Java Reference
In-Depth Information
The
UndoManager
manages the edit operations on an editable object by serving as an
UndoableEditListener
and responding to the creation of each
UndoableEditEvent
. When
an
UndoableEdit
can't be undone, a
CannotUndoException
is thrown. In addition, when an
UndoableEdit
can't be redone, a
CannotRedoException
is thrown.
If you want to create objects that support undoable and redoable operations, the objects
need to implement the
StateEditable
interface, and they can use the
UndoableEditSupport
class to help manage the list of
UndoableEdit
objects.
Before going into the details of the individual pieces of the Undo Framework, let's explore
how to use it with the Swing text components. If this is all you want to do, you don't need to
understand how the rest works.
Using the Undo Framework with Swing Text
Components
The Swing text components already support the necessary undo and redo capabilities. You
merely need to manage them with an
UndoManager
and tell the manager when to undo/redo
something.
As an example, consider a program that includes a
JTextArea
with two toolbar buttons for
undoing and redoing a text operation, as shown in Figure 21-1.
Figure 21-1.
Undo Swing text component usage example
To enable the
JTextArea
shown in Figure 21-1 to support undoable operations, you must
attach an
UndoableEditListener
to the
Document
of the component. Using an
UndoManager
as
the listener is all you need to do. First, you create the manager, and then you attach it.
UndoManager manager = new UndoManager();
textArea.getDocument().addUndoableEditListener(manager);
Once the manager is attached to the document of the
JTextArea
, it will monitor all changes
to the contents of the text area. Because each of the Swing text components has a
Document
data
model, you can associate an
UndoManager
with each of these components directly.
After attaching the manager to the text component, you must provide some means to tell
the manager to undo/redo an operation. Normally this would be done through a menu selec-
tion or a toolbar button selection. In Figure 21-1, this is done with the help of buttons on a
JToolBar
, with one button for each command. For the Undo button, you want the manager to
undo an operation. Therefore, the
ActionListener
for the button should call the
public void
undo()
method of the
UndoManager
. The Redo button's
ActionListener
should call the manager's
