Event Handling and Listeners in SWING - PowerPoint PPT Presentation

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Event Handling and Listeners in SWING

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To write an Action Listener, follow the steps given below: ... Compound components ... Potential problems ... – PowerPoint PPT presentation

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Title: Event Handling and Listeners in SWING

1
Event Handling and Listeners in SWING

The practice of event handling

2
Some swing component events and listeners
Act that results in event Listener
User clicks a button, presses return while typing in a text field, or chooses a menu item ActionListener
User closes a window WindowListener
User presses a mouse button while the cursor is over a component MouseListener
User moves the mouse over a component MouseMotionListener
Component becomes visible ComponentListener
Component gets the keyboard focus FocusListener
Table or list selection changes ListSelectionListener
3
Implementing listeners

To write an Action Listener, follow the steps
given below
Declare an event handler class and specify that
the class either implements an ActionListener
interface or extends a class that implements an
ActionListener interface.
For example public class MyClass implements
ActionListener
Register an instance of the event handler class
as a listener on one or more components.
For example someComponent.addActionListener(insta
nceOfMyClass)
Include code that implements the methods in
listener interface.
For example public void actionPerformed(ActionEve
nt e) ...//code that reacts to the action...

Three key bits of code
1) add interface
2) register
3) handle
Components can have multiple listeners
A simple JButton ActionListener

4
Implementing listeners (2)

public class myClass implements ActionListener
// where setting up occurs (e.g. constructor)
JButton button new JButton(I am a button)
button.addActionListener(this)
public void actionPerformed(ActionEvent e)
// respond to event
// end response method
// end class

5
Implementing listeners (2)

import java.awt.
import java.awt.event.
public class AL extends Frame implements
WindowListener,ActionListener
TextField text new TextField(20)
Button b
private int numClicks 0
public static void main(String args)
AL myWindow new AL("My first window")
myWindow.setSize(350,100)
myWindow.setVisible(true)
public AL(String title)
super(title)
setLayout(new FlowLayout())
addWindowListener(this)

Set up things public class AL extends Frame
implements windowListener,ActionListener
TextField text new TextField(20) Button b
private int numClicks 0
6
Implementing listeners (2)
We would like to handle the button-click event,
so we add an action listener to the button b as
below b new Button("Click me")
b.addActionListener(this)

import java.awt.
import java.awt.event.
public class AL extends Frame implements
WindowListener,ActionListener
TextField text new TextField(20)
Button b
private int numClicks 0
public static void main(String args)
AL myWindow new AL("My first window")
myWindow.setSize(350,100)
myWindow.setVisible(true)
public AL(String title)
super(title)
setLayout(new FlowLayout())
addWindowListener(this)

7
Implementing listeners (2)
public void actionPerformed(ActionEvent e)
numClicks text.setText("Button Clicked "
numClicks " times") Now, when the user clicks
the Button b, the button fires an action event
which invokes the action listener's
actionPerformed method.

import java.awt.
import java.awt.event.
public class AL extends Frame implements
WindowListener,ActionListener
TextField text new TextField(20)
Button b
private int numClicks 0
public static void main(String args)
AL myWindow new AL("My first window")
myWindow.setSize(350,100)
myWindow.setVisible(true)
public AL(String title)
super(title)
setLayout(new FlowLayout())
addWindowListener(this)

8
Types of event listeners

Global component listeners
may be used for any Swing components
Types of global listeners
ComponentListener (changes in size, position,
visibility)
FocusListener (whether ability for keyboard
input)
KeyListener (key press events, only with focus)
MouseListener (clicks and movement into/out of
component area)
MouseMotionListener (changes in position over
component)

9
Types of event listeners (2)

Component-specific listeners
relevant to specific components actions
Types
ActionListener
CaretListener
ChangeListener
DocumentListener
ItemListener
ListSelectionListener
WindowListener
etc.
See
http//java.sun.com/docs/books/tutorial/uiswing/ev
ents/eventsandcomponents.html

10
Working with event listeners

Getting event information
Low-level events
Semantic events
Adapters for event handling
Inner classes for event handling

11
Getting event information

EventObject class - use sub classes of this to
determine whats happened.
Get the firing object with getSource()
Actual event classes sometimes have specific
types
e.g. the ComponentListener uses a sub-class of
EventObject ComponentEvent that has
getComponent()
Event classes may define methods that return more
information
e.g. ActionEvent has a method for getting
modifiers (Shift, Alt, Ctrl)

12
Low-level and semantic events

Low-level events - window-system level
e.g. mouse, key, component, container, focus,
window
trigger component-independent
Semantic events
everything else! e.g. action, item, list
selection
trigger can differ by component
e.g. button click and textfield return action
events

13
Low-level and semantic events

Listen for semantic events whenever possible
Gives robust and portable code
eg Button - listen for action event rather than
mouse event. Means that button responds to
keyboard shortcuts.
Compound components
eg combo box - no real way of guaranteeing low
level listeners on all look and feel specific
components used to form the compound component.

14
Adapters for event handling

Classes which implement listener interfaces must
implement all listener methods
e.g. MouseListener has 5 methods mouseClicked,
mouseReleased, mousePressed, mouseEntered,
mouseExited
This leads to cluttered code
Say you only want mouseClicked to do something
then all others have to be implemented but empty
Alternative.

15
Adapters for event handling (2)

... is to extend a MouseAdapter class
inherits empty definitions of all five
mouseListener methods. Eg
public class MyClass extends MouseAdapter
...
someObject.addMouseListener(this)
...
public void mouseClicked(MouseEvent e)
//Event handler implementation goes here...

16
Inner classes for event handling

Dont want to / cant inherit from an adapter
class?
theres no multiple inheritance in Java
eg cant extend JPanel AND MouseAdapter
Solution use an inner class
public class MyClass extends JPanel
anObject.addMouseListener(new myAdapter())
class myAdapter extends MouseAdapter
public void mouseClicked(MouseEvent e)
// blah
// end mouseClicked
// end inner class
// end MyClass

17
Inner classes for event handling (2)

Anonymous classes -
used to simplify code
good when only 1 instance will ever be needed
public class MyClass extends JPanel
...
someObject.addMouseListener(new MouseAdapter()
public void mouseClicked(MouseEvent e)
//Event handler implementation goes here
)
...

18
Threads and Swing

Why use them?
Improved perceived performance
Can remove time consuming task from event thread
to keep GUI responsive
Initialisation of program so GUI appears faster
Potential problems
Deadlock the application if access any realised
swing components from non event threads.

19
Threads and Swing

Remember the rule
Once a Swing component has been realised, all
code that might affect or depend on the state of
that component should be executed in the
event-dispatching thread.
If code does not need to be in event thread then
public void actionPerformed(ActionEvent e)
final SwingWorker worker new SwingWorker()
public Object construct()
//---code that might take a while to execute
is here...
return someValue
worker.start() //required for SwingWorker 3

20
Threads and Swing

invokeLater()
requests that event thread runs certain code
can be called from any thread
code goes in run method of Runable object
returns immediately without waiting for event
thread to execute code.
Runnable updateAComponent new Runnable()
public void run() component.doSomething()
SwingUtilities.invokeLater(updateAComponent)

21
Threads and Swing (4)

invokeAndWait()
identical to invokeLater() except doesnt return
till event thread has finished executing the
code.
Should use this if possible - less chance of
deadlock.
void showHelloThereDialog() throws Exception
Runnable showModalDialog new Runnable()
public void run()
JOptionPane.showMessageDialog(myMa
inFrame,
"Hello There")
SwingUtilities.invokeAndWait(showModalDialog)