Transcript Event Handling

Agenda
• Introduction.
• Event Model.
• Creating GUI Application.
• Event Examples.
Introduction
Event-Driven Programming
• In Procedural programming :
 Code is executed in procedural order.
• In event-driven programming :
 Code is executed upon activation of events.
7-4
What’s Event
• Event is a type of signal to tell the program that
something has happened.
• Event is generated by
 External user actions such as mouse movements, mouse
clicks, and keystrokes .
 Operating system, such as a timer.
7-5
Event Source
• The component on which an event is fired is called:
 Source object or source component.
• Examples
 A button is the source object for a button-clicking event.
 A ComboBox it the source object for selecting an item.
7-6
Event Object
• All event information are stored in object called Event
object.
• Information examples :
 Source of event.
 Time when event occurred.
7-7
Event Object Classes
7-8
Event Object Examples
7-9
Event Model
Delegation Event Model
• Model used by Java to handle user interaction with
GUI components.
• Describes how your program can respond to user
interaction.
7 - 11
Event Model Components
• Event model contains three important players:
 Event Source.
 Event Listener/Handler.
 Event Object
7 - 12
Event Source
• Event source GUI component that generates the event
• Examples: Button , TextField and ComboBox.
7 - 13
Event Listener/Handler
• Receives and handles events.
• Contains business logic.
• Examples:
 Computing the summation of two numbers.
 Computing the maximum of two numbers.
 Displaying information useful to the user.
7 - 14
Listener Methods
7 - 15
Event Object
• Created when an event occurs - user interacts with a
GUI component.
• Contains all necessary information about the event
that has occurred.
 Type of event that has occurred.
 Source of the event.
 Time when event occurred.
• Represented by an Event class.
7 - 16
Registration
• A listener should be registered with a source.
• Once registered, listener waits until an event occurs.
• When an event occurs:
 An event object created by the event source.
 Event object is fired by the event source to the registered
listeners.
 Method of event listener is called with an event object as a
parameter(Business logic).
7 - 17
Registration cont.
• Once the listener receives an event object from the
source:
 Understand the event.
 Processes the event that occurred.
Note
 Event source can be registered to one or more listener.
7 - 18
Delegation Model Flow
Event Source
1- Source Register
Listener
2- Fire an Event
Object
Event Listener
3- Reacts to
the Event
7 - 19
Creating GUI Application
GUI Example
• To create A GUI Application with event handling
1.
2.
3.
Create a GUI.
Create Listener.
Register listener with the source.
7 - 21
1- Create GUI
• In this step we create the GUI and adding all needed
components.
• The GUI Describes and displays the appearance of the
application.
7 - 22
2- Create Listener
• Listener is a class implementing the appropriate
listener interface.
 Override all methods of the appropriate listener interface.
 Describe in each method how you would like the event to be
handled.
 May give empty implementations for methods you don't
need.
7 - 23
2- Create Listener cont.
7 - 24
3- Registeration
• Register the listener object with the event source.
 The object is an instantiation of the listener class in step 2
 Use the add<Type>Listener method of the event source.
• Example:
 JButton b =new JButton(“ADD”);
 b.addActionListener(listener);
7 - 25
Event Examples
Action Event
• An action event occurs when the user :
 Clicks a button.
 Chooses a menu item.
 Presses Enter in a text field .
java.awt.event.ActionEvent
+getActionCommand( ) : String
+getSource( ) : Object
+getWhen( ) : long
7 - 27
Handling Action Event
• Java provides a listener interface ActionListener to
handle action events.
• You must override actionPerformed (ActionEvent e)
method.
7 - 28
Item Event
• An action event occurs when the user :
 Check a Checkbox, RadioButton, menu items.
 Select a ComboBox .
java.awt.event.ItemEvent
+getSource( ) : Object
+getWhen( ) : long
+getStateChange( ) : int
7 - 29
Handling Item Event
• Java provides a listener interface ItemListener to
handle Item events.
• You must override itemStateChanged (ItemEvent e)
method.
7 - 30
Mouse Event
• An action event occurs when the user uses the mouse
to interact with a component.
java.awt.event.MouseEvent
+getWhen( ) : long
+getClickCount( ) : int
+getX( ) : int
+getY( ) : int
+getButton( ) : int
7 - 31
Handling Mouse Event
• Java provides two listener interfaces, MouseListener
and MouseMotionListener to handle mouse events.
• The MouseMotionListener listens for actions such as
dragging or moving the mouse.
java.awt.event.MouseMotionListener
+mouseDragged (e : MouseEvent ) : void
+mouseMoved (e : MouseEvent ) : void
7 - 32
Handling Mouse Event
• The MouseListener listens for actions such as when
the mouse is pressed, released, entered, exited, or
clicked.
java.awt.event.MouseListener
+mousePressed (e : MouseEvent ) : void
+mouseReleased (e : MouseEvent ) : void
+mouseClicked (e : MouseEvent ) : void
+mouseEntered (e : MouseEvent ) : void
+mouseExit (e : MouseEvent ) : void
7 - 33
Focus Event
• An action event occurs when the Component :
 Gain focus.
 Lose focus.
java.awt.event.FocusEvent
+getSource() : Object
7 - 34
Handling Focus Event
• Java provides a listener interface ItemListener to
handle Item events.
• You must override focusLost (focusEvent e) and
focusGained (focusEvent e) methods.
7 - 35
Key Event
• An action event occurs when the user is typing on
keyboard.
• Keys
 KeyEvent.VK_Home for home.
 KeyEvent.VK_1 for 1.
 KeyEvent.VK_H for h etc.. .
java.awt.event.KeyEvent
+getKeyChar( ) : char
+getKeyCode( ) : int
7 - 36
Handling Key Event
• Java provides a listener interface KeyListener to
handle Item events.
• The KeyListener listens for actions such as when the
key is pressed, released, or typed.
java.awt.event.KeyListener
+keyPressed (e : KeyEvent ) : void
+keyReleased (e : KeyEvent ) : void
+keyTyped (e : KeyEvent ) : void
7 - 37
Questions
Thanks