Event Handling and Listeners in SWING

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Event Handling and Listeners in SWING

• The practice of event handling

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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
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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

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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

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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
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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)

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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)

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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)

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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

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Working with event listeners

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

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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)

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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

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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.

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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.

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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...

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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

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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
• )
• ...

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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.

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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

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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)

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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)
  

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Summary

• Implementing event listeners
• Types of event listeners
• Handling event listeners
• getting event information
• low-level and semantic events
• adapters
• inner classes - named and anonymous
• Threads

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A simple Swing program

• Uses components in containers
• Lays components out correctly
• Listens for events
• An example
• SwingExample.java (revisited)
• Code on Course Website

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A (Slightly) More Complex Swing program

• Uses components in containers (again)
• Lays components out correctly (again - but more
  complex)
• Listens for events - Multiple listeners
• Another example
• SwingExample2.java