Difference between revisions of "Projects/Nepomuk/QuickStart"

Revision as of 17:53, 30 July 2012

Nepomuk Quickstart

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.

Initializing the Resource Manager

The ResourceManager is the central KMetaData configuration point. For KDE 4.2 and newer, we must explicitly initialize it in order to connect to the Nepomuk service.

Nepomuk::ResourceManager::instance()->init();

This method returns an int. If it succeeds (returns 0), the connection to the Nepomuk service has been established and we can work with it. If it fails (returns negative integer), the Nepomuk service was unreachable and we can't continue working with any Nepomuk related code--this may occur if the user has disabled the Nepomuk service for his current session.

Retrieving Metadata

Let's get the metadata for a file, assuming the URL or the file is stored in uri.

Nepomuk::Resource res( uri );
QHash<QUrl, Nepomuk::Variant> properties = res.properties();

This allows us to obtain all the properties assigned to the file.

We can now use Nepomuk to get human-readable labels for the properties, and display the properties in a generic way:

for( QHash<QUrl, Nepomuk::Variant>::const_iterator it = properties.constBegin();
     it != properties.constEnd(); ++it ) {
   QUrl propertyUri = it.key();
   Nepomuk::Variant value = it.value();
 
   Nepomuk::Types::Class propertyType( propertyUri );
 
   someList->appendItem( propertyType.label() + ": " + value.toString() );
}

Setting Metadata

Similarly, assume url is the URL of a file that we want to set some metadata to. This time we want to set a tag and a comment and will do this using two slightly different methods:

Let's start with the tag and use the easy Nepomukish way:

Nepomuk::Tag tag( "This is my nice tag name" );
Nepomuk::Resource res( url );
res.addTag( tag );

Simple! Actually, if the tag already exists, it will be reused.

Now let's set a comment for the file without the use of the convenience methods in Nepomuk:

Nepomuk::Resource res( url );
QString comment = getFancyFileComment();
res.setProperty( Soprano::Vocabulary::NAO::description(), comment );

That's all. The comment is saved and will now be searchable via Nepomuk.

Includes and Linking

For the example you need the following includes:

#include <Nepomuk2/ResourceManager>
#include <Nepomuk2/Variant>
#include <Nepomuk2/Types/Class>
#include <Nepomuk2/Tag>
#include <Soprano/Vocabulary/NAO>

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_LIBRARIES} )