INFT316 Event Handling

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INFT316Event Handling
Tables from Wigglesworth and Lumby prepared by Dr
Michael Rees
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Observers

• The Observer model is a state change notification
  system
• Objects can monitor other objects for change
• Monitoring objects must implement interface
  Observer
• void update( Observable obj, Object arg )
• Monitored objects must extend class Observable
• void addObserver( Observer obj )
• void notifyObservers( )
• void notifyObservers( Object obj )
• void setChanged( )
• boolean hasChanged( )

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Observer Example

• public class Pump
• public static void main ()
• Pump p new Pump()
• Valve v new Valve()
• Buzzer b new Buzzer()
• v.addObserver(b)

• public class Buzzer implements Observer
• public void update ( Observable obj, Object arg)
  
• Valve v (Valve) obj
• if (v.getPressure( ) gt threshold )

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AWT and Swing Components
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Painting Swing Components

• Painting renders the graphics image of JFC
  components
• All paint operations controlled by the central
  event dispatch thread
• Painting occurs when display is updated, e.g.,
• window re-sized
• window covered or uncovered by another
• window maximised

• Swing painting
• All Swing components go in the containers
  contentPane (a JPanel)
• Therefore
• public void paint(Graphics g)
• this.getContentPane().paint(g)
• // your paint code here
• or
• put some central component on the content pane
  for drawing upon, e.g., a Canvas or JPanel
• or
• make the containers content pane your own JPanel

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Event-driven Programming

• The Hollywood model Dont call us, well call
  you.
• Rather than continuously polling the system for
  events, a better model is to have user events
  trigger certain actions
• Requires Events and Event handlers
• Two steps required to handle an event
• Define a class that listens for an occurrence of
  the event (by implementing the appropriate
  listener interface)
• Register an instance of the listener class with
  the component affect by the event

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Events

• package java.awt.event
• all AWT events
• class java.awt.event.AWTEvent
• superclass of all AWT events
• class java.util.EventObject
• superclass of all Java platform events
• package javax.swing.event
• swing events
• mostly subclasses of java.util.EventObject

• Common events
• AdjustmentEvent for Adjustable object, eg
  JScrollBar
• ComponentEvent move, show, hidden, or resize
• ContainerEvent notification events
• FocusEvent gets or loses focus
• InputEvent superclass of mouse and keyboard
  events
• ItemEvent eg JCheckBox, JComboBox and Jlist
• KeyEvent
• MouseEvent
• PaintEvent needs update() call
• TextEvent eg JTextField and JTextArea
• WindowEvent
• ChangeEvent for notification
• InternalFrameEvent
• ListSelectionEvent
• MenuEvent
• TableModelEvent
• TreeSelectionEvent

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Select Methods of the Event Classes
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Select Methods of the Event Classes
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Listeners

• The AWT and Swing APIs define a set of interfaces
  called listeners
• Each kind of event has a listener interface, and
  each listener has methods for every event that
  can occur in its event class
• Use listener interfaces to create handlers for
  events

http//java.sun.com/j2se/1.4.2/docs/api/java/awt/e
vent/MouseListener.html
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Listener Example

• public class MyMouseListener implements
  MouseListener
• public void mouseClicked(MouseEvent e)
• Component clicked e.getComponent()
• clicked.setBackground(new Color((float)
  Math.random(), (float) Math.random(), (float)
  Math.random()))
• // need to implement all methods
  of interface

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Listener Shortcuts