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<div id="header">
<h1>How to provide context properties</h1>
<div id="toc">
<div id="toctitle">Table of Contents</div>
<noscript><p><b>JavaScript must be enabled in your browser to display the table of contents.</b></p></noscript>
</div>
</div>
<div id="preamble">
<div class="sectionbody">
<div class="paragraph"><p>Any component can provide its own key/value pairs and make them appear
as a <tt>ContextProperty</tt> for C++ (and in other forms for other
languages).</p></div>
<div class="paragraph"><p>As a provider of context properties, you need to drop one or more
<em>property declaration</em> files into <tt>/usr/share/contextkit/providers/</tt>
to register your properties with the context framework. This file
follows a format described below and is used by <tt>libcontextsubscriber</tt>
and others to find you when someone subscribes to your properties.
The file is also used by the Context Commander to show descriptions of
your properties and to learn about the possible values that a property
can legally have.</p></div>
<div class="paragraph"><p>The property declaration files also inform the context framework how
you want to be contacted. Right now, you have to implement the
<tt>org.freedesktop.ContextKit</tt> D-Bus interface, and register yourself on
either the system or the session D-Bus with a suitable bus name. The
choice of system or session bus and your bus name go into the property
declaration file.</p></div>
<div class="paragraph"><p>The only supported way right now to implement the
<tt>org.freedesktop.ContextKit</tt> interface is to use the
<a href="libcontextprovider"><tt>libcontextprovider</tt></a> library.</p></div>
<div class="paragraph"><p>The name of the property declaration file must be
"<em>bus-name</em><tt>.context</tt>".</p></div>
</div>
</div>
<h2 id="_the_property_declaration_file">The property declaration file</h2>
<div class="sectionbody">
<div class="paragraph"><p>The property declaration file contains XML and must follow
<a href="context-provider-schema.html">this XML schema</a>. A simple example
for the <tt>Example.Random</tt> property looks like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><?xml version="1.0"?>
<provider bus="session"
service="com.example.RandomProvider">
<key name="com.example.Random">
<type>double</type>
<doc>
A random number between 0 and 1 that changes every now and then.
</doc>
</key>
</provider></tt></pre>
</div></div>
<div class="paragraph"><p>This file declares the single property <tt>com.example.Random</tt> and
instructs the Content Framework to connect to the
<tt>com.example.RandomProvider</tt> bus name on the session D-Bus. It would
be stored as
<tt>/usr/share/contextkit/providers/com.example.RandomProvider.context</tt></p></div>
<div class="paragraph"><p>You need to be careful when choosing property name; see the
"Guidelines for property providers" section for how to avoid
conflicts.</p></div>
<div class="paragraph"><p>When providing properties from the <a href="context-properties.html">core
list</a>, you need to follow additional rules to make sure that your
property declarations and the centrally maintained core list do not
fall out of synch. See the "Providing core properties" section for
more about them.</p></div>
<div class="paragraph"><p>After installing a property declaration file into a directory <tt>$dir</tt>,
you should usually execute <tt>update-contextkit-providers</tt>. This will
update the various caches that clients like the <tt>libcontextsubscriber</tt>
library use.</p></div>
<div class="paragraph"><p>But, if a property declaration file is installed via a Debian package,
you should not call <tt>update-contextkit-providers</tt> in your maintainer
scripts. Triggers in the relevant packages take care of running
update-contextkit-providers at the right time and only as often as
necessary.</p></div>
<div class="paragraph"><p>The XML element tree in the property declaration file simply consists
of a list of <tt>key</tt> elements.</p></div>
<div class="paragraph"><p>A <tt>key</tt> element in the property declaration tree can have a <tt>doc</tt>
child element. The contents of this element should be plain text
without any additional markup.</p></div>
<h3 id="_types">Types</h3><div style="clear:left"></div>
<div class="paragraph"><p><strong>NOTE:</strong> This is preview of things that might come, or not. For now,
just use a <tt>type</tt> attribute in your <tt>key</tt> elements with one of
<tt>"TRUTH"</tt>, <tt>"STRING"</tt>, <tt>"INT"</tt>, or <tt>"DOUBLE"</tt> as the value.</p></div>
<div class="paragraph"><p>ContextKit uses the experimental <a href="context-types.html">Desktop Type
System</a>. A property declaration should have a <tt>type</tt> element that
conforms to the Desktop Type System.</p></div>
<div class="paragraph"><p>For example, a property that is a enumeration would be declared like
this:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><key name="Economy">
<type>
<int-enum>
<up val="1" doc="Getting better"/>
<down val="2" doc="Getting worse"/>
<even val="3" doc="Getting boring"/>
</int-enum>
</type>
<doc>
The current economic trend, as an enumerated value.
</doc>
</key></tt></pre>
</div></div>
</div>
<h2 id="_guidelines_for_property_providers">Guidelines for property providers</h2>
<div class="sectionbody">
<h3 id="_names">Names</h3><div style="clear:left"></div>
<div class="paragraph"><p>Context property names can contain any character except "/".</p></div>
<div class="paragraph"><p>Only core property names are allowed to start with a capital ASCII
letter ("A" to "Z").</p></div>
<div class="paragraph"><p>When defining a new non-core property, you need to choose a unique
prefix. Start with a reversed fully qualified domainname that you
control, such as "com.nokia." or "org.gnome.". Then, if the first
component is "org" and the second component is not a top-level domain,
drop "org". Then convert the first character of the prefix to
lower-case if it is one of "A" to "Z".</p></div>
<div class="paragraph"><p>For example, the GNOME project can use "gnome." as their prefix, KDE
can use "kde.", and Nokia can use "com.nokia.".</p></div>
</div>
<h2 id="_providing_core_properties">Providing core properties</h2>
<div class="sectionbody">
<div class="paragraph"><p>The Context Framework project maintains a list of <em>core properties</em>.
These core properties are intended to cover the needs of most
applications and be meaningful for many different devices.</p></div>
<div class="paragraph"><p>When you are implementing a provider for some of the core properties,
you must of course make sure that you follow the specification of that
property. You can not redefine its type or description in your
property declaration file, obviously.</p></div>
<div class="paragraph"><p>Thus, for core properties, you should not include any <tt>type</tt> or <tt>doc</tt>
elements in your key declarations, just a <tt>name</tt> element. The list of
core properties is also known to context subcribers at run-time and
will be used to fill in the missing details.</p></div>
<div class="paragraph"><p>For example, a property declaration file for core properties could
look like this:</p></div>
<div class="listingblock">
<div class="content">
<pre><tt><?xml version="1.0"?>
<provider bus="session"
service="com.example.RandomProvider">
<key name="Example.Random"/>
</provider></tt></pre>
</div></div>
<div class="paragraph"><p>This just declares that the <tt>Example.Random</tt> property can be
subscribed to by contacting <tt>com.example.RandomProvider</tt> on the
session bus. It’s type and documentation are found elsewhere.</p></div>
</div>
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<div id="footer-text">
Last updated 2009-11-26 11:20:27 EEST
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