The EventHandler class provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.

  The EventHandler class provides support for dynamically generating event listeners whose methods execute a simple statement involving an incoming event object and a target object.

The EventHandler class is intended to be used by interactive tools, such as application builders, that allow developers to make connections between beans. Typically connections are made from a user interface bean (the event source) to an application logic bean (the target). The most effective connections of this kind isolate the application logic from the user interface. For example, the EventHandler for a connection from a JCheckBox to a method that accepts a boolean value can deal with extracting the state of the check box and passing it directly to the method so that the method is isolated from the user interface layer.

Inner classes are another, more general way to handle events from user interfaces. The EventHandler class handles only a subset of what is possible using inner classes. However, EventHandler works better with the long-term persistence scheme than inner classes. Also, using EventHandler in large applications in which the same interface is implemented many times can reduce the disk and memory footprint of the application.

The reason that listeners created with EventHandler have such a small footprint is that the Proxy class, on which the EventHandler relies, shares implementations of identical interfaces. For example, if you use the EventHandler create methods to make all the ActionListeners in an application, all the action listeners will be instances of a single class (one created by the Proxy class). In general, listeners based on the Proxy class require one listener class to be created per listener type (interface), whereas the inner class approach requires one class to be created per listener (object that implements the interface).

You don't generally deal directly with EventHandler instances. Instead, you use one of the EventHandler create methods to create an object that implements a given listener interface. This listener object uses an EventHandler object behind the scenes to encapsulate information about the event, the object to be sent a message when the event occurs, the message (method) to be sent, and any argument to the method. The following section gives examples of how to create listener objects using the create methods.

Examples of Using EventHandler

The simplest use of EventHandler is to install a listener that calls a method on the target object with no arguments. In the following example we create an ActionListener that invokes the toFront method on an instance of javax.swing.JFrame.

myButton.addActionListener(
    (ActionListener)EventHandler.create(ActionListener.class, frame, "toFront"));

When myButton is pressed, the statement frame.toFront() will be executed. One could get the same effect, with some additional compile-time type safety, by defining a new implementation of the ActionListener interface and adding an instance of it to the button:

//Equivalent code using an inner class instead of EventHandler.
myButton.addActionListener(new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        frame.toFront();
    }
});

The next simplest use of EventHandler is to extract a property value from the first argument of the method in the listener interface (typically an event object) and use it to set the value of a property in the target object. In the following example we create an ActionListener that sets the nextFocusableComponent property of the target (myButton) object to the value of the "source" property of the event.

EventHandler.create(ActionListener.class, myButton, "nextFocusableComponent", "source")

This would correspond to the following inner class implementation:

//Equivalent code using an inner class instead of EventHandler.
new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        myButton.setNextFocusableComponent((Component)e.getSource()); 
    }
}

It's also possible to create an EventHandler that just passes the incoming event object to the target's action. If the fourth EventHandler.create argument is an empty string, then the event is just passed along:

EventHandler.create(ActionListener.class, target, "doActionEvent", "")

This would correspond to the following inner class implementation:

//Equivalent code using an inner class instead of EventHandler.
new ActionListener() {
    public void actionPerformed(ActionEvent e) {
        target.doActionEvent(e);
    }
}

Probably the most common use of EventHandler is to extract a property value from the source of the event object and set this value as the value of a property of the target object. In the following example we create an ActionListener that sets the "label" property of the target object to the value of the "text" property of the source (the value of the "source" property) of the event.

EventHandler.create(ActionListener.class, myButton, "label", "source.text")

This would correspond to the following inner class implementation:

//Equivalent code using an inner class instead of EventHandler.
new ActionListener {
    public void actionPerformed(ActionEvent e) {
        myButton.setLabel(((JTextField)e.getSource()).getText()); 
    }
}

The event property may be "qualified" with an arbitrary number of property prefixes delimited with the "." character. The "qualifying" names that appear before the "." characters are taken as the names of properties that should be applied, left-most first, to the event object.

For example, the following action listener

EventHandler.create(ActionListener.class, target, "a", "b.c.d")

might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):

//Equivalent code using an inner class instead of EventHandler.
new ActionListener {
    public void actionPerformed(ActionEvent e) {
        target.setA(e.getB().getC().isD()); 
    }
}

The target property may also be "qualified" with an arbitrary number of property prefixs delimited with the "." character. For example, the following action listener:

   EventHandler.create(ActionListener.class, target, "a.b", "c.d")
 

might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):

   //Equivalent code using an inner class instead of EventHandler.
   new ActionListener {
     public void actionPerformed(ActionEvent e) {
         target.getA().setB(e.getC().isD()); 
    }
}

As EventHandler ultimately relies on reflection to invoke a method we recommend against targeting an overloaded method. For example, if the target is an instance of the class MyTarget which is defined as:

   public class MyTarget {
     public void doIt(String);
     public void doIt(Object);
   }
 

Then the method doIt is overloaded. EventHandler will invoke the method that is appropriate based on the source. If the source is null, then either method is appropriate and the one that is invoked is undefined. For that reason we recommend against targeting overloaded methods.