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4.3.3 Create Your Own Plugin

Written by Jorrit Tyberghein, jorrit.tyberghein@gmail.com and Eric Sunshine, sunshine@sunshineco.com.

Making a plugin in Crystal Space is not very hard but nevertheless there are still a few issues that are often forgotten. Here in this article we show you how you can write a simple plugin and use it in your application.

Defining your Plugin API

The first thing that you need to do when making a plugin is to define the API for it. The API is what your application is going to use to talk to the plugin. It is the interface to the plugin so it is very important to get this right. In the Crystal Space framework the Shared Class Facility (see section Shared Class Facility (SCF)) is used to define the API. With this facility you create an abstract interface containing only the methods from the API. An abstract class in C++ means that all methods are pure virtual. This means that no implementation is given; only method declarations. The implementation will come later in the code of the plugin.

This concept is completely analogous to the Java interface mechanism. The advantage of using this paradigm is that you have a clear separation between the API and the implementation. This allows one to easily replace an implementation of some API or even provide multiple implementations (for example, the software and OpenGL renderers are two implementations of the same 3D rendering API).

Here is the API definition for our sample plugin:

 
#ifndef __GAME_MYAPI_H__
#define __GAME_MYAPI_H__

#include <csutil/scf.h>
#include <csutil/scf_implementation.h>

class csVector3;

/**
 * This is the API for our plugin. It is recommended
 * that you use better comments than this one in a
 * real situation.
 */
struct iMyApi : public virtual iBase
{
SCF_INTERFACE (iMyApi, 1, 0, 0);
/// Do something.
virtual void DoSomething (int param, const csVector3&) = 0;
/// Get something.
virtual int GetSomething () const = 0;
};

#endif // __GAME_MYAPI_H__

The above text should be put in a header file. Let's put it in `myapi.h'.

First we include `csutil/scf.h'. This is a Crystal Space header for SCF which we need to get the definition of `iBase' and the definition of the SCF_INTERFACE() macro.

Then we declare `csVector3' as a class. We do this so that we can later use `csVector3' as a parameter in one of the API methods. We do not need the complete definition of `csVector3' since we are going to define the method so that it passes the vector by reference.

In the interface declaration we use the SCF_INTERFACE() macro to define the version of this interface. This versioning can be used to query for specific versions of an interface. This can be useful later when you want to extend the API without breaking existing apps. The version has three parts: major, minor, and micro.

Finally we define the API by making a structure that inherits from `iBase'. We use `struct' instead of `class' simply because, for structures, the default visibility is `public' instead of `private' as for classes. This is just a convenience. There is no other difference between a `struct' or a `class' in C++. Note that you have to use virtual inheritance for SCF to work properly.

The name `iMyApi' is not random. Crystal Space uses this naming convention (starting a name with `i') for SCF interfaces so that it is easy to see that they refer to SCF interfaces.

We inherit from `iBase' because it is the basis of all SCF interfaces. All SCF interfaces must inherit from `iBase' either directly or indirectly. This will ensure that we have reference counting (more on that later) and also takes care of the other internal SCF issues.

In that structure we define two methods: DoSomething() and GetSomething(). Note that every method is defined as follows:

 
virtual ... = 0;

The `= 0' means that we will not give an implementation here. The implementation will be provided by the plugin (see later).

Note that it is good practice to use `const' wherever applicable. In the declaration of GetSomething() we added `const' at the end to indicate that this method will not change the object. This is useful for a number of reasons:

Creating the Plugin Implementation (header)

After you defined the API for your plugin it is now time to actually make the plugin implementation. First you define a header called `myplug.h' with the following contents:

 
#ifndef __GAME_MYPLUG_H__
#define __GAME_MYPLUG_H__

#include <iutil/comp.h>
#include <csgeom/vector3.h>
#include <myapi.h>

struct iObjectRegistry;

/**
* This is the implementation for our API and
* also the implementation of the plugin.
*/
class MyPlugin : public scfImplementation2<MyPlugin,iMyApi,iComponent>
{
private:
  iObjectRegistry* object_reg;
  csVector3 store_v;

public:
  MyPlugin (iBase* parent);
  virtual ~MyPlugin ();

  // From iComponent.
  virtual bool Initialize (iObjectRegistry*);

  // From iMyApi.
  virtual void DoSomething (int param, const csVector3&);
  virtual int GetSomething () const;
};

#endif // __GAME_MYPLUG_H__

This requires a little explanation. The Crystal Space plugin framework requires that every named SCF class which will be requested by name from a plugin module via the Crystal Space plugin manager/loader must implement the `iComponent' interface. This interface has a single method, Initialize(), with which the class will be initialized after it is instantiated. This gives the instance a chance to perform various initialization operations and it also provides the instance with a pointer to the global object registry.

But, our plugin also needs to implement its own native `iMyApi' interface. So here is a situation where the same class needs to implement two interfaces at the same time. By using the `scfImplementation2' templated class we can easily declare the class `MyPlugin' to implement both `iComponent' and `iMyApi'.

In the declaration of `MyPlugin' we then have to implement all functions from both `iComponent' and `iMyApi'. To do that the method declarations from both interfaces are repeated here but the `= 0' is removed. This means that we'll actually give a concrete implementation here.

Note that `MyPlugin' needs a constructor that accepts an `iBase*' parameter. Otherwise SCF will not be able to intantiate this class.

Creating the Plugin Implementation (source)

Now we create the main source file containing the implementation of our plugin. Let's call this `myplug.cpp':

 
#include <cssysdef.h>
#include <myplug.h>
#include <iutil/objreg.h>
#include <iutil/plugin.h>

CS_IMPLEMENT_PLUGIN
SCF_IMPLEMENT_FACTORY (MyPlugin)

MyPlugin::MyPlugin (iBase* parent) :
	scfImplementationType (this, parent),
	object_reg(0)
{
}

MyPlugin::~MyPlugin ()
{
}

bool MyPlugin::Initialize (iObjectRegistry* r)
{
  object_reg = r;
  return true;
}

void MyPlugin::DoSomething (int param, const csVector3& v)
{
  // Just some behavior.
  if (param == 1)
    store_v = v;
  else
    store_v = -v;
}

int MyPlugin::GetSomething () const
{
  return (int)store_v.x + (int)store_v.y + (int)store_v.z;
}

The first macro is CS_IMPLEMENT_PLUGIN(). This indicates to the Crystal Space framework that this module will end up as a plugin (as opposed to an application or library). On some platforms this actually makes a difference; on others it does not. For best portability, you should use this macro in exactly one C++ file within each plugin module.

The SCF_IMPLEMENT_FACTORY() says that C++ class `MyPlugin' represents an SCF factory which allows SCF to instantiate objects of this class. In addition to some other administrative tasks, this macro defines a function capable of instantiating an object of class `MyPlugin'. Note that one plugin module can in fact define several distinct named SCF classes. In that case you need multiple SCF_IMPLEMENT_FACTORY() lines; one for each exported SCF class.

In the constructor of `MyPlugin' you must also call the constructor of the templated superclass by using scfImplementationType(). The first parameter to scfImplementationType() is always `this'.

The rest of the plugin is very straightforward. It is important to realize that you should do most initialization of the plugin in the Initialize() function and not in the constructor. The reason for this is that, at construction time, you cannot depend on the entire Crystal Space framework being ready. Also when Initialize() is called you get a pointer to the object registry which is essential for locating other modules and plugins loaded by the Crystal Space framework.

Telling SCF About Your Plugin

SCF discovers plugins automatically and dynamically. It determines which plugin modules implement which SCF classes by consulting meta-information associated with each plugin. The meta-information file for your plugin must have the same basename as your built plugin module, but with extension `.csplugin'. For instance, if the example plugin is built with the name `myplugin.dll' (Windows) or `myplugin.so' (Unix), then the associated meta-information file should be named `myplugin.csplugin'. At build-time, the meta-information may be embedded directly into the plugin module if supported by the platform and if embedding is enabled. If not, then then the `.csplugin' file will be laid down alongside the built plugin module.

The meta-information file is a structured XML-format document, and can contain any information relevant to the plugin module; it is not limited only to SCF information. SCF itself expects to find a node named <scf>, which contains SCF-related information about the plugin module.

The `myplugin.csplugin' meta-information file for our example plugin module might look like this:

 
<?xml version="1.0"?>
<!-- myplugin.csplugin -->
<plugin>
  <scf>
    <classes>
      <class>
        <name>crystalspace.mygame.myplugin</name>
        <implementation>MyPlugin</implementation>
        <description>My Special Game Plugin</description>
        <requires>
          <class>crystalspace.graphics3d.</class>
        </requires>
      </class>
    </classes>
  </scf>
</plugin>

Each named SCF class exported by the plugin should be presented in a <class> node within the <classes> group. Each class has a <name>, which is the SCF name of the class; an <implementation>, which is the name of the C++ class implementing the SCF class; a <description>; and optionally a <requires> node, which lists the other SCF classes upon which this class depends. Any number of classes may appear in the <requires> group. If your plugin depends only upon a certain type of class, rather than a specific SCF class, then you list only the prefix portion of the desired class type, as shown in this example (where we desire any 3D renderer).

Compiling the Plugin

Depending on the development tools that you use, you should refer to one of the HOWTO's on the subject of building an external Crystal Space module.

Loading the Plugin in Your Application

First, include the header defining the API of the plugin:

 
#include <myapi.h>

Do not include the `myplug.h' header file since it is implementation specific and you should not use the implementation of the plugin directly. Doing this invalidates the entire reason to use plugins in the first place.

To load the plugin there are a few possibilities. First, you can load the plugin manually using csLoadPlugin like this:

 
csRef<iPluginManager> plugin_mgr = 
  csQueryRegistry<iPluginManager> (object_reg);
csRef<iMyApi> myapi = csLoadPlugin<iMyApi> (plugin_mgr,
  "crystalspace.mygame.myplugin");        
if (myapi.IsValid())
{
  ...
}

This will get the plugin manager from the object registry. This is the module that is responsible for loading and unloading plugins. The code then uses the plugin manager to load your plugin. Note that this can fail. You should always check the returned value to see if it is valid.

Another way to load the plugin is through RequestPlugins(), which is called at initialization time:

 
if (!csInitializer::RequestPlugins (object_reg,
  CS_REQUEST_VFS,
  CS_REQUEST_SOFTWARE3D,
  CS_REQUEST_ENGINE,
  ...
  CS_REQUEST_PLUGIN("crystalspace.mygame.myplugin", iMyApi),
  CS_REQUEST_END))
{
  ...
}
...

csRef<iMyApi> myapi = csQueryRegistry<iMyApi> (object_reg);

This way has several advantages. First, it allows the user to override your plugin at the command line or in the configuration file (if your program has one). In cases where there are multiple implementations for the same API this can be an important consideration. It is by doing this, for example, that it is possible to switch between software and OpenGL renderers with the command-line `--video=' option, or via configuration file.

Secondly it registers the plugin with the object registry so that it is easier to find your module later. This also allows other plugins to find your plugin by doing a query on the object registry.

Using the Plugin in Your Application

After loading the plugin you can use the plugin simply by calling the methods defined in the API:

 
myapi->DoSomething (1, csVector3 (2, 3, 4));
printf ("%d\n", myapi->GetSomething ());

This should print out 9.


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