Java Reference
In-Depth Information
«interface»
XListener
Trigger an event
User
Action
source: SourceClass
+add
X
Listener(listener:
X
Listener)
+
handler(event: XEvent)
(2) Register by invoking
source.add
X
Listener(listener);
(1) A listener object is an
instance of a listener interface
listener:
ListenerClass
(a) A generic source object with a generic listener
«interface»
java.awt.event.ActionListener
source: javax.swing.JButton
+addActionListener(listener: ActionListener)
+
actionPerformed(event: ActionEvent)
(2) Register by invoking
source.add
Action
Listener(listener);
(1) An action event listener is an
instance of
ActionListener
listener:
CustomListenerClass
(b) A
JButton
source object with an
ActionListener
F
IGURE
16.5
A listener must be an instance of a listener interface and must be registered with a source object.
three columns in Table 16.1 list event types, the corresponding listener interfaces, and
the methods defined in the listener interfaces. The listener interface contains the
method(s), known as the
event handler(s)
, for processing the event. For example, as
shown in the first line of this table, the corresponding listener interface for
ActionEvent
is
ActionListener
; each listener for
ActionEvent
should imple-
ment the
ActionListener
interface; the
ActionListener
interface contains the
handler
actionPerformed(ActionEvent)
for processing an
ActionEvent
.
event handler
2.
The listener object must be registered by the source object
. Registration methods
depend on the event type. For
ActionEvent
, the method is
addActionListener
. In
general, the method is named
addXListener
for
XEvent
. A source object may fire
several types of events, and for each event the source object maintains a list of regis-
tered listeners and notifies them by invoking the
handler
of the listener object to
respond to the event, as shown in Figure 16.6. (Note that this figure shows the internal
implementation of a source class. You don't have to know how a source class such as
JButton
is implemented in order to use it, but this knowledge will help you understand
the Java event-driven programming framework.)
register listener
source: SourceClass
source: javax.swing.JButton
+add
X
Listener(
X
Listener listener)
+add
Action
Listener(
Action
Listener listener)
An event is
triggered
An event is
triggered
Store in a list
Store in a list
event:
X
Event
event:
Action
Event
listener1
listener2
...
listener
n
listener1
listener2
...
listener
n
Invoke
listener1.handler(event)
listener2.handler(event)
...
listener
n
.handler(event)
Invoke
listener1.actionPerformed(event)
listener2.actionPerformed(event)
...
listener
n
.actionPerformed(event)
(b) Internal function of a
JButton
object
(a) Internal function of a generic source object
F
IGURE
16.6
The source object notifies the listeners of the event by invoking the listener object's handler.
