Event-driven Programming Robin Burke IT 130 Fall 2004

Outline The event-driven model Web applications Form HTML Handlers forms buttons text boxes accessing form content Handlers onClick onChange

Imperative programming defines a series of operations User invokes program waits for result "Batch" processing

Event-driven programming Modern applications are event-driven Different kind of programming from sequential (batch) execution programmer does not control when code is executed user controls that Programmer provides capabilities the user invokes them may be multiple ways to do the same thing Basic idea = "event handlers" small bits of code that the application calls when certain events occur

Event-driven programming cont'd Imperative program load payroll data do payroll computation output checks Event-oriented program establish payroll application interface associate loading routine with "load" menu item associate payroll computation with "compute" menu option associate check printing operation with "print" menu options User is in charge of the sequence

Basic idea Events are generated To use an event, the programmer user actions (clicking, moving mouse) browser actions (loading, closing) To use an event, the programmer writes JavaScript code associates it with the appropriate document element associates it with the appropriate event

Pieces we need New HTML elements New HTML attributes forms interface widgets buttons, text boxes, etc. New HTML attributes event generation New JavaScript capabilities associate code with events

Web application Application Web application "A complete, self-contained program that performs a specific function directly for the user" Web application an application delivered over the WWW

Web applications Core of web development Not in our scope But 90% of the web's "pages" are generated from user input only 10% are static pages Not in our scope IT 230 teaches how to build those kinds of applications we will not use forms as input to applications But web applications need input input comes from HTML forms we can use form element to explore event handling

Example applications Course online Any search engine

What all these share... The need to get user input from a web page Format of input name-value pair Interaction idea web form How to get from user? depends on the data

Form elements Form element Input element identify a part of the page where the user can input data Input element specific data items name and value attributes user can change the value attribute

Input elements name and value attributes user can change the value attribute how depends on the type text field selection list radio button, etc

Example

Input elements Treated like other page elements can be organized into paragraph, tables, etc. Form layout is almost always table-based All use the same base element INPUT what kind of input controlled by the type attribute

Input elements

Textarea for multi-line input <textarea name="message" rows=4 cols=40 wrap="virtual"></textarea>

Select / Option for menus and lists <select size="3"> <option>BS CGA-DES</option> <option>BS CGA-DEV</option> <option>BA DC</option> <option>BS DC</option> ... </select>

Input elements we will use type="button" rendered as a "raised" rectangle user can click type="text" rendered as an "indented" box can display text user can enter text

Examples Button Text box more typical (with label) <input type="button" value="Click Here for Lucky Number" /> Text box <input type="text" name="fahrBox" size="10" value "" /> more typical (with label) Enter a temperature in degrees Fahrenheit: <input type="text" name="fahrBox" size="10" value "" />

Events Any HTML element can generate events Commonly-used many different types Commonly-used button  click event when the user clicks on the button text  change event when the user changes the value

Linking element and event event handler attribute when the event occurs on that element the JavaScript statement will execute Example onClick="Convert();" In Element <input type="button" value="Convert to Celsius" onClick="Convert();" />

Example lucky.html

Programming rollem.html

Interacting with form values Form data name / value pairs processed by the server can also be manipulated by JavaScript Change the value of a form element changes what the user sees Linked to events we can create simple browser-based applications

Syntax Getting a value Setting a value Refer to subparts using dot (.) syntax Format document.FORM_NAME.ELEMENT_NAME.value Getting a value tempF = document.TempForm.fahrBox.value; Setting a value document.TempForm.celsiusBox.value = tempC;

Examples lucky1.html convert1.html

Programming convert2.html

Dynamic HTML JavaScript can alter many HTML properties Example we see more of this later in the class Example alter the src attribute of an img element pics.html

onChange event onClick event For text elements Example user presses a button For text elements onChange event when the user alters the contents and then clicks outside the element Example copybox.html

Homework #5 Metric conversions You need to add two conversions work inches <--> cm feet <--> meters You need to add miles <--> km

Structure convert.js hwk5.html Tasks a library with the conversion functions hwk5.html the page that interacts with the user Tasks add lines for miles / km and text elements add functions that handle onChange events add conversion functions to convert.js

Advice Start early Use cut, paste and edit! Don't forget documentation 45-50 lines of code to write Use cut, paste and edit! Don't forget documentation Test each step

Wednesday Lab (Rm 722) Forms and Events Good practice for the homework