xml storage of object state

7/31/2019 XML Storage of Object State

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Mobile Development

Storing State with XML

RobMilesDepartment of Computer Science


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Introduction

An introduction to the problem

Writing an XML file> The XMLTextWriter class> Attributes and Elements in XML

> Text Encoding in XML files

Reading an XML document

> The XMLDocument class XML and namespaces

Storing the XML on a Smartphone/PocketPC


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Storing High Score Data

I wanted to store the high score of a player in

a Smartphone game> Name of the player

> Name of the game

> Score reached in the game

The data may need to be exported and used

in other systems, for example league tables

XML is the obvious choice for this


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Writing a PC Version

Unless you use some ofthe Mobile Powertoys it is

hard to see what aSmartphone application isdoing

I therefore decided to write

a PC version of the code I can port the important

bits later


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XML Namespaces

To get direct access to the XML methods and

the text encoding types I have to use two

namespaces:using System.Xml;

using System.Text; Once I have these I can write a method to

save the values in an XML document


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Writing an XML document

publicvoid SaveXML ( string filename )

{

XmlTextWriter writer ;

writer = new XmlTextWriter( filename, Encoding.ASCII);writer.Formatting = Formatting.Indented ;

writer.WriteStartDocument();

writer.WriteStartElement("highscore");

writer.WriteEndElement();

writer.WriteEndDocument();

writer.Close();

}

document with an empty highscore element


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Empty XML Document

The header of the document simply describes the

version of xml and the encoding being used

The score element is shown as empty

This is a completely legal XML document> but it does not contain any data.


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XML Attributes and Elements

There are two types of information in an XML file> Element: a lump of data about something; may contain other

elements

> Attribute: used to further describe a particular element.

The document being created presently has one

element, called highscore.

I can add an attribute to the highscore element which

identifies the game the score is for


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Adding an Attribute

publicvoid SaveXML ( string filename )

{

XmlTextWriter writer ;

writer = new XmlTextWriter (filename,Encoding.ASCII);writer.Formatting = Formatting.Indented;



writer.WriteAttributeString( "game", "Breakout");


writer.WriteEndDocument();writer.Close();

}


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Elements and Attribute Output

The game attribute identifies the name of the game

for which the high score was reached

This attribute is attributed to a given highscore

element


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Adding the Player and Score

Now we need to add the data about the player

and the score reached

There are two ways to do this:> add two more attributes to the highscore element. These

would be called playerand score and would hold the required

values.

> add two new elements, playerand score inside the highscoreelement


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Elements vs. Attributes

I have decided that player and score should be elements ratherthan attributes

It is easier for me to extend the player and score storage;

> I could add the address of the player and the date and time the score wasachieved

> Those attributes should bind to the player and score items, not the highscoreitself

Information directly about the high score data, such as the gameit applies to, should be an attribute

Another use for an attribute would be as an id tag of an element,or perhaps a version number (which you can see in the headerof the XML file itself)


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Writing the High Score

XmlTextWriter writer;

writer = new XmlTextWriter( filename, Encoding.ASCII) ;

writer.Formatting = Formatting.Indented;



writer.WriteAttributeString( "game", "Breakout");

writer.WriteElementString("playername",playerName);

writer.WriteElementString("score", score.ToString());


writer.WriteEndDocument();writer.Close();


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High Score XML

Rob Miles

1500


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XML & Meanings

Before we read the XML it is important to

have discussion about the meaning of things

The program that we write will ascribe

meaning to the elements it gets:>A score which is a big value is good

> In golf this would not be true..

There is nothing in the XML which gives the

meaning of the data itself


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Element Namespace

Not to be confused with the C# namespace

(although the intention is similar)

Allows an element to state the context in

which this element has meaning

This means that two programmers using the

same name for an element could ensure thatpeople using their elements can determine

the proper context/ontology


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Adding a Namespace

writer.WriteStartElement("highscore",

"www.mygameuri.com/highscore");

The uri gives the user of this element aunique identifier for this element

This ensures that my highscore element can

be identified as unique There does not have to be a web page at the

uri


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The Namespace in XML

The xmlns attribute identifies the namespace

for this element

I can create a set of namespaces based at a

particular uri We will see how these are tested later on


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Reading XML

We are going to use the XmlDocument class

provided by .NET:

// get a new document

document =new XmlDocument();

// load it from a file

document.Load(filename);

We can then read properties off the documentto get at the data


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XmlDocument structure

Name : highscore

NamespaceURI: www.mygameuri.com/highscore

game: Breakout

playername score

Rob Miles 1234


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Getting the Root element

System.Xml.XmlElement rootElement =

document.DocumentElement;

// make sure it is the right elementif ( rootElement.Name != "highscore" )

{

Console.WriteLine ( "Wrong data" );

} This gets the root element for the document and

makes sure it is the right one


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Checking a namespace

// make sure it is in the right namespace

if ( rootElement.NamespaceURI !=

"www.mygameuri.com/highscore" )

{

Console.WriteLine ("Wrong namespace");

}

All elements have a namespace property which maps

on to the namespace attribute


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Reading an attribute

// check to see if the name is correct

string gameName =

rootElement.GetAttribute("game");if ( gameName != "Breakout" )

{

return false ;

} Attributes are accessed by their name using theGetAttribute method


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Getting a Child Element

The simplest way to get hold of a child element is to

use the name as an indexer:

XmlNode playerNameNode = rootElement["player"];if ( playerNameNode == null )

{

Console.WriterLine ( "Missing player name" );

}

This gets the element with the given name, or null if

the name is not found.


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Get the value of an element

The value of an element is a child of thatelement:

playerName = playerNameNode.FirstChild.Value; The FirstChild member of the element in

this case is the data payload of that element

We can set the player name to this

All the values are returned as strings This means that we need to parse the score

value to get an integer


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Setting Values

You can set values in an element as well

There is also a method call which will save an

element (and all of its children)

This can be used if you have updated values

in the document that you want to save

ou want


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File Storage

You need to decide where to store the XML

files themselves

The obvious place to put them is the samedirectory as the binary program

On the Compact Framework you need to do a

bit of work to get the location of this


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Getting the Current Directory

System.Reflection.Assembly execAssem =

System.Reflection.Assembly.GetExecutingAssembly();

// set up the application filename

// from the current assembly

string appFilePath =execAssem.GetModules()[0].FullyQualifiedName;

// now strip the path from this filename

applicationDirectory =

System.IO.Path.GetDirectoryName(appFilePath);

// make sure we have a path separator on the end

if (!applicationDirectory.EndsWith(@"\"))

{

applicationDirectory += @"\";

}


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Demo 01 Saving and Loading

Scores

This project runs on a Smartphone and saves

the score values which are entered by the

user It can also load them back


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XML and Data

XML data elements always end up as text

If you want to store text which may contain

things like < and > you will have to investigateCDATA which lets you write strings containing

arbitrary text

You must also perform parsing to recover anynumeric values which you have stored> Remember to handle errors!


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XML and Programs

The XMLDocument is a very good way to hold

a structure in memory

But if you simply want to read object statesyou may decide instead to just read through

the XML elements and pull out the properties

you want


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XML is Fun!

No, really..

It provides a very easy way to manage

program data in a flexible and extensiblemanner

> For very little effort on your part

Whenever you are storing program data, and

you arent putting it in a database, you should

put it in XML!

xml storage of object state

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