public class EventHandler extends Object implements InvocationHandler
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.
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.
This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, myButton, "nextFocusableComponent", "source")
//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:
This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, target, "doActionEvent", "")
//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.
This would correspond to the following inner class implementation:EventHandler.create(ActionListener.class, myButton, "label", "source.text")
//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
might be written as the following inner class (assuming all the properties had canonical getter methods and returned the appropriate types):EventHandler.create(ActionListener.class, target, "a", "b.c.d")
//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.Proxy,
EventObject| Constructor and Description |
|---|
EventHandler(Object target,
String action,
String eventPropertyName,
String listenerMethodName)
Creates a new
EventHandler object;
you generally use one of the create methods
instead of invoking this constructor directly. |
| Modifier and Type | Method and Description |
|---|---|
static <T> T |
create(Class<T> listenerInterface,
Object target,
String action)
Creates an implementation of
listenerInterface in which
all of the methods in the listener interface apply
the handler's action to the target. |
static <T> T |
create(Class<T> listenerInterface,
Object target,
String action,
String eventPropertyName)
/**
Creates an implementation of
listenerInterface in which
all of the methods pass the value of the event
expression, eventPropertyName, to the final method in the
statement, action, which is applied to the target. |
static <T> T |
create(Class<T> listenerInterface,
Object target,
String action,
String eventPropertyName,
String listenerMethodName)
Creates an implementation of
listenerInterface in which
the method named listenerMethodName
passes the value of the event expression, eventPropertyName,
to the final method in the statement, action, which
is applied to the target. |
String |
getAction()
Returns the name of the target's writable property
that this event handler will set,
or the name of the method that this event handler
will invoke on the target.
|
String |
getEventPropertyName()
Returns the property of the event that should be
used in the action applied to the target.
|
String |
getListenerMethodName()
Returns the name of the method that will trigger the action.
|
Object |
getTarget()
Returns the object to which this event handler will send a message.
|
Object |
invoke(Object proxy,
Method method,
Object[] arguments)
Extract the appropriate property value from the event and
pass it to the action associated with
this
EventHandler. |
@ConstructorProperties(value={"target","action","eventPropertyName","listenerMethodName"}) public EventHandler(Object target, String action, String eventPropertyName, String listenerMethodName)
EventHandler object;
you generally use one of the create methods
instead of invoking this constructor directly. Refer to
the general version of create for a complete description of
the eventPropertyName and listenerMethodName
parameter.target - the object that will perform the actionaction - the name of a (possibly qualified) property or method on
the targeteventPropertyName - the (possibly qualified) name of a readable property of the incoming eventlistenerMethodName - the name of the method in the listener interface that should trigger the actionNullPointerException - if target is nullNullPointerException - if action is nullEventHandler,
create(Class, Object, String, String, String),
getTarget(),
getAction(),
getEventPropertyName(),
getListenerMethodName()public Object getTarget()
EventHandler(Object, String, String, String)public String getAction()
EventHandler(Object, String, String, String)public String getEventPropertyName()
EventHandler(Object, String, String, String)public String getListenerMethodName()
null signifies that all methods in the
listener interface trigger the action.EventHandler(Object, String, String, String)public Object invoke(Object proxy, Method method, Object[] arguments)
EventHandler.invoke in interface InvocationHandlerproxy - the proxy objectmethod - the method in the listener interfacearguments - an array of objects containing the values of the
arguments passed in the method invocation on the proxy instance,
or null if interface method takes no arguments.
Arguments of primitive types are wrapped in instances of the
appropriate primitive wrapper class, such as
java.lang.Integer or java.lang.Boolean.EventHandlerpublic static <T> T create(Class<T> listenerInterface, Object target, String action)
listenerInterface in which
all of the methods in the listener interface apply
the handler's action to the target. This
method is implemented by calling the other, more general,
implementation of the create method with both
the eventPropertyName and the listenerMethodName
taking the value null. Refer to
the general version of create for a complete description of
the action parameter.
To create an ActionListener that shows a
JDialog with dialog.show(),
one can write:
EventHandler.create(ActionListener.class, dialog, "show")
T - the type to createlistenerInterface - the listener interface to create a proxy fortarget - the object that will perform the actionaction - the name of a (possibly qualified) property or method on
the targetlistenerInterfaceNullPointerException - if listenerInterface is nullNullPointerException - if target is nullNullPointerException - if action is nullcreate(Class, Object, String, String)public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName)
listenerInterface in which
all of the methods pass the value of the event
expression, eventPropertyName, to the final method in the
statement, action, which is applied to the target.
This method is implemented by calling the
more general, implementation of the create method with
the listenerMethodName taking the value null.
Refer to
the general version of create for a complete description of
the action and eventPropertyName parameters.
To create an ActionListener that sets the
the text of a JLabel to the text value of
the JTextField source of the incoming event,
you can use the following code:
This is equivalent to the following code:EventHandler.create(ActionListener.class, label, "text", "source.text");
//Equivalent code using an inner class instead of EventHandler.
new ActionListener() {
public void actionPerformed(ActionEvent event) {
label.setText(((JTextField)(event.getSource())).getText());
}
};
T - the type to createlistenerInterface - the listener interface to create a proxy fortarget - the object that will perform the actionaction - the name of a (possibly qualified) property or method on
the targeteventPropertyName - the (possibly qualified) name of a readable property of the incoming eventlistenerInterfaceNullPointerException - if listenerInterface is nullNullPointerException - if target is nullNullPointerException - if action is nullcreate(Class, Object, String, String, String)public static <T> T create(Class<T> listenerInterface, Object target, String action, String eventPropertyName, String listenerMethodName)
listenerInterface in which
the method named listenerMethodName
passes the value of the event expression, eventPropertyName,
to the final method in the statement, action, which
is applied to the target. All of the other listener
methods do nothing.
The eventPropertyName string is used to extract a value
from the incoming event object that is passed to the target
method. The common case is the target method takes no arguments, in
which case a value of null should be used for the
eventPropertyName. Alternatively if you want
the incoming event object passed directly to the target method use
the empty string.
The format of the eventPropertyName string is a sequence of
methods or properties where each method or
property is applied to the value returned by the preceding method
starting from the incoming event object.
The syntax is: propertyName{.propertyName}*
where propertyName matches a method or
property. For example, to extract the point
property from a MouseEvent, you could use either
"point" or "getPoint" as the
eventPropertyName. To extract the "text" property from
a MouseEvent with a JLabel source use any
of the following as eventPropertyName:
"source.text",
"getSource.text" "getSource.getText" or
"source.getText". If a method can not be found, or an
exception is generated as part of invoking a method a
RuntimeException will be thrown at dispatch time. For
example, if the incoming event object is null, and
eventPropertyName is non-null and not empty, a
RuntimeException will be thrown.
The action argument is of the same format as the
eventPropertyName argument where the last property name
identifies either a method name or writable property.
If the listenerMethodName is null
all methods in the interface trigger the action to be
executed on the target.
For example, to create a MouseListener that sets the target
object's origin property to the incoming MouseEvent's
location (that's the value of mouseEvent.getPoint()) each
time a mouse button is pressed, one would write:
This is comparable to writing aEventHandler.create(MouseListener.class, target, "origin", "point", "mousePressed");
MouseListener in which all
of the methods except mousePressed are no-ops:
//Equivalent code using an inner class instead of EventHandler.
new MouseAdapter() {
public void mousePressed(MouseEvent e) {
target.setOrigin(e.getPoint());
}
};
T - the type to createlistenerInterface - the listener interface to create a proxy fortarget - the object that will perform the actionaction - the name of a (possibly qualified) property or method on
the targeteventPropertyName - the (possibly qualified) name of a readable property of the incoming eventlistenerMethodName - the name of the method in the listener interface that should trigger the actionlistenerInterfaceNullPointerException - if listenerInterface is nullNullPointerException - if target is nullNullPointerException - if action is nullEventHandler Submit a bug or feature
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