Handling Event (Part 3)
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Event Handling
One event source and one event listener
Button
Action Event
ColorAction
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Event Handling
Multiple ways to activate the same command link all the event sources to the same listener
Menu item (Blue) Button (Blue) Keystroke (CTRL+B)
Menu Item Button Keystroke
Action Event Action Event Action Event
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ColorAction
Command Abstraction
Swing uses the Action interface to encapsulate commands and to attach them to multiple event sources An action is an object that encapsulates
A description of the command (as a text string and an optional icon) Parameters that are necessary to carry out the command (such as the requested color in the example)
It enables information concerning the command to be located in one place It also enables the command to be enabled or disabled in one place
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The Action Interface
The Action interface extends the ActionListener interface
void actionPerformed(ActionEvent event) void setEnabled(boolean b) boolean isEnabled() void putValue(String key, Object value) Object getValue(String key) void addPropertyChangeListener (PropertyChangeListener listener) void removePropertyChangeListener (PropertyChangeListener listener)
Implemented by AbstractAction class
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Action Interface Methods
setEnabled() and isEnabled()
Enable or disable the action and check whether the action is currently enabled If an action is attached to a menu or toolbar, the option is grayed out when the action is disabled
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Action Interface Methods
putValue and getValue methods store and retrieve arbitrary name/value pairs in the action object There are a few predefined strings
Action.NAME Action.SMALL_ICON (for toolbars and menus) Action.SHORT_DESCRIPTION (as tool tips) Action.MNEMONIC_KEY Action.ACCELERATOR_KEY
putValue(Action.NAME, Blue) putValue(Action.SMALL_ICON, new ImageIcon(blue_ball.gif));
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Action Interface Methods
addPropertyChangeListener and removePropertyChangeListener
Allow other objects, in particular menus or toolbars that trigger the action, to be notified when the properties of the action object change For example, if a menu is added as a property change listener of an action object and the action object is subsequently disabled, then the menu is called and can gray out the action name
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AbstractAction Class
Action is an interface, not a class
You have to implement all seven methods
The AbstractAction class implements all methods except for actionPerformed
It takes care of storing all name/value pairs and managing the property change listeners We simply extend it and supply an actionPerformed method
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AbstractAction Class
public class ColorAction extends AbstractAction { public ColorAction(String name, Icon icon, Color c) { putValue(Action.NAME, name); putValue(Action.SMALL_ICON, icon); putValue(color, c); putValue(Action.SHORT_DESCRIPTION, Set panel color to + name.toLowerCase()); } public void actionPerformed(ActionEvent event) { Color c = (Color) getValue(color); setBackground(c); repaint(); } }
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AbstractAction Class
Create action objects
Action blueAction = new ColorAction(Blue, new ImageIcon(blue_ball.gif), Color.BLUE);
Associate the action with a button
There is a JButton constructor that takes an Action object
JButton blueButton = new JButton(blueAction);
The constructor reads the name and icon from the action, sets the short description as tool tip, and sets the action as the listener
Associate the action with a menu item
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Add Action to Keystrokes
Keystrokes are delivered to the component that has focus
What are the components in this example? At any time, any one of the three buttons (Yellow, Blue, and Red) may have focus Each button needs to handle key events and listen to CTRL+Y, CTRL+B, and CTRL+R keys
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Add Action to Keystrokes
Generate objects of the KeyStroke class
Use the static method getKeyStroke Specify virtual key code and flags (such as SHIFT and CONTROL key combinations)
KeyStroke ctrlBKey = KeyStroke.getKeyStroke(KeyEvent.VK_B, InputEvent.CTRL_MASK); KeyStroke ctrlBKey = KeyStroke.getKeyStroke(ctrl B);
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Add Action to Keystrokes
Each JComponent has three input maps that map KeyStroke objects to associated actions
WHEN_FOCUSED (when this component has focus) WHEN_ANCESTOR_OF_FOCUSED_COMPONENT (when this component contains the component that has focus) WHEN_IN_FOCUSED_WINDOW (when this component is contained in the same window as the component that has focus)
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Add Action to Keystrokes
Keystroke processing check these maps in the following order
Check WHEN_FOCUSED map of the component with input focus. If keystroke exists, execute the corresponding action. If action is enabled, stop processing Starting from the component with input focus, check the WHEN_ANCESTOR_OF_FOCUSED_COMPONENT maps of its parent components. As soon as a map with the keystroke is found, execute the action. If action is enabled, stop processing Look at all visible and enabled components in the window with focus that have this keystroke registered in a WHEN_IN_FOCUSED_WINDOW map. Give these components a chance to execute the action. As soon as the first enabled action is executed, stop processing
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Add Action to Keystrokes
Obtain an input map from the component with getInputMap method
InputMap imap = panel.getInputMap(JComponent.WHEN_FOCUSED);
The WHEN_FOCUSED condition means that this map is consulted when the current component has the keyboard focus
Is this the map that we want?
In our example, one of the buttons, not the panel, has the focus; so either of the other two maps would work
InputMap imap = panel.getInputMap(JComponent. WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
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Add to Action Keystrokes
The InputMap does NOT map KeyStroke objects to Action objects
It maps them to arbitrary objects
The ActionMap maps objects to actions
Easier to share the same actions among keystrokes coming from different input maps
Each component has three input maps and one action map
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Add Action to Keystrokes
Tie the input maps and the action map
Each action needs a name
imap.put(KeyStroke.getKetStroke(ctrl Y), panel.yellow); ActionMap amap = panel.getActionMap(); amap.put(panel.yellow, yellowAction);
Use the string none for do-nothing action to deactivate a key
imap.put(KeyStroke.getKeyStroke(ctrl C), none);
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Summary
How to carry out the same action in response to a button, a menu item, or a keystroke
Make a class that extends AbstractAction class Make an object of the action class Construct a button or menu item from the action object For actions triggered by keystrokes
Locate the top-level component of the window (such as a panel) Get WHEN_ANCESTOR_OF_FOCUSED_COMPONENT of the top-level component Make a KeyStroke object for the desired keystroke Make an action key object (such as a string describing the action) Add the pair (keystroke, action key) into the input map Get the action map of the top-level component and add the pair (action key, action object) into the map
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ActionTest
import java.awt.*; import java.awt.event.*; import javax.swing.*; public class ActionTest { public static void main(String[] args) { ActionFrame frame = new ActionFrame(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } }
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ActionTest
class ActionFrame extends JFrame { public ActionFrame() { setTitle("ActionTest"); setSize(WIDTH, HEIGHT); ActionPanel panel = new ActionPanel(); Container contentPane = getContentPane(); contentPane.add(panel); } public static final int WIDTH = 300; public static final int HEIGHT = 200; }
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ActionTest
class ActionPanel extends JPanel { public ActionPanel() { // define actions Action yellowAction = new ColorAction("Yellow", new ImageIcon("yellow-ball.gif"), Color.YELLOW); Action blueAction = new ColorAction("Blue", new ImageIcon("blue-ball.gif"), Color.BLUE); Action redAction = new ColorAction("Red", new ImageIcon("red-ball.gif"), Color.RED); // add buttons for these actions add(new JButton(yellowAction)); add(new JButton(blueAction)); add(new JButton(redAction));
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ActionTest
// associate the Y, B, and R keys with names InputMap imap = getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT); imap.put(KeyStroke.getKeyStroke("ctrl Y"), "panel.yellow"); imap.put(KeyStroke.getKeyStroke("ctrl B"), "panel.blue"); imap.put(KeyStroke.getKeyStroke("ctrl R"), "panel.red"); // associate the names with actions ActionMap amap = getActionMap(); amap.put("panel.yellow", yellowAction); amap.put("panel.blue", blueAction); amap.put("panel.red", redAction); }
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ActionTest
public class ColorAction extends AbstractAction { /** Constructs a color action. @param name the name to show on the button @param icon the icon to display on the button @param c the background color */ public ColorAction(String name, Icon icon, Color c) { putValue(Action.NAME, name); putValue(Action.SMALL_ICON, icon); putValue(Action.SHORT_DESCRIPTION, "Set panel color to " + name.toLowerCase()); putValue("color", c); } public void actionPerformed(ActionEvent event) { Color c = (Color)getValue("color"); setBackground(c); repaint(); } } }
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ActionTest2
import java.awt.*; import java.awt.event.*; import javax.swing.*; public class ActionTest2 { public static void main(String[] args) { ActionFrame2 frame = new ActionFrame2(); frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE); frame.setVisible(true); } }
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ActionTest2
class ActionFrame2 extends JFrame { public ActionFrame2() { setTitle("ActionTest2"); setSize(640, 480); JPanel panel = new JPanel(); class ColorAction extends AbstractAction { public ColorAction(String name, Color c, Component comp) { putValue(Action.NAME, name); putValue("Color", c); target = comp; } public void actionPerformed(ActionEvent evt) { Color c = (Color)getValue("Color"); target.setBackground(c); target.repaint(); } private Component target; }
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ActionTest2
Action blueAction = new ColorAction("Blue", Color.BLUE, panel); Action yellowAction = new ColorAction("Yellow", Color.YELLOW, panel); Action redAction = new ColorAction("Red", Color.RED, panel); class ActionButton extends JButton { public ActionButton(Action a) { setText((String)a.getValue(Action.NAME)); Icon icon =(Icon)a.getValue(Action.SMALL_ICON); if (icon != null) setIcon(icon); addActionListener(a); } } panel.add(new ActionButton(yellowAction)); panel.add(new ActionButton(blueAction)); panel.add(new ActionButton(redAction));
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ActionTest2
panel.registerKeyboardAction(yellowAction, KeyStroke.getKeyStroke("ctrl Y"), JComponent.WHEN_IN_FOCUSED_WINDOW); panel.registerKeyboardAction(blueAction, KeyStroke.getKeyStroke("ctrl B"), JComponent.WHEN_IN_FOCUSED_WINDOW); panel.registerKeyboardAction(redAction, KeyStroke.getKeyStroke("ctrl R"), JComponent.WHEN_IN_FOCUSED_WINDOW); Container contentPane = getContentPane(); contentPane.add(panel); JMenu m = new JMenu("Color"); m.add(yellowAction); m.add(blueAction); m.add(redAction); JMenuBar mbar = new JMenuBar(); mbar.add(m); setJMenuBar(mbar); } }
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