| The Action interface provides a useful extension to the
ActionListener
interface in cases where the same functionality may be accessed by
several controls.
In addition to the actionPerformed method defined by the
ActionListener interface, this interface allows the
application to define, in a single place:
- One or more text strings that describe the function. These strings
can be used, for example, to display the flyover text for a button
or to set the text in a menu item.
- One or more icons that depict the function. These icons can be used
for the images in a menu control, or for composite entries in a more
sophisticated user interface.
- The enabled/disabled state of the functionality. Instead of having
to separately disable the menu item and the toolbar button, the
application can disable the function that implements this interface.
All components which are registered as listeners for the state change
then know to disable event generation for that item and to modify the
display accordingly.
This interface can be added to an existing class or used to create an
adapter (typically, by subclassing AbstractAction).
The Action object
can then be added to multiple Action-aware containers
and connected to Action-capable
components. The GUI controls can then be activated or
deactivated all at once by invoking the Action object's
setEnabled method.
Note that Action implementations tend to be more expensive
in terms of storage than a typical ActionListener,
which does not offer the benefits of centralized control of
functionality and broadcast of property changes. For this reason,
you should take care to only use Actions where their benefits
are desired, and use simple ActionListeners elsewhere.
Swing Components Supporting Action
Many of Swing's components have an Action property. When
an Action is set on a component, the following things
happen:
- The
Action is added as an ActionListener to
the component.
- The component configures some of its properties to match the
Action.
- The component installs a
PropertyChangeListener on the
Action so that the component can change its properties
to reflect changes in the Action's properties.
The following table describes the properties used by
Swing components that support Actions.
In the table, button refers to any
AbstractButton subclass, which includes not only
JButton but also classes such as
JMenuItem. Unless otherwise stated, a
null property value in an Action (or a
Action that is null) results in the
button's corresponding property being set to null.
| Component Property
| Components
| Action Key
| Notes
|
enabled
| All
| The isEnabled method
|
|
toolTipText
| All
| SHORT_DESCRIPTION
|
|
actionCommand
| All
| ACTION_COMMAND_KEY
|
|
mnemonic
| All buttons
| MNEMONIC_KEY
| A null value or Action results in the
button's mnemonic property being set to
'\0'.
|
text
| All buttons
| NAME
| If you do not want the text of the button to mirror that
of the Action, set the property
hideActionText to true. If
hideActionText is true, setting the
Action changes the text of the button to
null and any changes to NAME
are ignored. hideActionText is useful for
tool bar buttons that typically only show an Icon.
JToolBar.add(Action) sets the property to
true if the Action has a
non-null value for LARGE_ICON_KEY or
SMALL_ICON.
|
displayedMnemonicIndex
| All buttons
| DISPLAYED_MNEMONIC_INDEX_KEY
| If the value of DISPLAYED_MNEMONIC_INDEX_KEY is
beyond the bounds of the text, it is ignored. When
setAction is called, if the value from the
Action is null, the displayed
mnemonic index is not updated. In any subsequent changes to
DISPLAYED_MNEMONIC_INDEX_KEY, null
is treated as -1.
|
icon
| All buttons except of JCheckBox,
JToggleButton and JRadioButton.
| either LARGE_ICON_KEY or
SMALL_ICON
| The JMenuItem subclasses only use
SMALL_ICON. All other buttons will use
LARGE_ICON_KEY; if the value is null they
use SMALL_ICON.
|
accelerator
| All JMenuItem subclasses, with the exception of
JMenu.
| ACCELERATOR_KEY
|
|
selected
| JToggleButton, JCheckBox,
JRadioButton, JCheckBoxMenuItem and
JRadioButtonMenuItem
| SELECTED_KEY
| Components that honor this property only use
the value if it is
non-null . For example, if
you set an
Action that has a
null value for
SELECTED_KEY on a
JToggleButton , the
JToggleButton will not update it's selected state in
any way. Similarly, any time the
JToggleButton 's
selected state changes it will only set the value back on
the
Action if the
Action has a
non-null value for
SELECTED_KEY .
Components that honor this property keep their selected state
in sync with this property. When the same
Action is used
with multiple components, all the components keep their selected
state in sync with this property. Mutually exclusive
buttons, such as
JToggleButton s in a
ButtonGroup ,
force only one of the buttons to be selected. As such, do not
use the same
Action that defines a value for the
SELECTED_KEY property with multiple mutually
exclusive buttons.
|
JPopupMenu, JToolBar and JMenu
all provide convenience methods for creating a component and setting the
Action on the corresponding component. Refer to each of
these classes for more information.
Action uses PropertyChangeListener to
inform listeners the Action has changed. The beans
specification indicates that a null property name can
be used to indicate multiple values have changed. By default Swing
components that take an Action do not handle such a
change. To indicate that Swing should treat null
according to the beans specification set the system property
swing.actions.reconfigureOnNull to the String
value true.
version:
1.42 05/05/07
author:
Georges Saab
See Also: AbstractAction |