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127 lines
3.4 KiB
Markdown
127 lines
3.4 KiB
Markdown
---
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language: xml
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filename: learnxml.xml
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contributors:
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- ["João Farias", "https://github.com/JoaoGFarias"]
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---
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XML is a markup language designed to store and transport data.
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Unlike HTML, XML does not specify how to display or to format data, just carry it.
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* XML Syntax
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```xml
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<!-- Comments in XML are like this -->
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<?xml version="1.0" encoding="UTF-8"?>
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<bookstore>
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<book category="COOKING">
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<title lang="en">Everyday Italian</title>
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<author>Giada De Laurentiis</author>
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<year>2005</year>
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<price>30.00</price>
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</book>
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<book category="CHILDREN">
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<title lang="en">Harry Potter</title>
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<author>J K. Rowling</author>
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<year>2005</year>
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<price>29.99</price>
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</book>
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<book category="WEB">
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<title lang="en">Learning XML</title>
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<author>Erik T. Ray</author>
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<year>2003</year>
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<price>39.95</price>
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</book>
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</bookstore>
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<!-- Above is a typical XML file.
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It starts with a declaration, informing some metadata (optional).
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XML uses a tree structure. Above, the root node is 'bookstore', which has
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three child nodes, all 'books'. Those nodes has more child nodes, and so on...
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Nodes are created using open/close tags, and childs are just nodes between
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the open and close tags.-->
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<!-- XML carries two kind of data:
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1 - Attributes -> That's metadata about a node.
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Usually, the XML parser uses this information to store the data properly.
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It is characterized by appearing in parenthesis within the opening tag
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2 - Elements -> That's pure data.
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That's what the parser will retrieve from the XML file.
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Elements appear between the open and close tags, without parenthesis. -->
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<!-- Below, an element with two attributes -->
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<file type="gif" id="4293">computer.gif</file>
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```
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* Well-Formated Document x Validation
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A XML document is well-formated if it is syntactically correct.
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However, it is possible to inject more constraints in the document,
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using document definitions, such as DTD and XML Schema.
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A XML document which follows a document definition is called valid,
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regarding that document.
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With this tool, you can check the XML data outside the application logic.
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```xml
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<!-- Below, you can see an simplified version of bookstore document,
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with the addition of DTD definition.-->
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE note SYSTEM "Bookstore.dtd">
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<bookstore>
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<book category="COOKING">
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<title >Everyday Italian</title>
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<price>30.00</price>
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</book>
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</bookstore>
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<!-- This DTD could be something like:-->
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<!DOCTYPE note
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[
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<!ELEMENT bookstore (book+)>
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<!ELEMENT book (title,price)>
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<!ATTLIST book category CDATA "Literature">
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<!ELEMENT title (#PCDATA)>
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<!ELEMENT price (#PCDATA)>
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]>
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<!-- The DTD starts with a declaration.
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Following, the root node is declared, requiring 1 or more child nodes 'book'.
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Each 'book' should contain exactly one 'title' and 'price' and an attribute
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called 'category', with "Literature" as its default value.
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The 'title' and 'price' nodes contain a parsed character data.-->
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<!-- The DTD could be declared inside the XML file itself.-->
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<?xml version="1.0" encoding="UTF-8"?>
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<!DOCTYPE note
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[
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<!ELEMENT bookstore (book+)>
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<!ELEMENT book (title,price)>
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<!ATTLIST book category CDATA "Literature">
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<!ELEMENT title (#PCDATA)>
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<!ELEMENT price (#PCDATA)>
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]>
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<bookstore>
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<book category="COOKING">
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<title >Everyday Italian</title>
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<price>30.00</price>
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</book>
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</bookstore>
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```
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