CSCI 444 / CIS 644 Event Driven Programming

Outline I.What is an event driven system? II.What is event driven programming? III.Course Overview

I. What is an event driven system? An event is an occurrence of interest –Events take place at a particular time An event driven system is a computer system where events are a driving force –Events from the user (mouse click) –Internal events (variable assignment, timers) –Events from other computing systems (any message arriving across the network)

Examples of Event Driven Systems Vending Machines Cruise Control System in a car TV Any modern windowing program –e.g. PowerPoint What are the events associated with each of these?

II. Event Driven Programming Fundamentally a different paradigm from procedural (or OOP or functional or logic) programming –System is made of objects –System spends much of its time in stasis –Events occur that propagate through the system and then it returns to statis

Basic Event Driven Programming Model Event SourceEvent Handler System Objects notifies modifies

What makes this model unique? 1.Loose Coupling between source and handler –Runtime registration –Multicasting –Multiplexing –Separate Compilation –Inverted Semantics

Differences continued 2.State based control Behavior of system depends on its state E.g. Coke machine only delivers pop after enough money has been inserted Good ways and bad ways to implement this

Differences continued 3.Concurrent and Distributed processing If a handler takes too much time, other events might be delayed/lost One solution is to have handler execute is a separate thread. Concurrency concerns now become important. E.g. what happens is multiple events arrive in rapid sequence? Might there be multiple threads executing in the same handler?

Concurrent and Distributed Processing - continued Distributed Processing –Source and Handler might be on different computing systems entirely. –E.g. a web browser makes a request for a web page from a server. –Client / Server computing is fairly well understood. There are many other models, however.

Course Outline Java Event Driven Programming Component Based Programming in Java Concurrent and Distributed Event Programming Software Engineering Event Driven Systems