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<h4 class="subsubsection"> 4.3.1.7 Plugin Meta-Information </h4>

<p>It is possible and common to use <small>SCF</small> interfaces and objects directly via
the C++ class names, via creation functions, or via facilities which return
pointers to pure <small>SCF</small> interfaces.  However, when an external plugin modules
publishes a class implementing an <small>SCF</small> interface, the client of that plugin
can not access the object directly via C++.  This is where the <small>SCF</small>
<em>class name</em> comes into play.  An <small>SCF</small> class name is an abitrary string
assigned to the implementation.  The string is passed to
<code>scfCreateInstance()</code> in order to create an instance of the named
object.  If necessary, <small>SCF</small> will load the plugin which implements the named
object in order to satisfy the request.  The <small>SCF</small> class name can be
anything you like, though it is often useful to impose a hierarchical
interpretation on the name.  For instance,
<samp>&lsquo;crystalspace.graphics3d.opengl&rsquo;</samp>.
</p>
<p>The data which describes a plugin module, such as the <small>SCF</small> class names
and C++ implementations which the plugin exports, is known as
<em>meta-information</em>.  Each plugin has an associated meta-information
resource which describes the plugin module.  This information can be accessed
without even loading the plugin module.  At development time, the
meta-information is maintained in an <small>XML</small>-format text file which has
the same name as the plugin module, but with extension <tt>&lsquo;.csplugin&rsquo;</tt>.  Here
is a sample meta-information resource for a plugin named <tt>&lsquo;myplugin&rsquo;</tt>
(<tt>&lsquo;myplugin.so&rsquo;</tt> on Unix, <tt>&lsquo;myplugin.dll&rsquo;</tt> on Windows):
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">&lt;?xml version=&quot;1.0&quot;?&gt;
&lt;!-- myplugin.csplugin --&gt;
&lt;plugin&gt;
  &lt;scf&gt;
    &lt;classes&gt;
      &lt;class&gt;
        &lt;name&gt;myproj.myplugin.foo1&lt;/name&gt;
        &lt;implementation&gt;MyClass1&lt;/implementation&gt;
        &lt;description&gt;My first custom foo class&lt;/description&gt;
      &lt;/class&gt;
      &lt;class&gt;
        &lt;name&gt;myproj.myplugin.foo2&lt;/name&gt;
        &lt;implementation&gt;MyClass2&lt;/implementation&gt;
        &lt;description&gt;My second custom foo class&lt;/description&gt;
        &lt;requires&gt;
          &lt;class&gt;myproj.myplugin.bar1&lt;/class&gt;
          &lt;class&gt;myproj.myplugin.bar2&lt;/class&gt;
        &lt;/requires&gt;
      &lt;/class&gt;
    &lt;/classes&gt;
  &lt;/scf&gt;
&lt;/plugin&gt;
</pre></td></tr></table>
<p>The top-level node of a meta-information file is named <code>&lt;plugin&gt;</code>.  All
<small>SCF</small>-related information is contained within an <code>&lt;scf&gt;</code> child node.
Plugin modules can export multiple named <small>SCF</small> classes.  Each exported class
is represented by a <code>&lt;class&gt;</code> node within the <code>&lt;classes&gt;</code> group.  The
<code>&lt;name&gt;</code> node of a <code>&lt;class&gt;</code> is the class' <small>SCF</small> name.  The
<code>&lt;implementation&gt;</code> node references the C++ class which actually implements
the named <small>SCF</small> class.  This is the same name that is privided as an
argument to the <code>SCF_IMPLEMENT_FACTORY()</code> macro.  When an <small>SCF</small> class
depends upon other <small>SCF</small> classes, the dependencies are indicated via the
optional <code>&lt;requires&gt;</code> group, which contains one <code>&lt;class&gt;</code> node per
dependency.
</p>
<p>The above example meta-information resource indicates that the plugin exports
two C++ classes, <samp>&lsquo;MyClass1&rsquo;</samp> and <samp>&lsquo;MyClass2&rsquo;</samp> under the <small>SCF</small> class
names <samp>&lsquo;myproj.myplugin.foo1&rsquo;</samp> and <samp>&lsquo;myproj.myplugin.foo2&rsquo;</samp>,
respectively.  Furthermore, the second exported class has a dependency upon two
other <small>SCF</small> classes, <samp>&lsquo;myproj.myplugin.bar1&rsquo;</samp> and
<samp>&lsquo;myproj.myplugin.bar2&rsquo;</samp>.  <small>SCF</small> will ensure that these other classes are
loaded before it instantiates <samp>&lsquo;MyClass2&rsquo;</samp>.
</p>
<p>Meta-information in the <tt>&lsquo;.csplugin&rsquo;</tt> file is extensible; it is not
restricted to <small>SCF</small>-only usage.  Plugin authors can choose to publish
supplementary information about plugins in addition to the <small>SCF</small> information
already published.  As a hypothetical example, image loading plugins might
desire to publish <em>image indentification</em> information which would allow the
image loading multiplexor to selectively request image loading plugins
<em>on-demand</em>, rather than requesting all plugins unconditionally, even if
they are not needed.  Here is a possible meta-information table for a <small>PNG</small>
image loader (with the <code>&lt;scf&gt;</code> node collapsed to <samp>&lsquo;<small class="dots">...</small>&rsquo;</samp> for the
sake of illustration):
</p>
<table><tr><td>&nbsp;</td><td><pre class="example">&lt;?xml version=&quot;1.0&quot;?&gt;
&lt;!-- cspngimg.csplugin --&gt;
&lt;plugin&gt;
  &lt;scf&gt;...&lt;/scf&gt;
  &lt;imageloader&gt;
    &lt;imagetype&gt;
      &lt;class&gt;crystalspace.graphic.image.io.png&lt;/class&gt;
      &lt;identify&gt;
        &lt;mimetype&gt;image/png&lt;/mimetype&gt;
        &lt;extension&gt;png&lt;/extension&gt;
        &lt;extension&gt;PNG&lt;/extension&gt;
        &lt;scan length=&quot;4&quot; bytes=&quot;\0x89PNG&quot;/&gt;
     &lt;/identify&gt;
    &lt;/imagetype&gt;
  &lt;/imageloader&gt;
&lt;/plugin&gt;
</pre></td></tr></table>
<p>In this example, the <small>PNG</small> loader meta-information tells the multiplexor
several different ways to identify a <small>PNG</small> image:
</p>
<ul>
<li>
By checking <small>MIME</small> type, if available.
</li><li>
By checking file extension, if available.
</li><li>
By checking for the <em>magic</em> identification string <samp>&lsquo;\0x89PNG&rsquo;</samp> in the
raw image data.
</li></ul>


<p>If the multiplexor identifies the image as <small>PNG</small>, only then will it actually
request the <small>PNG</small> loader plugin.
</p>
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