ch.randelshofer.util
Class EventLoop

java.lang.Object
  ch.randelshofer.util.EventLoop

Direct Known Subclasses:

EventDispatcher, SequentialDispatcher

public abstract class EventLoop
extends java.lang.Object

An EventLoop can process events on a separate worker thread. It consists of two parts: the event collector and the event processor.

The event collector collects all incoming events and puts them into a queue.
The event processor removes the events from the queue and processes them.

The key feature of the EventLoop is, that clients don't have to wait until an event has been processed. Clients are free to proceed as soon as the collector has put the event into the queue.

Other important features are that events are processed in the same sequence as they have been collected and that only one thread is used to process the events.

Usage

This is an abstract class. It does all the queue handling, but does no processing. To use it, you have to create a subclass which overrides the methods #collectEvent and #processEvent.

Example

An EventLoop, which outputs Strings on a background thread could look like this:

 public class AsyncDisplay
 extends AbstractEventLoop {
     public void display(String string) {
         collectEvent(string);
     }
     protected void processEvent(Object event) {
          System.out.println((String) event);
    }
 }
 

To use the class proceed like this:

 AsyncDisplay a = new AsyncDisplay();
  a.display("Hello World");
 

Version:

1.2 2001-09-24 Support for coalescing of events added.
1.1 2001-08-24 Reworked for JDK 1.3.
1.0.2 2000-03-03 Catch SecurityException's (To make this class work in Netscape Navigator).
history 1.0.1 14.10.1998 Added versioning info to class comments.
history 1.0 Datum ? Threads are not being reused anymore.

Author:

Werner Randelshofer

Constructor Summary

EventLoop()
Creates a new EventLoop which processes events at Thread.NORM_PRORITY.

EventLoop(int priority)
Creates a new EventLoop which processes events at the desired thread priority.

Method Summary

void	clear() Clears the event queue.
protected void	collectEvent(java.lang.Object event) Collects an event and puts it into the event queue for later processing.
boolean	isCoalesce() Returns true if the EventLoop coalesces multiple pending events.
protected abstract void	processEvent(java.lang.Object event) This method processes an event on the event processor thread.
protected void	processEvents() This method removes events from the event queue and proceses them until the queue is empty or until #stop is called.
void	setCoalesce(boolean b) Sets whether the EventLoop coalesces multiple pending events.
void	start() Starts the event processor.
void	stop() Stops the event processor.

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait

Constructor Detail

EventLoop

public EventLoop()

Creates a new EventLoop which processes events at Thread.NORM_PRORITY.

EventLoop

public EventLoop(int priority)

Creates a new EventLoop which processes events at the desired thread priority.

Parameters:

priority - The Thread priority of the event processor.

Method Detail

collectEvent

protected void collectEvent(java.lang.Object event)

Collects an event and puts it into the event queue for later processing.

Parameters:

event - The event to be put into the queue.

setCoalesce

public void setCoalesce(boolean b)

Sets whether the EventLoop coalesces multiple pending events. A busy application may not be able to keep up with event generation, causing multiple events to be queued. Coalescing is based on equality tests of event objects. More formally, coalesces an event o if and only if the queue contains at least one element e such that (o==null ? e==null : o.equals(e)).

EventLoops do not coalesce events by default.

Parameters:

b - Specify true to turn on coalescing.

isCoalesce

public boolean isCoalesce()

Returns true if the EventLoop coalesces multiple pending events.

See Also:

setCoalesce(boolean)

start

public void start()

Starts the event processor.
The event processor is started by default.

stop

public void stop()

Stops the event processor.

clear

public void clear()

Clears the event queue.

processEvent

protected abstract void processEvent(java.lang.Object event)

This method processes an event on the event processor thread.

Parameters:

event - An event from the queue.

processEvents

protected void processEvents()

This method removes events from the event queue and proceses them until the queue is empty or until #stop is called.

This method must be called from the event processor thread only.