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<h3 class="subsection"> 4.1.1 Plugins and Modules </h3>
<p>Crystal Space is a package of components and libraries which can all be useful
for creating computer games. Although some of the packages are intended more
for 3D gaming this is not as such a restriction of the package. Components
such as the sound driver function just as well in a 2D gaming environment, and
the Virtual File System plugin (see section <a href="VFS.html#0">Virtual File System (<small>VFS</small>)</a>) can be used in non-graphical
applications. This highlights one of the important characteristics of Crystal
Space: the components and libraries are more or less independent of each other.
If you do not want “real” physics, then just don't use the physics plugin.
If you don't want scripting then don't include that. All packages are designed
to be functional on their own or with a minimal number of other packages.
There are some dependencies of course. For example, the 3D engine requires a
3D renderer to display its output, and a renderer requires a canvas onto which
to draw. On the other hand, you could very well use the 3D renderer without
the 3D engine.
</p>
<p>Although there is a high level of independence, there is also a high level of
integration. The components and libraries were also designed to be used
together and as such offer a flexible scheme of integration.
</p>
<p>If you are interested in developing a program using Crystal Space, then it is
important to understand that Crystal Space is not a single monolithic library,
but instead consists of several <em>libraries</em> and <em>plug-in modules</em>.
This manual uses the term <em>module</em> for both libraries and plug-ins when
the distinction between the two is not important.
</p>
<p>A plug-in module is similar to a library, but has some advantages over a
library. All plug-ins can be accessed in a common fashion. They have a pure
interface, and they can provide this interface even if they are extracted to a
dynamic load library. So, they are the better solution as long as speed is not
the dictator of all decisions as in the math library (access to plug-in
functions uses virtual methods). The main difference between the interface of
a library and a plug-in is that a library may use <small>SCF</small>, the Shared Class
Facility (see section <a href="SCF.html#0">Shared Class Facility (<small>SCF</small>)</a>), and C++ classes; plug-ins may only use <small>SCF</small>.
</p>
<ul>
<li>
What is <small>SCF</small>?
<p>The main <small>SCF</small> object is the <em>interface</em>. An interface is the
solution to strictly decouple the public methods of an object from their
implementation. You only get a pointer to an abstract class with only virtual
methods, called the interface, so your program doesn't know about the actual
object behind the pointer. This object is called an <em>implementation</em> of
the interface. You as the user of Crystal Space will call functions that
create the actual object, but only return the interface. After that you can
use the interface like a C++ object. When you don't need it anymore, don't
<samp>‘delete’</samp> it, but call <code>DecRef()</code>. When you pass a pointer to the
interface to anywhere, call <code>IncRef()</code> from there, and <code>DecRef()</code>
when you don't need the interface there anymore.
</p>
<p>Starting with Crystal Space version 0.95 we also have smart pointers.
A smart pointer is an instance of the class <code>csRef</code>. This class takes
care of <code>IncRef()</code> and <code>DecRef()</code> for you
(see section <a href="Smart-Pointers.html#0">Correctly Using Smart Pointers</a>). It is very important to use smart pointers
now. They are making life a lot easier and in the future they may become
required usage (i.e. no longer optional).
</p>
<p>As the user you'll only have to include a header that defines the interface,
but not the implementation. Despite the obvious advantage of having a very
clear structure in your code, there is the advantage that linking the
interface to the implementation can be done at run-time, but more about this
later.
</p>
</li><li>
What are the characteristics of a library?
<p>A library is just a normal C++ library as you know them. A library can
optionally provide <small>SCF</small> interfaces. In the case of a library this is just
a way to define a clear structure. But as their main interface, libraries
provide C++ classes.
</p>
</li><li>
What are the characteristics of a plug-in?
<p>A plug-in, on the other hand, will only provide <small>SCF</small> interfaces, no normal
C++ classes. The plug-in itself will also appear as an <small>SCF</small> interface.
This is part of the definition of a plug-in. A plug-in can be organized as
static library or <small>DLL</small>; this only makes a small difference in how you use
it in your program.
</p>
<p>As the user of Crystal Space, you have to do
the following steps to use a plug-in:
</p>
<ul class="toc">
<li>-
First, do all these steps for the dependencies (other plug-ins) that this
plug-in relies on.
</li><li>-
Register the library that contains the plug-in.
</li><li>-
Load the plug-in. This will also initialize the plug-in. It will fail if you
forgot any dependencies.
</li><li>-
Query the plug-in interface. This is an <small>SCF</small> interface, so you now have
access to the functions of the plug-in.
</li></ul>
</li><li>
How can I register the plug-in library?
<p><em>Registering</em> means to tell <small>SCF</small> the name of the plug-in that was given
to it by its author, and a class or a dynamic library to associate it with.
Plugin libraries can contain multiple named <small>SCF</small> classes. For example, to
use the software graphics renderer <small>SCF</small> must know that the <small>SCF</small> class
<samp>‘crystalspace.graphics3d.software’</samp> can be found in <tt>‘soft3d.dll’</tt>
(Windows) or <tt>‘soft3d.so’</tt> (Unix). <small>SCF</small> determines dynamically which
plugin libraries contain which <small>SCF</small> classes by consulting meta-information
associated with each plugin. Depending upon the platform, the associated
meta-information may be bundled into the plugin itself, or it might exist in a
separate file with a <tt>‘.csplugin’</tt> extension alongside the plugin module.
</p>
<p>How you register a library depends on whether it is a static library or a
dynamic library (<tt>‘.dll’</tt> or <tt>‘.so’</tt>). For a static library, that is,
one which is linked directly into the application rather than loaded
dynamically at run-time, put the following macro invocation at top-level in one
of your C++ files once for each <small>SCF</small> class contained in your library:
</p>
<table><tr><td> </td><td><pre class="example">SCF_REGISTER_STATIC_CLASS(
<var>cxx-class</var>,
<var>scf-name</var>,
<var>description</var>,
<var>dependencies</var>)
</pre></td></tr></table>
<p>Here, <var>cxx-class</var> is the name of the C++ class which implements the
factory for this particular <small>SCF</small> class. <var>cxx-class</var> is the same name
given to the <code>SCF_IMPLEMENT_FACTORY()</code> macro. <var>scf-name</var> is the
<small>SCF</small> class name corrsponding to <var>cxx-class</var>, <var>description</var> is a
human-readable string describing the purpose of the class, and
<var>dependencies</var> is a comma-separated list of other <small>SCF</small> class upon which
this class depends.
</p>
<p>For a dynamic library, <small>SCF</small> will discover the plugin and register the
contained classes automatically by consulting the associated meta-information.
</p>
</li><li>
How can I load the plug-in?
<p>To load a plug-in, you must tell the plug-in manager the name
of the plug-in as it was registered in the previous step. In the most
common case you will probably use the plug-in loader to load the plug-in.
This is a convenience class that is able to load plug-in as specified
in a config file, from the commandline, or as requested by the application
programmer. In addition to loading the plug-in (the plug-in loader will
use the plug-in manager to do that), the plug-in loader will optionally
also register the plug-in with the <em>Object Registry</em>.
</p>
<p>The object registry is a central location in the Crystal Space framework
where any module can register <small>SCF</small> objects. The object registry is not
only for plug-ins. It can contain any <small>SCF</small> object. Objects in the
object registry also have a tag name associated with them. By convention
the <em>default</em> object for a given <small>SCF</small> interface will be
added to the object registry with the tag name equal to the interface
name. For example, a 3D renderer is a plug-in module that
implements (among others) the <samp>‘iGraphics3D’</samp> interface. At the same
time there can be multiple 3D renderers in memory (for example, for
procedural textures). But the default 3D renderer will be registered
with the tag name <samp>‘iGraphics3D’</samp>.
</p>
<p>Note that the decision which graphics driver you use (e.g. Software or
OpenGL) is done at the time you load the plug-in by passing the name of
that driver. At the time you ask for the plug-in interface and use it in
your program, this does not make a difference anymore. This makes it possible
to exchange the driver simply by loading another driver, but without changing
your main program.
</p>
<p>The Crystal Space modules themselves will use the standard plug-ins with the
default tag name as explained above. For example, the 3d engine looks
for the graphics driver by looking in the object registry for an object
with the tag <samp>‘iGraphics3D’</samp>.
</p>
<p>Now how can you actually load the plug-in? You can either load them
manually in the code using the <code>csLoadPlugin</code> function or else
you can use the plugin loader. As explained above, the plugin loader
can load plugins specified on the commandline, a config file, or else
explicitly requested in the code. This is done in a specific way:
The commandline has highest priority. i.e. if the user specified the
OpenGL video driver on the commandline then this will be the plugin that
is loaded on the <samp>‘iGraphics3D’</samp> tag. The config file and plugins requested
from the code are ignored then (for that tag at least). The plugins
requested from the code have lowest priority and only serve as a default
if neither the commandline nor the config file specified a plug-in for
the given tag.
</p>
<p>There is a class called <code>csInitializer()</code> which contains various
convenience routines to help initialize a Crystal Space application.
This class also contains a routine (<code>RequestPlugins()</code>) which will
use the plugin loader automatically.
</p>
<p>There are several advantages to using the plugin loader as opposed
to manually loading plug-ins using <code>csLoadPlugin</code>:
</p>
<ul class="toc">
<li>-
The plugin loader will sort all plug-ins based on dependencies.
For example, the engine depends on a 3D rasterizer so the plugin
loader will make sure that the engine is loaded later. If you manually
load plugins you risk that the loading will fail because a plugin
that it needs is not yet present.
</li><li>-
The user is able to override the plug-ins loaded by the plugin loader.
He or she can modify the config file or specify an alternative
plug-in on the commandline. There is no way to change what plugin is
loaded using <code>csLoadPlugin</code> unless by recompilation (unless of course
you use some other way to read the config file and find out what plugin
to load).
</li></ul>
</li><li>
How can I query the plug-in interface?
<p>This is the last step before you can use the plug-in. It means that inside
your program you ask the object registry for an object registered with
the desired tag. The easiest way to do this is as follows:
<code>csQueryRegistry<iInterface>(<var>object_reg</var>)</code>.
This macro will search the object registry for the default object
implementing the given interface (i.e. the object registered with the
tag equal to the interface name). Alternatively you can also use
<code>csQueryRegistryTag</code> to get an object with a specific tag.
</p>
</li><li>
Conclusion
<p>To sum it up, <small>SCF</small> is mainly used to provide common interfaces for
<small>DLL</small>s, but it can also be used by statically linked libraries. If you
want to know how to write a plug-in yourself, you should read the complete
<small>SCF</small> documentation. See section <a href="SCF.html#0">Shared Class Facility (<small>SCF</small>)</a>.
</p></li></ul>
<p>For further information about modules and plug-in drivers, see the sections
on using Crystal Space (see section <a href="Using-Crystal-Space.html#0">Using Crystal Space</a>).
</p>
<p>Now that you have learned some basics about the Crystal Space environment,
you can try writing your first program.
</p>
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