Event handling62
- 1. Event handling http://improvejava.blogspot.in/ 1
- 2. Objectives On completion of this period, you would be able to know • Event handling • Delegation event model • Events • Event Sources • Event Listeners http://improvejava.blogspot.in/ 2
- 3. Recap In the previous class, you have leant • The AWT classes http://improvejava.blogspot.in/ 3
- 4. Event Handling • The AWT based programs and applets are event-driven programs • Event handling is at the core of success of these programs • The modern approach to handling events is based on the ‘delegation event model’ http://improvejava.blogspot.in/ 4
- 5. The Delegation Event Model • This model defines – standard and consistent mechanisms to generate and process events • Its concept is – A source generates an event and sends it to one or more listeners. – The listener simply waits until it receives an event – Once received, the listener processes the event and then returns – Listeners must register with a source in order to receive an event notification http://improvejava.blogspot.in/ 5
- 6. The Delegation Event Model contd.. • The advantages of this model are – The application logic that processes events is cleanly separated from the user interface logic – Notifications are sent only to listeners that want to receive them http://improvejava.blogspot.in/ 6
- 7. Events • An event is an object that describes a state change in a source • It can be generated as a consequence of a person interacting with the elements in a GUI • Examples include – pressing a button – entering a character via the keyboard – selecting an item in a list – clicking the mouse http://improvejava.blogspot.in/ 7
- 8. Event Classes • The classes that represent events are at the core of Java’s event handling mechanism • EventObject – Defined in java.util package – It is the superclass for all events • AWTEvent – Defined within the java.awt package – It is a subclass of EventObject – It is a superclass of all AWT events that are handled by the delegation event model http://improvejava.blogspot.in/ 8
- 9. Event Classes contd.. • The package java.awt.event defines several types of events that are generated by various user interface elements Event Name Generated when ActionEvent A button is pressed, a list item is double-clicked, or a menu item is selected AdjustmentEvent A scroll bar is manipulated ComponentEvent A component is hidden, moved, resized, or becomes visible ContainerEvent A component is added to or removed from a container FocusEvent A component gains or loses keyboard focus Table 62.1 Event classes http://improvejava.blogspot.in/ 9
- 10. Event Classes contd.. InputEvent Abstract super class for all component input event classes ItemEvent A check box or list item is clicked; also occurs when a choice selection is made or a checkable menu item is selected or deselected KeyEvent Input is received from the keyboard MouseEvent The mouse is dragged, moved, clicked, pressed, or released; also generated when the mouse enters or exits a component TextEvent The value of a text area or text field is Changed WindowEvent Window is activated, closed, deactivated, deiconified, iconified, opened, or quit Table 62.2 Event classes http://improvejava.blogspot.in/ 10
- 11. Event Sources • A source is an object that generates an event • e.g. – A button – A TextField • Sources may generate more than one type of event • e.g . – Click on button – Double click on button http://improvejava.blogspot.in/ 11
- 12. Event Sources contd.. • A source must register listeners in order for the listeners to receive notifications about a specific type of event • Each type of event has its own registration method • the general form: – public void addTypeListener(TypeListener el) – Here, Type is the name of the event and el is a reference to the event listener. – e.g., – the method that registers a keyboard event listener is called addKeyListener( ) – The method that registers a mouse motion listener is called addMouseMotionListener( ) http://improvejava.blogspot.in/ 12
- 13. Event Sources contd.. • When an event occurs – All registered listeners are • Notified about the event • As well as, receive a copy of the event object • This is known as multicasting the event http://improvejava.blogspot.in/ 13
- 14. Event Sources contd.. Event Source Generates Button action events when the button is pressed Checkbox item events when the check box is selected or deselected Choice item events when the choice is changed List action events when an item is double-clicked; generates item events when an item is selected or deselected Menu Item action events when a menu item is selected; Item events when a checkable menu item is selected or deselected Scrollbar adjustment events when the scroll bar is manipulated Text components text events when the user enters a character Window window events when a window is activated, closed, deactivated, deiconified, iconified, opened, or quit. Table 62.3 Event sources http://improvejava.blogspot.in/ 14
- 15. Event Listeners • A listener is an object that is notified when an event occurs • It has two major requirements – it must have been registered with one or more sources to receive notifications – it must implement methods to receive and process these notifications http://improvejava.blogspot.in/ 15
- 16. Event Listeners contd.. • Listeners are created by implementing one or more of the interfaces • When an event occurs, the event source invokes the appropriate method defined by the listener and provides an event object as its argument http://improvejava.blogspot.in/ 16
- 17. Event Listeners contd.. Interface Description ActionListener Defines one method to receive action events AdjustmentListener Defines one method to receive adjustment events ComponentListener Defines four methods to recognize when a component is hidden, moved, resized, or shown ContainerListener Defines two methods to recognize when a component is added to or removed from a Container FocusListener Defines two methods to recognize when a component gains or loses keyboard focus ItemListener Defines one method to recognize when the state of an item changes KeyListener Defines three methods to recognize when a key is pressed, released, or typed Table 62.4 Event Listeners http://improvejava.blogspot.in/ 17
- 18. Event Listeners contd.. Interface Description MouseListener Defines five methods to recognize when the mouse is clicked, enters a component, exits a component, is pressed, or is released MouseMotionListener Defines two methods to recognize when the mouse is dragged or moved TextListener Defines one method to recognize when a text value Changes WindowListener Defines seven methods to recognize when a window is activated, closed, deactivated, deiconified, iconified, opened, or quit Table 62.5 Event Listeners http://improvejava.blogspot.in/ 18
- 19. Summary • In this class we discussed – Event handling – Delegation event model – Events – Event Sources – Event Listeners http://improvejava.blogspot.in/ 19
- 20. Quiz 1. What is the event source for AdjustmentEvent ? a) TextField b) List c) ScrollBar d) Button http://improvejava.blogspot.in/ 20
- 21. Quiz contd.. 2. EventObject is defined in which package ? a) java.awt b) java.awt.event c) java.util d) java.applet http://improvejava.blogspot.in/ 21
- 22. Frequently Asked Questions 1. Explain the delegation model of event handling 2. List some example for event sources 3. What is event listeners? List some examples for event listener http://improvejava.blogspot.in/ 22