Projects/Nepomuk/Resources

Introduction

The Nepomuk Resource class provides a convenient wrapper over the Nepomuk data. It does so by keeping a cache of the relevant data and using the Resource Watcher to keep it up to date.

Saving Resources

Resources are a real-time view of the data in the virtuoso database. They are automatically saved after each change, and all other resources across the different applications are updated.

Saving New Resources

A new resource will automatically be saved the first time it is used.

    Nepomuk2::Resource res;
    res.exists(); // Will return false
    res.uri(); // Will be blank

    res.setRating( 5 ); // Will be created and saved
    res.exists(); // Will return true
    res.uri(); // Will return its unique identifier

The following examples show the behavior of storing resources in different cases. It uses the HistoryItem ontology used in KGet as an example of a typed resource.

  /**
   * This resource won't be saved to the database because
   * there is no property assigned to it
   */
  Nepomuk2::Resource notSaved("file:///tmp/notsaved.txt");
  
  /**
   * This resource will be saved to the database because we
   * set a property to it
   */
  Nepomuk2::Resource saved("file:///tmp/saved.txt");
  
  saved.addProperty(Nepomuk2::Vocabulary::NIE::comment(), Nepomuk2::Variant("This will be saved"));
  
  /**
   * This is saved to a resource with the Nepomuk2::HistoryItem type
   * because we set a property of it
   */
  Nepomuk2::HistoryItem typedResource("file:///tmp/typedResource.txt");

  typedResource.setSize(50);

Managing Properties

When using the Resource class, one can easily access certain properties such as ratings and tags via convenience functions. However, if one wishes to access more properties then that is performed via the property, setProperty and addProperty functions.

It's important to know which properties are available in each resource. The simplest way would be to check they keys in properties.

Accessing the Vocabularies

In order to modify any of the properties in Nepomuk, one must access the ontologies. The recommended way of doing that is by using the pre-generated ontologies headers which are shipped with Soprano and NepomukCore.

#include <Soprano/Vocabulary/RDF>
#include <Soprano/Vocabulary/RDFS>
#include <Soprano/Vocabulary/NRL>
#include <Soprano/Vocabulary/NAO>

#include <Nepomuk2/Vocabulary/NIE>
#include <Nepomuk2/Vocabulary/NFO>
#include <Nepomuk2/Vocabulary/NMM>
#include <Nepomuk2/Vocabulary/NCO>
#include <Nepomuk2/Vocabulary/PIMO>
#include <Nepomuk2/Vocabulary/NCAL>

using namespace Neppomuk2::Vocabulary;
using namespace Soprano::Vocabulary;

Setting Properties

When setting a property of a resource. It is imperative that the domain, range and cardinality of the property be correct. Otherwise the setProperty call will silently fail and output an error as a debug message. In future releases we will provide other ways to get notified of the errors.

Example -

    Nepomuk2::Resource res;
    res.addType( NCO::Contact() );

    res.setProperty( NFO::fileName(), "Fire" ); // Will silently fail
    res.property( NFO::fileName() ).toString(); // Will be empty

    res.addProperty( NCO::fullname(), QLatin1String("Tom Marvolo Riddle") ); // Will work
    res.addProperty( NCO::fullname(), QLatin1String("Lord Voldemort") ); // Will fail - nco:fullname has a max cardinality of 1

    QString origName = res.property( NCO::fullname() ).toString(); // will return "Tom Marvolo Riddle"
    res.setProperty( NCO::fullname(), QLatin1String("Lord Voldemort") ); // will work this time - we're using setProperty

    QString newName = res.property( NCO::fullname() ).toString(); // Will return You-know-who

Nepomuk Resource Generator

While it is fairly convenient to use the setProperty and getProperty methods. It requires you to explicitly define the property which you need modify. This can get quite cumbersome and destroys readability of the code.

In order to improve this situation we have provided a resource generator, which generates custom Resource classes from the ontologies. It relies on a CMake macro to accomplish that.

Usage in CMake

The resource generator is best used through the CMake macro provided by Nepomuk. The syntax is fairly simple and similar to the macro for adding ui files to a list of sources:

NEPOMUK2_ADD_ONTOLOGY_CLASSES(<sources-var>
         [ONTOLOGIES] <onto-file1> [<onto-file2> ...]
         [CLASSES <classname1> [<classname2> ...]]
         [VISIBILITY <visibility-name>]
       )

The optional CLASSES parameter allows to specify the classes to be generated (RDF class names) in case one does not want all classes in the ontologies to be generated. If omitted all classes in the ontology files will be generated.

The optional VISIBILITY parameter can only be used in non-fast mode and allows to set the gcc visibility to make the generated classes usable in a publically exported API. The <visibility-name> is used to create the name of the export macro and the export include file. Thus, when using "VISIBILITY foobar" include file "foobar_export.h" needs to define FOOBAR_EXPORT.

Example

include(Nepomuk2AddOntologyClasses)

nepomuk2_add_ontology_classes (SRCS
    ONTOLOGIES
    ${SHAREDDESKTOPONTOLOGIES_ROOT_DIR}/nie/nie.trig
    ${SHAREDDESKTOPONTOLOGIES_ROOT_DIR}/nie/nco.trig
    ${SHAREDDESKTOPONTOLOGIES_ROOT_DIR}/pimo/pimo.trig
)

This will generate a C++ class for each type defined in the mentioned ontologies.

#include "personcontact.h"

Nepomuk2::PersonContact spiderman;
spiderman.setFullName( QLatin1String("Peter Parker") );
QString name = spiderman.fullname();