written code previously that handles callback chaining, I would rather poke myself in the eye with a fork!

The Image Processing Toolbox has the nice pair of iptaddcallback and iptremovecallback functions that largely handle these issues. But for general Matlab, there seemed to be no alternative until I remembered that events trigger callbacks. I decided to use a listener for the WindowButtonMotion event to detect the mouse motion. Event listeners were briefly explained two weeks ago and deserve a dedicated future article. The advantage of using an event listener is that we don’t disturb any existing WindowButtonMotionFcn callback. We still need to be somewhat careful that our listeners don’t do conflicting things, but it’s a lot easier than trying to manage everything through the single WindowButtonMotionFcn.

A demonstration of this appears below with some comments following (note that this code uses the FindJObj utility):

function inactiveBtnToolTip
  %Illustrates how to make a tooltip appear on an inactive control
  h = figure('WindowButtonMotionFcn',@windowMotion,'Pos',[400,400,200,200]);
  col = get(h,'color');
  lbl = uicontrol('Style','text', 'Pos',[10,160,120,20], ...
                  'Background',col, 'HorizontalAlignment','left');
  btn = uicontrol('Parent',h, 'String','Button', ...
                  'Enable','inactive', 'Pos',[10,40,60,20]);
  uicontrol('Style','check', 'Parent',h, 'String','Enable button tooltip', ...
            'Callback',@chkTooltipEnable, 'Value',1, ...
            'Pos',[10,80,180,20], 'Background',col);
  drawnow;
 
  %create the tooltip and postTip action
  jBtn = findjobj(btn);
  import java.awt.event.ActionEvent;
  javaMethodEDT('setToolTipText',jBtn,'This button is inactive');
  actionMap = javaMethodEDT('getActionMap',jBtn);
  action = javaMethodEDT('get',actionMap,'postTip');
  actionEvent = ActionEvent(jBtn, ActionEvent.ACTION_PERFORMED, 'postTip');
 
  %get the extents plus 2 pixels of the control to compare to the mouse position
  btnPos = getpixelposition(btn)+[-2,-2,4,4]; %give a little band around the control
  left = btnPos(1);
  right = sum(btnPos([1,3]));
  btm = btnPos(2);
  top =  sum(btnPos([2,4]));
 
  % add a listener on mouse movement events
  tm = javax.swing.ToolTipManager.sharedInstance; %tooltip manager
  pointListener = handle.listener(h,'WindowButtonMotionEvent',@figMouseMove);
 
  %inControl is a flag to prevent multiple triggers of the postTip action
  %while mouse remains in the button
  inControl = false;
 
  function figMouseMove(src,evtData) %#ok
    %get the current point
    cPoint = evtData.CurrentPoint;
 
    if cPoint(1) >= left && cPoint(1) <= right &&...
       cPoint(2) >= btm  && cPoint(2) <= top
 
      if ~inControl %we just entered
        inControl = true;
        action.actionPerformed(actionEvent); %show the tooltip
      end %if
    else
      if inControl %we just existed
        inControl = false;
        %toggle to make it disappear when leaving button
        javaMethodEDT('setEnabled',tm,false);
        javaMethodEDT('setEnabled',tm,true);
      end %if
    end %if
  end %gpMouseMove
 
  function windowMotion(varargin)
    %illustrate that we can still do a regular window button motion callback
    set(lbl,'String',sprintf('Mouse position: %d, %d',get(h,'CurrentPoint')));
    drawnow;
  end %windowMotion
 
  function chkTooltipEnable(src,varargin)
    if get(src,'Value')
      set(pointListener,'Enable','on');
    else
      set(pointListener,'Enable','off');
    end %if
  end %chkTooltipEnable
end %inactiveBtnToolTip

Tooltip on an inactive button

Comments on the code:

1. The code illustrates that we can successfully add an additional listener to listen for mouse motion events while still carrying out the original WindowButtonMotionFcn callback. This makes chaining callbacks much easier.
2. The handle.listener object has an Enable property that we can use to temporarily turn the listener on and off. This can be seen in the chkTooltipEnable() callback for the check box in the code above. If we wanted to permanently remove the listener we would simply use delete(pointListener). Note that addlistener adds a hidden property to the object being listened to, so that the listener is tied to the object’s lifecycle. If you create a listener directly using handle.listener you are responsible for it’s disposition. Unfortunately, addlistener fails for HG handles on pre-R2009 Matlab releases, so we use handle.listener directly.
3. The code illustrates a good practice when tracking rapidly firing events like mouse movement of handling reentry into the callback while it is still processing a previous callback. Here we use a flag called inControl to prevent the postTip action being continuously fired while the mouse remains in the control.
4. I was unable to determine if there is any corresponding action for the postTip to dismiss tips so I resorted to using the ToolTipManager to toggle its own Enable property to cleanly hide the tooltip as the mouse leaves the control.

Each Matlab callback has an associated event with it. Some of the ones that might be immediately useful at the figure-level are WindowButtonDown, WindowButtonUp, WindowKeyPress, and WindowKeyRelease. They can all be accessed through handle.listener or addlistener as in the code above.

Unfortunately, events do not always have names that directly correspond to the callback names. In order to see the list of available events for a particular Matlab object, use the following code, which relies on another undocumented function – classhandle. Here we list the events for gcf:

>> get(get(classhandle(handle(gcf)),'Events'),'Name')
ans = 
    'SerializeEvent'
    'FigureUpdateEvent'
    'ResizeEvent'
    'WindowKeyReleaseEvent'
    'WindowKeyPressEvent'
    'WindowButtonUpEvent'
    'WindowButtonDownEvent'
    'WindowButtonMotionEvent'
    'WindowPostChangeEvent'
    'WindowPreChangeEvent'

Note that I have made extensive use of the javaMethodEDT function to execute Java methods that affect swing components on Swing’s Event Dispatch Thread. I plan to write about this and related functions in my next article.

1. Matlab and the Event Dispatch Thread (EDT) The Java Swing Event Dispatch Thread (EDT) is very important for Matlab GUI timings. This article explains the potential pitfalls and their avoidance using undocumented Matlab functionality....
2. GUI integrated browser control A fully-capable browser component is included in Matlab and can easily be incorporated in regular Matlab GUI applications. This article shows how....
3. Multi-line tooltips Multi-line tooltips are very easy to set up, once you know your way around a few undocumented hiccups....
4. Spicing up Matlab uicontrol tooltips Matlab uicontrol tooltips can be spiced-up using HTML and CSS, including fonts, colors, tables and images...

There are three distinct simple ways to achieve continuous callbacks:

Using Java callbacks

As explained in an earlier article, Matlab uicontrols are basically Java Swing objects that possess a large number of useful callbacks. Matlab sliders’ underlying Java objects, which are really not JSliders but JScrollBars, have an AdjustmentValueChangedCallback property that is useful for our purposes and is accessible using the FindJObj utility. Simply download FindJObj from the File Exchange, and then:

hSlider = uicontrol('style','slider', ...);
jScrollBar = findjobj(hSlider);
jScrollBar.AdjustmentValueChangedCallback = @myCbFcn;
% or: set(jScrollBar,'AdjustmentValueChangedCallback',@myCbFcn)

Where myCbFcn is the Matlab callback function that will be invoked continuously when the arrow buttons are depressed or the slider’s thumb is dragged.

Using an event listener

An alternative to the Java route is to use Matlab’s undocumented handle.listener function to listen to the slider’s Action event, as follows:

hListener = handle.listener(hSlider,'ActionEvent',@myCbFcn);

This alternative is used by Matlab’s own imscrollpanel function:

if isJavaFigure
   % Must use these ActionEvents to get continuous events fired as slider
   % thumb is dragged. Regular callbacks on sliders give only one event
   % when the thumb is released.
   hSliderHorListener = handle.listener(hSliderHor,...
      'ActionEvent',@scrollHorizontal);
   hSliderVerListener = handle.listener(hSliderVer,...
      'ActionEvent',@scrollVertical);
   setappdata(hScrollpanel,'sliderListeners',...
      [hSliderHorListener hSliderVerListener]);
else
   % Unfortunately, the event route is only available with Java Figures,
   % so platforms without Java Figure support get discrete events only
   % when the mouse is released from dragging the slider thumb.
   set(hSliderHor,'callback',@scrollHorizontal)
   set(hSliderVer,'callback',@scrollVertical)
end

Using a property listener

The handle.listener function can also be used to listen to property value changes. In our case, set a post-set listener, that gets triggered immediately following Value property updates, as follows:

hhSlider = handle(hSlider);
hProp = findprop(hhSlider,'Value');  % a schema.prop object
hListener = handle.listener(hhSlider,hProp,'PropertyPostSet',@myCbFcn);

In addition to ‘PropertyPostSet’, we could also listen on ‘PropertyPreSet’, which is triggered immediately before the property is modified. There are also corresponding ‘*Get’ options. In relatively old Matlab releases (I believe R2007b and earlier, but I’m not certain), the option names were simply ‘PostSet’, ‘PreSet’ etc., without the ‘Property’ prefix.

Do you know of any other way to achieve continuous callbacks? If so, I would be delighted to hear in the comments section below.

1. Controlling callback re-entrancy Callback reentrancy is a major problem for frequently-fired events. Luckily, it can easily be solved....
2. Inactive Control Tooltips & Event Chaining Inactive Matlab uicontrols cannot normally display their tooltips. This article shows how to do this with a combination of undocumented Matlab and Java hacks....
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4. Uicontrol callbacks This post details undocumented callbacks exposed by the underlying Java object of Matlab uicontrols, that can be used to modify the control's behavior in a multitude of different events...

So the prompt can (probably) not be modified in advance, but what if it could be modified after being displayed? It is true that my cprintf utility modifies the Command-Window contents in order to display formatted text in a variety of font colors. But this case is different since cprintf runs once synchronously (user-invoked), whereas the prompt appears asynchronously multiple times.

There are two methods of handling multiple asynchronous events in Matlab: setting a callback on the object, and setting a PostSet handle.listener (or schema.listener) on the relevant object property. The first of these methods is a well-known Matlab practice, although we shall see that it uses an undocumented callback and functionality; the PostSet method is entirely undocumented and not well-known and shall be described in some later article. I decided to use the callback method to set the prompt – interested readers can try the PostSet method.

Setting the Command Window’s callback

The solution involved finding the Command-Window reference handle, and setting one of its many callbacks, in our case CaretUpdateCallback. This callback is fired whenever the desktop text is modified, which is an event we trap to replace the displayed prompt:

% Get the reference handle to the Command Window text area
jDesktop = com.mathworks.mde.desk.MLDesktop.getInstance;
try
  cmdWin = jDesktop.getClient('Command Window');
  jTextArea = cmdWin.getComponent(0).getViewport.getComponent(0);
catch
  commandwindow;
  jTextArea = jDesktop.getMainFrame.getFocusOwner;
end
 
% Instrument the text area's callback
if nargin && ~isempty(newPrompt) && ~strcmp(newPrompt,'>> ')
  set(jTextArea,'CaretUpdateCallback',{@setPromptFcn,newPrompt});
else
  set(jTextArea,'CaretUpdateCallback',[]);
end

Now that we have the Command-Window object callback set, we need to set the logic of prompt replacement – this is done in the internal Matlab function setPromptFcn. Here is its core code:

% Does the displayed text end with the default prompt?
% Note: catch a possible trailing newline
try jTextArea = jTextArea.java;  catch,  end  %#ok
cwText = get(jTextArea,'Text');
pos = strfind(cwText(max(1,end-3):end),'>> ');
if ~isempty(pos)
  % Short prompts need to be space-padded
  newLen = jTextArea.getCaretPosition;
  if length(newPrompt)<3
    newPrompt(end+1:3) = ' ';
  elseif length(newPrompt)>3
    fprintfStr = newPrompt(1:end-3);
    fprintf(fprintfStr);
    newLen = newLen + length(fprintfStr);
  end
 
  % The Command-Window text should be modified on the EDT
  awtinvoke(jTextArea,'replaceRange(Ljava.lang.String;II)',...
            newPrompt(end-2:end), newLen-3, newLen);
  awtinvoke(jTextArea,'repaint()');
end

In this code snippet, note that we space-pad prompt string that are shorter than 3 characters: this is done to prevent an internal-Matlab mixup when displaying additional text – Matlab “knows” the Command-Window’s text position and it gets mixed up if it turns out to be shorter than expected.

Also note that I use the semi-documented awtinvoke function to replace the default prompt (and an automatically-appended space) on the Event-Dispatch Thread (more on this in a future article). Since Matlab R2008a, I could use the more convenient javaMethodEDT function, but I wanted my code to work on all prior Matlab 7 versions, where javaMethodEDT was not yet available.

Preventing callback re-entry

The callback snippet above would enter an endless loop if not changed: whenever the prompt is modified the callback would have been re-fired, the prompt re-modified and so on endlessly. There are many methods of preventing callback re-entry – here’s the one I chose:

function setPromptFcn(jTextArea,eventData,newPrompt)
 
  % Prevent overlapping reentry due to prompt replacement
  persistent inProgress
  if isempty(inProgress)
    inProgress = 1;  %#ok unused
  else
    return;
  end
 
  try
    % *** Prompt modification code goes here ***
 
    % force prompt-change callback to fizzle-out...
    pause(0.02);
  catch
    % Never mind - ignore errors...
  end
 
  % Enable new callbacks now that the prompt has been modified
  inProgress = [];
 
end  % setPromptFcn

Handling multiple prompt types

I now wanted my function to handle both static prompt strings (like: ‘[Yair] ‘) and dynamic prompts (like: ‘[25-Jan-2010 01:00:51] ‘). This is done by accepting string-evaluable strings/functions:

% Try to evaluate the new prompt as a function
try
  origNewPrompt = newPrompt;
  newPrompt = feval(newPrompt);
catch
  try
    newPrompt = eval(newPrompt);
  catch
    % Never mind - probably a string...
  end
end
if ~ischar(newPrompt) && ischar(origNewPrompt)
  newPrompt = origNewPrompt;
end

File Exchange submission

I then added some edge-case error handling and wrapped everything in a single utility called setPrompt that is now available on the File Exchange.

In the future, if I find time, energy and interest, maybe I’ll combine cprintf‘s font-styling capabilities, to enable setting colored prompts.

Setting a continuously-updated timestamp prompt

Using the code above, we can now display a dynamic timestamp prompt, as follows:

setPrompt usage examples

However, the displayed timestamp is somewhat problematic in the sense that it indicates the time of prompt creation rather than the time that the associated Command-Window command was executed. In the screenshot above, [25-Jan-2010 01:29:42] is the time that the 234 command was executed, not the time that the setPrompt command was executed. This is somewhat misleading. It would be better if the last (current) timestamp was continuously updated and would therefore always display the latest command’s execution time. This can be done using a Matlab timer as follows:

% This is entered in the main function before setting the prompt:
stopPromptTimers;
if nargin && strcmpi(newPrompt,'timestamp')
  % Update initial prompt & prepare a timer to continuously update it
  newPrompt = @()(['[',datestr(now),'] ']);
  start(timer('Tag','setPromptTimer', 'Name','setPromptTimer', ...
              'ExecutionMode','fixedDelay', 'ObjectVisibility','off', ...
              'Period',0.99, 'StartDelay',0.5, ...
              'TimerFcn',{@setPromptTimerFcn,jTextArea}));
end
 
% Stop & delete any existing prompt timer(s)
function stopPromptTimers
  try
    timers = timerfindall('tag','setPromptTimer');
    if ~isempty(timers)
      stop(timers);
      delete(timers);
    end
  catch
    % Never mind...
  end
end  % stopPromptTimers
 
% Internal timer callback function
function setPromptTimerFcn(timerObj,eventData,jTextArea)
  try
    try jTextArea = jTextArea.java;  catch,  end  %#ok
    pos = getappdata(jTextArea,'setPromptPos');
    newPrompt = datestr(now);
    awtinvoke(jTextArea,'replaceRange(Ljava.lang.String;II)',...
              newPrompt, pos, pos+length(newPrompt));
    awtinvoke(jTextArea,'repaint()');
  catch
    % Never mind...
  end
end  % setPromptTimerFcn

Can you come up with some innovative prompts? If so, please share them in a comment below.

Update 2010-Jan-26: The code in this article was updated since it was first published yesterday.

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