Nepomuk Quickstart
Tutorial Series   Nepomuk
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What's Next   Handle Resource Metadata with Nepomuk
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Reading or setting simple metadata in your application with Nepomuk, can be very easy. However, keep in mind that the process described here may have performance drawbacks when changing a lot of metadata. We will now take a look at a simple way to access a resource's metadata.

Some Basics

Nepomuk is centred around a main 'Resource' class. For simple, non-high performance access to Nepomuk, it is recommended that you use the Resource class.

Everything in Nepomuk is a Resource. Each Resource has a number of properties associated with it, which are stored as (key, value) pairs. They keys are referred to as properties or predicates, and the values are referred to as the objects.

Every file, folder, contact or tag is a Resource.

Dealing with Tags

Every tag in Nepomuk is a Resource. In fact the Tag class is also derived from the Resource class.

Setting Tags

Nepomuk2::Tag tag( "Awesome-Tag-Name" );
Nepomuk2::Resource res( url );
res.addTag( tag );

The 'Nepomuk2::Tag' class will automatically look for for a tag called "Awesome-Tag-Name", it is finds it then Tag::exists() will return true. Otherwise, the tag will be saved the first time the tag is used. In our case, when we add the tag to the Resource via Resource::addTag(), the tag will be saved.

Retrieving Tags

using namespace Nepomuk2;

Resource res( url );
QList<Tag> tags = res.tags();
foreach(const Tag& tag, tags)
    kDebug() << tag.genericLabel();

Every resource has a predefined function for retrieving all the Tags any resource is tagged with - Resource::tags(). The Resource::genericLabel() function tries to find a presentable name for any resource. In the case of tags, it returns the tags name.

Listing all the Tags

The simplest way to list all the tags that are present in Nepomuk is via a static function - Tag::allTags(). You, however, need to be careful using this in a production environment as it executes a blocking query in the background. Depending on the number of tags on the system, it could take some time.

using namespace Nepomuk2;

QList<Tag> tags = Tag::allTags();
foreach(const Tag& tag, tags)
    kDebug() << tag.genericLabel();

Dealing with files

Every file in Nepomuk is represented as a resource. Since dealing with files is very common, we provide a special File class which is derived from the Resource class. We also provide convenience functions for checking if a resource is a file.

using namespace Nepomuk2;

File fileRes( urlOfTheFile );
kDebug() << fileRes.url();

Resource res( urlOfTheFile );
if( res.isFile() )
    kDebug() << res.toFile().url();

Internally Nepomuk Files are not very different from other Resources.


Every resource in Nepomuk can be given a numeric rating. It is generally done on a scale of 0-10.

using namespace Nepomuk2;

File fileRes( urlOfTheFile );
int rating = fileRes.rating();

rating = rating + 1;
fileRes.setRating( rating );

Includes and Linking


For the example you need the following includes:

#include <Nepomuk2/Tag>
#include <Nepomuk2/Resource>
#include <Nepomuk2/File>
#include <KDebug>


In your CMakeLists.txt, you need the find macro for Nepomuk and link to it of course ;)

find_package(NepomukCore REQUIRED)
target_link_libraries(myfile ${NEPOMUK_CORE_LIBRARY} )

This page was last edited on 9 February 2018, at 12:35. Content is available under Creative Commons License SA 4.0 unless otherwise noted.