Event-Driven Applications

Event-Driven Applications
An Introduction with Examples in Java

Prof. David Bernstein
James Madison University

Computer Science Department

bernstdh@jmu.edu

Motivation

Multi-Threaded Applications:
- It can be very difficult to ensure that the right things happen at the right time
Overcoming this Difficulty:
- Focuses on how classes should handle events rather than on how or when these events will occur

Designing Event-Driven Applications

Focus On:
- The events that can occur (e.g., mouse clicks, timing signals, key presses);
- The classes that can generate events of different kinds (often called event producers; and
- The classes that need to respond to events of different kinds (often called event handlers.
Do Not Focus On:
- The order in which events occur

Events

One Simple Implementation

javaexamples/events/Event.java

Event Handlers

The Requirements

javaexamples/events/EventHandler.java

The Event Queue

A Central Repository for Events

Implementing an Event Queue

javaexamples/events/EventQueue.java

Firing/Dispatching Events

Event Bubbling:
- There is a (usually pre-defined) hierarchy of objects that handle events
- Examples: Lingo, HTML Events, AWT
Event Listening:
- The event queue keeps track of which objects want to be notified of which events
- Examples: Swing

Event Bubbling

A Document

Event Bubbling (cont.)

A Directory

Event Bubbling (cont.)

Expanding the Requirements of Event Handlers

javaexamples/events/BubblingEventHandler.java

Event Bubbling (cont.)

An Abstract Event Handler that "Bubbles"

javaexamples/events/AbstractBubblingEventHandler.java

Event Bubbling (cont.)

A Simple Bubbling Event Queue

javaexamples/events/SimpleBubblingEventQueue.java

Event Bubbling (cont.)

A More Flexibile Bubbling Event Queue

javaexamples/events/BubblingEventQueue.java

Event Bubbling (cont.)

An Example: A Document

javaexamples/events/DocumentElement.java

javaexamples/events/Word.java

javaexamples/events/Paragraph.java

javaexamples/events/Section.java

Event Bubbling (cont.)

An Example: A Driver that uses the Document

javaexamples/events/Driver1.java

Event Listening

Overview

Event Listenening (cont.)

A Simple Listener Event Queue

javaexamples/events/SimpleListenerEventQueue.java

Event Listening (cont.)

A More Flexible Listener Event Queue

javaexamples/events/ListenerEventQueue.java

Event Listenening (cont.)

An Example: A Messaging System

javaexamples/events/MessageLogger.java

javaexamples/events/SituationBoard.java

Event Listenening (cont.)

An Example: A Driver

javaexamples/events/Driver2.java

There's Always More to Learn