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12 High Level UI Event Handling
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12 High Level UI Event Handling

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  • 1. High Level UI Components Event Handling Cornelius Koo - 2005
  • 2. What is Event Handling• Process of recognizing when an event occurs and taking an action based on that event.
  • 3. 3 Steps in Event Management1.The hardware must recognize that something has occured : button pressed, mouse clicked, mouse hover, an adapter plugged in etc.2.The software on the device needs to be notified of the event.3. A message from the application manager will be sent to the MIdlet. The message would contain information about the event, so that we can process it.
  • 4. Scenario
  • 5. Listener• Before MIDlet can accept and process an event, it must implements Listener interfaces.• There are 2 Listeners in MIDP : 1. CommandListener 2. ItemStateListener
  • 6. public class TestCommandListener extends MIDlet implements CommandListener { ... public void commandAction(Command c, Displayable s) {…}}
  • 7. public class TestItemStateListener extends MIDlet implements ItemStateListener { ... public void itemStateChanged(Item item) {}}
  • 8. Command Object• Command object is an object that holds information about an event.
  • 9. Processing Events• Processing events steps :1. Create a Command object to hold information about an event.2. Add the Command to a Form, Textbox, List or Canvas.3. Add a "listener" to the above Form, Textbox, etc.
  • 10. private Display display;private Command cmExit;private Form fmMain;public TestDisplayable() { super(); display = Display.getDisplay(this); fmMain = new Form("Displayable Form"); cmExit = new Command("Exit", Command.EXIT, 1); fmMain.addCommand(cmExit); fmMain.setCommandListener(this);}…
  • 11. Item Object• Any components that can be added to a Form.• ChoiceGroup, DateField, Gauge and TextField are all subclasses of Item and each can process events.
  • 12. StringItem & ImageItem• StringItem and ImageItem are also subclasses of Item however once allocated, these objects are static and thus do not receive/acknowledge events.
  • 13. Command1. Label2. Type3. Priority
  • 14. label• Specifies the text you would associate with the text.
  • 15. type• Specify the type of specific soft button.
  • 16. priority• Used by application manager when arranging items that appear in a menu or for ordering soft buttons on the display.
  • 17. Command Types
  • 18. Command & CommandListener API
  • 19. Item• Item is any component that can be added to the form.• ChoiceGroup, CustomItem, DateField, Gauge, ImageItem, Spacer, StringItem, TextField.
  • 20. ItemListener• When an item’s value changed, it will generate an item event and sent it to ItemListener. (Except for StringItem and ImageItem)
  • 21. When it is called ?• If an Item has changed, itemStateChanged() must be called for the changed Item before it will acknowledge changes in a subsequent Item.• If a MIDlet makes a change to an Item (not a user interaction), itemStateChanged() will not be called.• If the device running the MIDlet can recognize when a user has moved from one Item to another, itemStateChanged() must be called when leaving one Item and before getting to the next.
  • 22. Item & ItemListener API Item ItemListener
  • 23. List Select Command• We can change the select command for List and set it to our own command.
  • 24. private List lsHero;private Command cmOpen = new Command(“Open",Command.ITEM,1);private String[] sound = { "Info", "Confirmation", "Warning", "Alarm", "Error“};
  • 25. ...lsHero = new List("Message Type", List.IMPLICIT, sound, null);lsHero.setSelectCommand(cmOpen);lsHero.setCommandListener(this);...
  • 26. public void commandAction(Command arg0, Displayable arg1) { if(arg0 == cmOpen){ System.out.println(“Open selected"); System.out.println( ((List)arg1) .getString(((List)arg1) .getSelectedIndex()) ); }}
  • 27. Reference• Core J2ME Technology and MIDP. John W. Muchow. Prentice Hall PTR, 2002.• Enterprise J2ME: Developing Mobile Java Applications. Michael Juntao Yuan. Prentice Hall PTR, 2003.• J2ME in A Nutshell. Kim Topley. Oreilly, 2002.

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