Java Reference
In-Depth Information
Note
Some developers might find it useful to extend the
UndoManager
to expose the
UndoableEdit
list
that it is managing. Then they can display the presentation names of the various edit commands. Fortunately
(or unfortunately), by default, these aren't exposed.
■
UndoableEditListener Interface and UndoableEditEvent Class
The
UndoManager
implements the
UndoableEditListener
interface so that it can be notified
when undoable operations happen. The listener has the following definition:
public interface UndoableEditListener extends EventListener {
public void undoableEditHappened(UndoableEditEvent undoableEditEvent);
}
The
UndoableEditListener
uses an
UndoableEditEvent
to tell interested objects when a
command that can be undone has happened. The
UndoableEditEvent
class includes one property,
edit
, which returns the
UndoableEdit
object for the event:
public UndoableEdit getEdit()
.
Of all the classes in the Swing-related packages, only the
AbstractDocument
class (as defined
in the
Document
interface) comes with built-in support to add these listeners. When creating
your own classes that support undoable operations, you'll need to maintain your own list of
listeners with the help of the
UndoableEditSupport
class, described next.
UndoableEditSupport Class
The
UndoableEditSupport
class is similar to the JavaBeans-related classes of
PropertyChangeSupport
and
VetoableChangeSupport
. All three of these classes manage
a list of a specific type of listener. In the case of the
UndoableEditSupport
class, that
type of listener is the
UndoableEditListener
. You add listeners with
public void
addUndoableListener(UndoableEditListener)
and remove them with
public void
removeUndoableListener(UndoableEditListener)
.
When you want to notify listeners that an
UndoableEdit
operation has happened, you call
the
public void postEdit(UndoableEdit)
method, which creates an
UndoableEditEvent
and
calls the
undoableEditHappened()
method of each listener.
■
Note
The
UndoableEditSupport
class also includes support for combining multiple undoable edit
commands into a
CompoundEdit
with the
public void beginUpdate()
and
public void endUpdate()
methods.
The basic framework for the class usage follows in Listing 21-3. Normally, you tie the undo-
able event to some other operation. In this example, it's tied to the moment an
ActionEvent
happens, and therefore any registered
ActionListener
objects need to be notified.
