XML Primer

XML Primer

General Remarks

XML means eXtensible Markup Language.
Introduction to XML by W3Schools
XML was designed to be self-descriptive.
Today XML has a reputation for being wordy; other data formats like JSON and YAML have become popular.
XML is used to describe user interface layouts, for example: Android and XAML for .Net desktop and phone apps.

Some Observations and Definitions

A sample XML file: employees1.xml
Observations about XML files:
1. XML files start with this XML declaration tag:
```
<?xml version="1.0" encoding="UTF-8"?>
```
2. An XML file has exactly one root note
3. The employee1.xml file has exactly one root node:
```
<employees> ... </employees>
```
4. A node consists of a start tag, and end tag, and everything in between.
  The "everything in between" is called the content of a node.
5. A child node is a node contained inside another node called the parent node.
6. The content can be child nodes, it can be text, or it can be empty.
7. Here is a node where the content is text:
```
<firstname>Alice</firstname>
```
  <firstname> is the start tag, </firstname> is the end tag, Alice is the content, which is text.
8. An empty node has no content, for example:
```
<lastname></lastname>
```
9. This empty node can be abbreviated as a combination start/end tag:
```
<lastname />
```
XML Elements [W3Schools]

Attributes

An attribute is data that is contained in a tag.
This version uses an attribute for id: employees2.xml
The attribute is id="11111"

The data in a node can be completely expressed using attributes:

<emp id="55555" lastname="Cheng"
     firstname="Alice" salary="68000" />

Whether to represent data in a node as child nodes or attributes is a judgement call. Often child nodes are use to represent supplimentary data, whereas attributes are used to represent data that is an integral part of the node, such as a primary key:
```
<emp id="55555" lastname="Cheng">
    <firstname>Alice</firstname>
    <salary>68000</salary>
</emp> 
```
XML Attributes [W3Schools]

Namespaces

XML namespaces provide a way to resolve name conflicts.
Look at these namespace examples from W3Schools

Here is a namespace definition and usage in the LinearLayout template:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical">

XML namespaces are often specified as a URL. The URL need not be a legal web address.

XML Entities

Some characters have special meanings in XML and HTML:
```
&  "  &  <  '
```
To represent these characters literally in an XML file, use the corresponding character entity:

Name Symbol Entity

Ampersand & &

Double Quote " "

Greater than > >

Less than < <

Single Quote ' '

For example:
```
<equation>y &lt; x + 5</equation>
```