internet programming and e application

1. RCS 301 INTERNET
PROGRAMMING AND
E-APPLICATIONS
CHAPTER 1
1

2. Course Introduction,
Agenda;
Introductions
Course Outline
Chapter One
2

3. cont..
Introductions
Who are you? (name, background).
How is God working in your life?
What's one interesting thing about you?
3

4. Course Outline
Web Technologies (Browser, HTML, The
Dynamic web, Dynamic Server Concepts, Web
Application Servers, XML, Plug-ins, and VRML)
Security and privacy on the Internet
(Cryptography, digital signatures, Client-based
Security, Server-based security)
Search engines and portals (Intelligent Network
Agents, Portal Sites, All-in-One Mega web sites)
4

5. cont..
Interactive Communication Experiences (Online
meetings, Internet Chat Solutions, Internet-based
Training (e-training, e-learning, portals)
Electronic Commerce (The payment business,
Comparing Payment Technologies (Postpaid
Payment Systems, Instant-paid Payment Systems,
Pre-paid Payment Systems, The future of Payment)
5

6. Method of Assessment
--Dave Cintron, Fast Track Web Programming: A
Programmer’s Guide to Mastering Web Technologies,
John Wiley & Sons.
--www.w3schools.com.
READING
--Coursework (quizzes, tests, assignments/ homework/
practical) 40%.
--End Semester Exam 60%.
6

7. CHAPTER ONE
WEB TECHNOLOGIES
Browser,
HTML,
The Dynamic web,
Dynamic Server Concepts,
Web Application Servers,
XML,
Plug-ins, and
VRML 7

8. BROWSER
-a computer application program with a
graphical user interface that allows a user to
locate, access, and display web pages.
-common web browsers include; internet
explorer, google chrome, safari, opera, mozilla
firefox etc...
8

9. Common Browser Error Messages
Page Not Found (404)
This error simply means that your browser is able to
communicate with the website or the server you are
trying to connect to, but the server could not find what
you are looking for thus responding with a 404 error
message.
There are many reasons like deleted or moved
content, behind the 404 error.
9

10. cont..
Bad Gateway (502)
This error message means while attempting to fulfill
your request, the server which is acting as a gateway
has received an invalid response from the upstream
server.
Most of the time, the problem is outside your
computer and the error is generated due to the
problems in the servers, and there is nothing much
you can do to solve this problem.
10

11. cont..
Service Unavailable (503)
This error message appears whenever the website you
are trying to reach is down at the current time, but the
server itself is responding to the requests with an error
message.
There is not much you can do in order to get access to
the server or the website. However, in rare cases, you
may receive an error like “Service Unavailable – DNS
Failure.” The reason for this might be an issue with
your router or with the DNS service itself. In those
cases, consider restarting your router or change the
DNS services you are using to resolve the issue.
11

12. cont..
Connection Timed Out
This Error message simply means that the request
sent by your computer isn’t being fulfilled within the
predetermined time frame.
The reasons for this type of error may be that the
actual server or computer you are trying to access is
not responding as it should, or there is a network
problem with your connection.
Try accessing other websites. If you can, then the
problem may be with the website or the server you
are visiting. If you can’t access the whole Internet,
then it might be a network problem.
12

13. cont..
Phishing or Malicious Content Warnings
These warnings are some of the best things about
the modern browser as they protect you from falling
prey to the Internet fraud. Whenever your browser
displays a phishing warning, it means that the
website you are visiting is not what you think it is and
is just faking to be the original site.
Crackers usually use these phishing techniques to
steal your passwords and other important
information. In those cases, quit clicking on the links
displayed on the web or in the emails and manually
enter the website URL to be sure.
13

14. How browsers work
The main function of a browser is to present the
web resources you choose by requesting it from
the server and displaying it in the browser
window.
14

15. The browser’s high level
structure
The browser’s main components are;
The user interface
The browser engine
The rendering engine
Networking
UI backend
JavaScript Interpreter
Data storage
15

16. The user interface: this includes the address
bar, back/forward button, bookmarking menu,
etc. Every part of the browser display except the
window where you see the requested page.
The browser engine: organizes actions between
the UI and the rendering engine.(the interface for
querying and manipulating the rendering engine.)
cont..
16

17. cont..
3. The rendering engine : responsible for displaying
requested content. For example if the requested
content is HTML, the rendering engine analyses
HTML and CSS, and displays the analysed content on
the screen.
4. Networking: for network calls such as HTTP
requests, using different implementations for different
platform behind a platform-independent interface.
17

18. cont..
5. UI backend: used for drawing basic widgets like
combo boxes and windows. This backend exposes a
generic interface that is not platform specific.
Underneath, it uses the operating system user
interface methods.
6. JavaScript interpreter. Used to analyse(parse)
and execute JavaScript code.
18

19. cont..
7. Data storage. The browser may need to
save all sorts of data locally. Browsers also
support storage mechanisms such as
localStorage, IndexedDB, WebSQL and
FileSystem. This is a persistence layer.
19

20. cont..
20

21. cont..
It is important to note that browsers such as
Chrome run multiple instances of the rendering
engine: one for each tab. Each tab runs in a
separate process.
21

22. The rendering engine
The responsibility of the rendering engine is well...
Rendering, that is display of the requested contents
on the browser screen.
By default the rendering engine can display HTML and
XML documents and images. It can display other
types of data via plug-ins or extension; for example,
displaying PDF documents using a PDF viewer plug-
in.
22

23. cont..
Rendering engines
Different browsers use different rendering engines:
Internet Explorer uses Trident, Firefox uses Gecko,
Safari uses WebKit. Chrome and Opera (from
version 15) use Blink, a fork of WebKit.
WebKit is an open source rendering engine which
started as an engine for the Linux platform and was
modified by Apple to support Mac and Windows.
See webkit.org for more details.
23

24. cont..
The main flow
The rendering engine will start getting the contents of
the requested document from the networking layer.
This will usually be done in 8kB chunks.After that, this
is the basic flow of the rendering engine:
Figure : Rendering engine basic flow
24

25. cont..
The rendering engine will start parsing the HTML
document and convert elements to DOM nodes in a
tree called the "content tree". The engine will parse the
style data, both in external CSS files and in style
elements. Styling information together with visual
instructions in the HTML will be used to create another
tree: the render tree.
The render tree contains rectangles with visual
attributes like color and dimensions. The rectangles are
in the right order to be displayed on the screen.
After the construction of the render tree it goes through
a "layout" process. This means giving each node the
exact coordinates where it should appear on the
screen. 25

26. cont.. The next stage is painting–the render tree will be
traversed and each node will be painted using the UI
backend layer.
It's important to understand that this is a gradual
process. For better user experience, the rendering
engine will try to display contents on the screen as
soon as possible.
It will not wait until all HTML is parsed before starting
to build and layout the render tree. Parts of the content
will be parsed and displayed, while the process
continues with the rest of the contents that keeps
coming from the network.
26

27. Main flow examples
WEBKIT
Rendering
Engine
27

28. GECKO
Rendering
engine
28

29. cont.
.
From the above figures, WebKit and Gecko use slightly
different terminology, the flow is basically the same.
Gecko calls the tree of visually formatted elements a
"Frame tree". Each element is a frame. WebKit uses
the term "Render Tree" and it consists of "Render
Objects".
WebKit uses the term "layout" for the placing of
elements, while Gecko calls it "Reflow". "Attachment"
is WebKit's term for connecting DOM nodes and visual
information to create the render tree. A minor
difference is that Gecko has an extra layer between
the HTML and the DOM tree. It is called the "content
sink" and is a factory for making DOM elements.
29

30. Hypertext Markup
Language (HTML)
-describes the content and structure of information on
a web page. not the same as the presentation
(appearance on screen)
-surrounds text content with opening and closing tags
-each tag's name is called an element
syntax: <element> content </element>
example: <p>This is a paragraph</p>
-whitespace is frequently insignificant in HTML (ignored
or collapsed to a single space)
30

31. Structure
of a
HTML
page
<!DOCTYPE html>
<html>
<head>
information about the page
</head>
<body>
page contents.
</body>
</html>
the header describes the page and the body
contains the page's contents.
HTML page is saved into a file ending with
extension .html or .htm
DOCTYPE tag tells browser to interpret our page's
code as HTML 31

32. Page title: <title>
describes the title of the web page.
<title>Chapter 1 part two: HTML Basics</title>
placed within the head of the page
displayed in the web browser's title bar and
when bookmarking the page
32

33. Paragraph: <p>
paragraphs of text (block)
<p>In the beginning was the Word,
and the Word was with God,
and the Word was God.
He was with God in the beginning.
</p>
placed within the body of the page.
33

34. Headings: <h1>, <h2>, ..., <h6>
headings to separate major areas of the page.
<h1>Ruaha catholic university</h1>
<h2>Department of Computer Science</h2>
<h3>Sponsored by Micro$oft</h3>
Ruaha catholic university
Department of Computer Science
Sponsored by Micro$oft
34

35. Horizontal rule: <hr>
a horizontal line to visually separate sections of a
page.
<p>First paragraph</p>
<hr />
<p>Second paragraph</p>
First paragraph
Second paragraph
should be immediately closed with />
35

36. More
about
HTML
tags
some tags can contain additional information
called attributes.
syntax: <element attribute="value"
attribute="value"> content </element>
example: <a href="page2.html">Next page</a>
some tags don't contain content; can be opened
and closed in one tag
syntax: <element attribute="value"
attribute="value" />
example: <hr />
example: <img src="bunny.jpg" alt="pic from
Easter" />
36

37. Links:
<a>
links, or "anchors", to other pages
<p>
Search
<a href="http://www.google.com/">Google</a> or
our
<a href="01_course_introduction.html">Lecture
Notes</a>.
</p>
Search Google or Lecture Notes.
-uses the href attribute to specify the destination URL
can be absolute (to another web site) or relative
(to another page on this site)
-anchors are inline elements; must be placed in a
block element such as p or h1 37

38. Block and inline elements
block elements contain an entire large region of
content
examples: paragraphs, lists, table cells
the browser places a margin of whitespace
between block elements for separation
inline elements affect a small amount of content
examples: bold text, code fragments, images
the browser allows many inline elements to
appear on the same line
must be nested inside a block element 38

39. Images: <img>
inserts a graphical image into the page (inline)
<img src="images/gollum.jpg" alt="Gollum from
LOTR" />
the src attribute specifies the image URL
HTML5 also requires an alt attribute describing the
image
39

40. More about images
<a href="http://theonering.net/">
<img src="images/gandalf.jpg" alt="Gandalf from
LOTR"
title="You shall not pass!" />
</a>
if placed in an a anchor, the image becomes a link
title attribute is an optional tooltip (on ANY
element)
40

41. Line break: <br>
forces a line break in the middle of a block element
(inline)
<p>The Lord is my shepherd, I lack nothing.<br />
He makes me lie down in green pastures,<br />
he leads me beside quiet waters,</p>
The Lord is my shepherd, I lack nothing.
He makes me lie down in green pastures,
he leads me beside quiet waters,
Warning: Don't over-use br they are hard to apply
css styles (more on this later)
br should not be used to separate paragraphs or
used multiple times in a row to create spacing 41

42. Phrase elements : <em>, <strong>
em: emphasized text (usually rendered in italic)
strong: strongly emphasized text (usually rendered in
bold)
<p>
HTML is <em>really</em>,
<strong>REALLY</strong> fun!
</p>
HTML is really REALLY fun!
as usual, the tags must be properly nested for a
valid page.
42

43. Nesting tags
<p>
HTML is <em>really,
<strong>REALLY</em> lots of</strong> fun!
</p>
tags must be correctly nested
(a closing tag must match the most recently
opened tag)
the browser may render it correctly anyway, but it
is invalid HTML
(how would we get the above effect in a valid
way?)
43

44. Comments
<!-- ... -->
comments to document your HTML file or "comment
out" text
<!-- My web page, by Anthony Jumanne 2015 -->
<p>Computer science courses are <!-- NOT --> a lot of
fun!</p>
Computer science courses are a lot of fun!
many web pages are not thoroughly commented (or
at all)
still useful at top of page and for disabling code
comments cannot be nested and cannot contain a --
HTML comments are visible to anyone that views
the page source code, but are not rendered when
the HTML document is rendered by a browser.
44

45. More HTML Elements
Unordered list: <ul>, <li>
ul represents a bulleted list of items (block)
li represents a single item within the list (block)
<ul>
<li>No shoes</li>
<li>No shirt</li>
<li>No soup for you!</li>
</ul>
No shoes
No shirt
No soup for you!
45

46. More
about
unorde
red
lists
a list can contain other lists:
<ul>
<li>Tribes:
<ul>
<li>Levi</li>
<li>Judah</li>
<li>Benjamin</li>
</ul>
</li>
<li>Disciples:
<ul>
<li>Peter</li>
<li>John</li>
</ul>
</li>
</ul>
Tribes:
Levi
Judah
Benjamin
Disciples:
-Peter
-John
46

47. Ordered list: <ol>
ol represents a numbered list of items (block)
<p>RIAA business model:</p>
<ol>
<li>Sue customers</li>
<li>???</li>
<li>Profit!</li>
</ol>
RIAA business model
Sue customers
???
Profit!
we can make lists with letters or Roman numerals
using CSS
47

48. Definition
list:<dl>
<dt>,
<dd>
dl represents a list of definitions of terms (block)
dt represents each term, and dd its definition
<dl>
<dt>newbie</dt>
<dd>one who does not have mad skills</dd>
<dt>own</dt>
<dd>to soundly defeat (e.gI owned that newbie!)</dd>
<dt>frag</dt>
<dd>a kill in a shooting game</dd>
</dl>
newbie
one who does not have mad skills
own
to soundly defeat (e.g. I owned that newbie!)
frag
a kill in a shooting game 48

49. Quotations: <blockquote>
a lengthy quotation (block)
<p>As Lincoln said in his famous Address:</p>
<blockquote>
<p>Fourscore and seven years ago, our fathers
brought forth on this continent a new nation,
conceived in liberty, and dedicated to the proposition
that all men are created equal.</p>
</blockquote>
As Lincoln said in his famous Address:
Fourscore and seven years ago, our fathers brought
forth on this continent a new nation, conceived in
liberty, and dedicated to the proposition that all men
are created equal.
49

50. Inline quotations: <q>
a short quotation (inline)
<p>Quoth the Raven, <q>Nevermore.</q></p>
Quoth the Raven, Nevermore.
Why not just write the following?
<p>Quoth the Raven, "Nevermore."</p>
We don't use " marks for two reasons:
HTML shouldn't contain literal quotation mark
characters; they should be written as &quot;
using <q> allows us to apply CSS styles to
quotations.
50

51. HTML Character Entities
a way of representing any Unicode character within a
web page
character(s) entity
< > &lt; &gt;
é è ñ &eacute; &egrave; &ntilde;
™ © &trade; &copy;
π δ Δ &pi; &delta; &Delta;
И &#1048;
® &reg;
list of HTML entities
https://www.w3schools.com/html/html_entities.asp)
51

52. cont..
Reserved characters in HTML must be replaced with
character entities. Characters that are not present on
your keyboard can also be replaced by entities.
example
If you use the less than (<) or greater than (>) signs in
your text, the browser might mix them with tags.
To display a less than sign (<) we must write: &lt; or
&#60;
52

53. What is new in html5
The DOCTYPE declaration for HTML5 is very simple:
<!DOCTYPE html>
New HTML5 Elements
The most interesting new HTML5 elements are:
New semantic elements like <header>, <footer>,
<article>, and <section>.
New attributes of form elements like number, date,
time, calendar, and range.
New graphic elements: <svg> and <canvas>.
New multimedia elements: <audio> and <video>.
53

54. Removed Elements in HTML5
Some of removed Elements in HTML5
The following HTML 4 elements have been removed
in html5;
Removed Element Use Instead
<acronym> <abbr>
<applet> <object>
<basefont> CSS
<big> CSS
<center> CSS
<dir> <ul>
<font> CSS
<strike> CSS
54

55. CHOOSING A HTML EDITOR
Both code and WYSIWYG html editors exist.
There is a debate over whether to use a WYSIWYG
editor instead of code. WYSIWYG stands for What
You See Is What You Get.
With WYSIWYG you can drag and drop, insert images
and create numbered or bullet lists using an editor just
like in Word.
The editor would then take that and create the HTML
code for you.
55

56. cont..
Some of the html code editors
Sublime text
Brackets
Aptana Studio 3
Notepad++
Some of the html WYSIWYG editors
Seamonkey
Amaya
kompozer
Trellian WebPage
56

57. The dynamic web
A website, or individual web page, can be static or
dynamic.
A static website contains information that does not
change. It remains the same, or static, for every
viewer of the site.
A dynamic website is a site that contains dynamic
(changing) contents depending on the viewer, the
time of the day, the viewer's native language, and
other factors.
57

58. Types of dynamic websites
A dynamic website can contain client-side scripting
or server-side scripting to generate the changing
content, or a combination of both scripting types.
Scripts
A script is a set of instructions. For Web pages
they are instructions either to the Web browser
(client-side scripting) or to the server (server-side
scripting). These are explained more below.
58

59. Client-side
The client is the system on which the Web browser is
running. JavaScript is the main client-side scripting
language for the Web.
Client-side scripts are interpreted by the browser. The
process with client-side scripting is:
the user requests a Web page from the server
the server finds the page and sends it to the user
the page is displayed on the browser with any
scripts running during or after display.
59

60. cont..
So client-side scripting is used to make Web pages
change after they arrive at the browser making pages
a bit more interesting and user-friendly.
Client-side scripts rely on the user's computer. If that
computer is slow they may run slowly. They may not
run at all if the browser does not understand the
scripting language. As they have to run on the user's
system the code which makes up the script is there in
the HTML for the user to look at (and copy or change).
60

61. Server-side
The server is where the Web page and other content
lives. The server sends pages to the user/client on
request. The process is:
the user requests a Web page from the server
the script in the page is interpreted by the server
creating or changing the page content to suit the
user and the occasion.
the page in its final form is sent to the user and
then cannot be changed using server-side
scripting
61

62. cont..
The use of HTML forms allow data to be sent to the
server and processed.
Server-side scripting tends to be used for allowing
users to have individual accounts and providing data
from databases. It allows a level of privacy, and
personalisation.
E-commerce, login pages, submission forms, and
social networking sites all rely heavily on server-side
scripting.
PHP, JSP, and ASP.net are the technologies for
server-side scripting.
62

63. The combination
A site such as Google, Amazon, or Facebook will
use both types of scripting:
server-side handles logging in, personal
information and preferences and provides the
specific data which the user wants (and allows
new data to be stored)
client-side makes the page interactive,
displaying or sorting data in different ways if the
user asks for that by clicking on elements with
event triggers
63

64. Dynamic Server Concepts
"Web server" can refer to hardware or software, or
both of them working together.
On the hardware side, a web server is a computer
that stores web server software and a website's
component files (e.g. HTML documents, images,
CSS stylesheets, and JavaScript files).
It is connected to the Internet and supports physical
data interchange with other devices connected to the
web.
64

65. cont..
2. On the software side, A web server is a specialized
type of file server. Its job is to retrieve files from the
server’s hard drive, format the files for the Web
browser, and send them out via the network.
The function of a typical Web server is shown below.
The user requests a web page. The Web Server finds
the web page file and sends it back out to the user.
The Web Server standards were originally designed to
publish static documents on the Internet.
65

66. 66

67. As the Internet and Web browsers became popular
in the mid-1990’s, organizations saw an opportunity
to provide web pages that contained dynamic
content like stock prices, weather information, and
shipping status for a package.
So, Web servers had to be extended to allow
software application development and access to
databases.
cont..
67

68. Then, What is a Web Application
Server?
Web application servers are designed specifically
to extend web servers to support dynamic content.
The application server software “hooks in” to the
web server software and automatically intercepts
any user requests for dynamic content.
The web server still sends out static web pages
and graphic files–Just like before. But now, the
application server can create dynamic content by
running programs, or by accessing databases.
68

69. 69

70. cont..
There are a variety of Web Application Server product
available. Some of the more popular products include:
Macromedia:
Cold Fusion
Cold Fusion is inexpensive and easy to learn resulting
in low cost for both development and implementation.
70

71. XML
XML, or Extensible Markup Language, is a markup
language that you can use to create your own tags.
It was created to overcome the limitations of HTML.
XML is a software- and hardware-independent tool.
It is extensible because it is not a fixed format like
HTML (which is a single, predefined markup
language). Instead, XML is a language which lets
you design your own markup languages.
71

72. cont..
A markup language is a set of words and symbols for
describing the identity or function of the parts of a
document (for example ‘this is a paragraph’, ‘this is a
heading’, ‘this is a list’, ‘this is the caption of this
figure’, etc).
XML is sometimes referred to as ‘self-describing’
because the names of the markup elements can
represent the type of content they hold (eg title,
chapter, link, etc).
72

73. Some key terminologies
Tag
A tag is a markup construct that begins with < and ends
with >. Tags come in three flavors:
start-tag, such as <section>;
end-tag, such as </section>;
empty-element tag, such as <line-break />.
Element
An element is a logical document component that either
begins with a start-tag and ends with a matching end-tag
or consists only of an empty-element tag. example
<greeting>Hello!</greeting>.
73

74. cont..
XML declaration
XML documents may begin with an XML
declaration that describes some information.
An example is;
<?xml version="1.0" encoding="UTF-8"?>.
74

75. So XML is Just Like HTML?
No. In HTML, the tag is fixed(predefined). An
<q> is always an inline quotation tag and the
tag <book> is meaningless.
XML provides a facility to define tags and the
relationships between them. Since there's no
predefined tag set, there can't be any
preconceived meaning.
75

76. Some XML editors
1. XML Notepad
2. EditiX
3. FontoXML
4. Oxygen XML Editor
76

77. XML Syntax
The syntax rules of XML are very simple and logical.
The rules are easy to learn, and easy to use.
<?xml version="1.0" encoding="UTF-8"?>
<root>
<child>
<subchild>.....</subchild>
</child>
</root>
77

78. EXAMPLE
<?xml version="1.0" encoding="UTF-8"?>
<note>
<to>Tove</to>
<from>Jani</from>
<heading>Reminder</heading>
<body>Don't forget me this weekend!</body>
</note>
78

79. cont..
To: Tove
From: Jani
Heading: Reminder
Body: Don't forget me this weekend!
79

80. cont..
In XML, it is illegal to omit the closing tag. All elements
must have a closing tag:
Note: The XML prolog does not have a closing tag!
This is not an error.
XML tags are case sensitive. The tag <Letter> is
different from the tag <letter>.
Opening and closing tags must be written with the
same case:<message>This is correct</message>
In XML, all elements must be properly nested within
each other:
<b><i>This text is bold and italic</i></b>
80

81. cont..
-In XML, the attribute values must always be quoted:
<note date="12/11/2007">
<to>Tove</to>
<from>Jani</from>
</note>
-XML does not truncate multiple white-spaces (HTML
truncates multiple white-spaces to one single white-
space):
XML: Hello Tove
HTML: Hello Tove
81

82. XML
Elements
An XML document contains XML Elements.
An XML element is everything from (including) the
element's start tag to (including) the element's end
tag. <price>29.99</price>
An element can contain: text, attributes, other
elements, or a mix of the above.
<bookstore>
<book category="children">
<title>Harry Potter</title>
<year>2005</year>
<price>29.99</price>
</book>
</bookstore>
82

83. cont..
In the example above:
<title>, <author>, <year>, and <price> have text
content because they contain text (like 29.99).
<bookstore> and <book> have element contents,
because they contain elements.
<book> has an attribute (category="children").
83

84. XML Tree Structure
XML documents form a tree structure that starts at "the
root" and branches to "the leaves".
84

85. XML Attributes
XML elements can have attributes, just like HTML.
They are designed to contain data related to a specific
element
Attribute values must always be quoted. Either single
or double quotes can be used.
<person gender="female"> or
<person gender='female'>
If the attribute value itself contains double quotes you
can use single quotes, like in this example:
<gangster name='George "Shotgun" Ziegler'>
or you can use character entities
85

86. XML Elements vs. Attributes
Take a look at these examples:
<person gender="female">
<firstname>Anna</firstname>
</person>
<person>
<gender>female</gender>
<firstname>Anna</firstname>
</person>
86

87. cont..
In the first example gender is an attribute.
In the last, gender is an element.
Both examples provide the same information.
There are no rules about when to use
attributes or when to use elements in XML.
87

88. Entity characters
There are 5 predefined entity references in XML:
&lt; < less than
&gt; > greater than
&amp; & ampersand
&apos; ' apostrophe
&quot; " quotation mark
88

89. cont..
If you place a character like "<" inside an XML
element, it will generate an error because it’s
interpreted as the start of a new element.
This will generate an XML error:
<message>salary < 1000</message>
To avoid this error, replace the "<" character with
an entity reference:
<message>salary &lt; 1000</message>
89

90. Comments in XML
The syntax for writing comments in XML is similar
to that of HTML:
<!-- This is a comment -->
Two dashes in the middle of a comment are not
allowed:
<!-- This is an invalid -- comment -->
90

91. PLUGINS
A plug-in (or plugin, add-in, add-on, or extension)
is a software component that adds a specific feature
to an existing computer program.
The common examples are the plug-ins used in web
browsers to add new features such as search
engines, virus scanners, or the ability to use a new file
type such as a new video format.
91

92. Why plug ins?
Applications support plug-ins for many reasons.
Some of the main reasons include:
to enable third-party developers to create
abilities which extend an application
to support easily adding new features
to reduce the size of an application
to separate source code from an application
because of incompatible software licenses.
92

93. How plugins work
The life cycle of a plug-in, unlike that of an
application, is completely controlled by the web page
that calls it.
When a browser starts, it looks for plugin on the
system.
When the user opens a page that contains
embedded data of a media type that invokes a plug-
in, the browser responds with the following sequence
of actions:
93

94. cont..
check for a plug-in with a matching MIME type
load the plug-in code into memory
initialize the plug-in
If you are browsing a page that has several
embedded RealAudio clips, for example, the browser
will create as many instances of the RealPlayer plug-
in as are needed (though of course playing several
RealAudio files at the same time would not be a good
idea).
94

95. cont..
When the user leaves the page or closes the
window, the plug-in instance is deleted. When
the last instance of a plug-in is deleted, the
plug-in code is unloaded from memory.
A plug-in consumes no resources other than
disk space when it is not loaded.
95

96. The following stages outline the life of a plug-in
from loading to deletion:
When Gecko encounters data of a MIME type
registered for a plug-in (either embedded in an
HTML page or in a separate file), it dynamically
loads the plug-in code into memory, if it hasn't
been loaded already, and it creates a new
instance of the plug-in.
Gecko calls the plug-in API function NPP_Initialize
when the plug-in code is first loaded.
By convention, all of the plug-in specific functions
have the prefix "NPP", and all of the browser-specific
functions have the prefix "NPN".
96

97. cont..
The browser calls the plug-in API function
NPP_New when the instance is created.
Multiple instances of the same plug-in can exist
(a) if there are multiple embedded objects on a single
page, or
(b) if several browser windows are open and each
displays the same data type.
A plug-in instance is deleted when a user leaves
the instance's page or closes its window; Gecko
calls the function NPP_Destroy to inform the plug-
in that the instance is being deleted.
97

98. cont..
When the last instance of a plug-in is
deleted, the plug-in code is unloaded from
memory. Gecko calls the function
NPP_Shutdown.
Plug-ins consume no resources (other than
disk space) when not loaded.
98

99. Using HTML to Display Plug-
Ins
When a user browses to a web page that invokes a
plug-in, how the plug-in appears (or does not appear)
depends on two factors:
The way the developer writes the plug-in
determines whether it is displayed in its own
window or is windowless.
The way the content provider uses HTML
elements to invoke the plug-in determines its
display mode: whether it is embedded in a page,
is part of a section, appears on its own separate
page, or is hidden.
99

100. VRML
(Virtual Reality Modeling Language)
Virtual Reality Modeling Language (VRML) is an
open-standard programming language created to
design three-dimensional (3-D) and Web-based
models, textures and illusion.
VRML is used to illustrate 3-D objects, buildings,
landscapes or other items requiring 3-D structure
and is very similar to Hypertext Markup Language
(HTML).
VRML also uses textual representation to define 3-D
illusion presentation methods. 100

101. What are the strengths of VRML?
VRML is a highly dynamic, extensible language. The
architects of VRML designed the language in a very
modular format.
Every object is created in its own world. For instance,
if a vase was designed in VRML, it would first be
constructed in its own world coordinates. If a designer
wanted to place the vase in a mansion that he was
creating, he could simply translate, rotate, and scale
the vase so it fit exactly where he wanted it to.
By specifying components such as the vase in a
generic coordinate system, large libraries of VRML
components can be created that developers can
reference.
101

102. What are the weaknesses of VRML?
Unfortunately, VRML is fairly slow.
Given VRML's architecture, it will always have
trouble competing with 3d implementations that
squeeze the last little bit of performance out of
the CPU and graphics hardware.
Since VRML is so abstract, it will always be one
step behind applications that are optimized for a
particular architecture.
102