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C H A P T E R T E N

Event-DrivenProgramming

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10-2

Background:

• Previous programming paradigms haveemphasized the process for solving a problem.

• Once the program was complete we would:

provide input

run the program display the answer(s)

• Things always happened in the same sequence.

• The program always terminated when theprocess was complete (except for some Prologprograms we will not mention).

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10-3

Background:

• Event-driven programs do not have a set

sequence of instructions to execute.• They also do not have a predetermined finish.

• The most common example of event-driven

programming is found in graphical user interfaces (GUIs):

M$ Windows

Apples Mac OS

X11 under *nix

• Other applications include embedded systems,control systems, sensor systems like home

security, etc.

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10-4

Event-driven Languages:

• Support of even-driven programming is often grafted on

to existing languages, for example X11 is implemented asa set of function libraries under C and class libraries underC++.

• Some languages were designed around the needs of

event-driven applications such as Visual Basic andTcl/Tk .

• With the advent of the word wide web (WWW) event-driven programming has gained in popularity as a way toadd interaction to web pages.

• Such interaction is programmed in a number oflanguages including JavaScript and Java .

• We will be using Java to implement Applets , programs

intended to run inside a web browser .

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10-5

Imperative vs. Event-Driven ParadigmsFigure 10.1

Computation is a function from its input to its output, made up of asequence of steps that produce some result as its goal.

* *

Computation is a community of persistent entities coupled togetherby their ongoing interactive behavior. Beginning and end are

special case that can often be ignored.

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10-6

Event-driven Programs

• Input to event-driven programs come from event

sources ; sensors, input devices, objects on a web page.• Events occur asynchronously and are placed in an event queue as they arise.

• Events are removed from the event queue and

processed (“handled ”) by the program’s main processingloop.

• As a result of handling an event the program mayproduce output or modify the value of a state variable .

• There is no predefined starting or stopping point.• Java provides support for event-driven programmingthrough certain classes and methods that can be used todesign program interaction.

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Java Class AWTEvent and Its Subclasses*Figure 10.2

Events in Java are defined by subclasses of theAWTEvent abstract class.

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Java Event Classes

• Every event source in a program can generate

an event that is a member of one of the classeson the previous slide.

• For example, if a Button is an event source, itgenerates events that are members of theActionEvent class.

• Objects that can be event sources are membersof subclasses of the Component abstract class.

• This class and its subclasses are shown on thenext slide, where we will find the Button object.

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Subclasses of Component That Can Be Sources of EventsFigure 10.3

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Java EventListener Class Interface:

• For a program to handle an event it must be

equipped with a listener that will recognize whena particular event has occurred.

• For example, for a program to be able to “hear”

that a button has been selected it must send anaddActionListener message to the button object.

• The EventListener class and its subclassesprovide the interface for recognizing events by

setting up appropriate listeners for componentobjects.

• The EventListener class interface and itssubclasses are shown on the next slide.

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Java EventListener Class Interface and Its Subclasses*Figure 10.4

To respond to eventswe need to implementspecial methods calledhandlers .

Each class of eventspredefines the namesof the handlers that canbe written for it…

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Handlers Required for Button, Menu,Text Typing, and Mouse EventsFigure 10.5

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Initial Frame Design for a Graphical Drawing ToolFigure 10.7

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Overall Structure of a Java AppletFigure 10.6

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Next time…

Writing AJava Applet

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