Java vs 'NET: Event Based Programming

1
Java vs .NETEvent Based Programming

• by Donal Lafferty, DSG TCD.
• Donal.Lafferty_at_cs.tcd.ie
• Wednesday, 13 June 2001

2
References

• Web examples
• The Java 1.1 Event Model by Gopalan Suresh Raj
• http//www.execpc.com/gopalan/java/dragger.html
• Books
• A Programmers Introduction to C by Eric
  Gunnerson, Apress
• Java in a Nutshell by By David Flanagan, OReilly
• Reference Manuals
• .NET documentation
• http//msdn.microsoft.com/library/default.asp?URL
  /library/dotnet/cpref/cpref_start.htm
• Examples
• tcdIO source code
• \\malinhead\projects\TORJC\

3
Java Event Model
Event Source Object

• Delegation Event Model
• Event message class
• Event listener interface
• Event source object
• Event listener object
• Adapter class

Message
Message
Message
Listener Object
Listener Interface
Listener Object
Listener Interface
Listener Object
Listener Interface
4
Defining an Event Message

• Example event object class
• public class AlarmEventObject extends EventObject
  
• public AlarmEventObject(Object source, String
  cause)
• super(source)
• alarmCause cause
• public String getCause() return alarmCause
• private String alarmCause
• //class

Message
5
Defining an Event Callback

• Example event listener
• interface
• public interface LevelAlarmListener extends
  EventListener
• public void announceAlarm(AlarmEventObject
  event)
• // interface

Listener Interface
6
Defining an Event Source

• Track subscribers
• public class LevelAlarmSource
• public LevelAlarmEventSource()
• private Vector LevelAlarmListeners new
  Vector()
• Support Register, Unregister methods.
• // Register / Unregister methods
• public void addLevelAlarmListener(LevelAlarmListe
  ner subscriber)
• locationListeners.addElement(subscriber)
• public void removeLevelAlarmListener(LevelAlarmLi
  stner unsubscriber)
• locationListeners.removeElement(unsubscriber)
  

Event Source Object
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Defining an Event Source

• Implement a notification method
• By convention, takes an event object as a
  parameter.
• Traverse subscriber list, send event message
• to each subscriber.
• // notify method
• protected notify(AlarmEventObject event)
• // Make copy to subscriber list to prevent
  add/removes
• Vector recipientList
• synchronized(this)
• recipientList (Vector) LevelAlarmListeners.clo
  ne()
• for (int i0 ilt recipientList.size() i)
• LevelAlarmListener listener
• (LocationListener) recipientList.elementAt(i)
• listener.announceAlarm(event)

Event Source Object
8
Defining an Event Listener

• Event listener
• Class definition
• public class AlarmListener implements
  LevelAlarmListener
• public AlarmListener()
• public void announceAlarm(AlarmEventObject
  event)
• System.out.println(event.getCause() )
• Usage
• public class MyProg
• public static void main(String args)
• LevelAlarmSource myEventSource
  LevelAlarmSource() // publisher
• AlarmListener myEventListener
  AlarmListener() // subscriber
• myEventSource.addLevelAlarmListener(
  myEventListener )// reg. myEventSource.notificati
  onTest() // quick test
• // main
• // class

Listener Object
Listener Interface
9
Other Event Concepts

• Adapter class
• Listener interface defines several callbacks
  logically related.
• Adapter class supplies a default implementation
  of several callbacks.

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C Event Model
Event Source Object

• Similar to Java
• 1st Class support for
• events as class members
• 3 parts
• Event class
• Event delegate
• Event source

Message
Message
Message
Listener Object
Delegate
Listener Object
Delegate
Listener Object
Delegate
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Delegates

• Analogous to a C style function pointer
• Type corresponding to a method signature
• delegate void RingAlarm(int alarmValue)
• Delegate-type variables can be instantiated.
• RingAlarm myAlarmRinger
• new RingAlarm(AlarmListener.AnnounceAlarm)
• Delegates can be passed as method parameters
• public void TestAlarm(RingAlarm myRingerMethod)
• myRingMethod(12)
• //2

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Delegates cond

• Multicast delegates
• RingAlarm myAlarmRinger new RingAlarm(AlarmListe
  ner.AnnounceAlarm)
• Operators to add and remove methods from a
  delegate
• // Add an anonymous delegate to myAlarmRinger
• myAlarmRinger new(AnotherListener.AnotherAnnoun
  ceAlarm)
• // Make a multicast call
• myAlarmRinger(12)
• // Remove a delegate
• myAlarmRinger - new(AnotherListener.AnotherAnnoun
  ceAlarm)
• //2

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Delegates cond

• Member delegates
• Specify access modifier public, protected,
  internal, private
• Supply delegate type
• Give the delegate an identifier.
• public class BoringDelegateExample
• public RingAlarm MyAlarmDelegate

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Defining an Event Message

• Example event object class
• public class LevelAlarmEventArgs
  System.EventArgs
• private string alarmCause
• public string Cause get() return cause
• public LevelAlarmEventArgs(string cause)
• alarmCause cause
• //class3

Message
15
Defining an Event Callback

• Create a delegate
• public delegate void LevelAlarmEventHandler(object
  source,
• LevelAlarmEventArgs
  e) //4

Delegate
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Defining an Event Source

• Event source class
• public class AlarmSource
• public AlarmSource()
• public delegate void LevelAlarmEventHandler(
• object source,
  LevelAlarmEventArgs e)
• public event LevelAlarmEventHandler
  OnLevelAlarmHandler
• protected void OnLevelAlarm(LevelAlarmEventArgs
  e)
• if (OnLevelAlarmHandler ! null) // I.e. at
  least one subscriber
• OnLevelAlarmHandler(e) // Dispatch handled
  by C
• public void TestEvent(LevelAlarmEventArgs e)
  OnLevelAlarm(e)
• //class

Event Source Object
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Defining Event Source cond

• Caveats
• Access modifier on event definition
• Events native to C, not CLR (as per delgates)
• If one callback throws an exeception, other
  delegates may not be called.
• Event callbacks not guaranteed to be delivered in
  a particular order.

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Defining an Event Listener

• Create listener class that implements the
  callback method
• Delegate
• public delegate void LevelAlarmEventHandler(object
  source,
• 
  LevelAlarmEventArgs e)
• Event listener (sink)
• public class Detector
• public Detector ()
• public void announceLevelAlarm (object source,
  LevelAlarmEventArgs e)
• System.Console.WriteLine (e.Cause )

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Registering an Event Listener

• Create delegate needed to subscribe to event.
• ...
• Detector myDetector new Detector()
• LevelAlarmEventHandler myHandler
• new LevelAlarmEventHandler(myDetector.announceAla
  rm)
• ...
• Register the delegate with the event
• ...
• AlarmSource mySource new AlarmSource()
• mySource.OnLevelAlarmHandler myHandler
• ...