Difference between revisions of "Guides/Platform Development/Tutorials/Writing a Plugin"

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== Conclusion ==
 
== Conclusion ==
  
This tutorial should have enabled you to create a new plugin for Sirikata which adds and exposes a new interface implementation to Sirikata. Frequently, after a basic implementation works one of the first needs is to control its behavior with options. See :ref:`platform-tutorial-options` for a tutorial on adding options to your plugin.
+
This tutorial should have enabled you to create a new plugin for Sirikata which adds and exposes a new interface implementation to Sirikata. Frequently, after a basic implementation works one of the first needs is to control its behavior with options. See [[Guides/Platform_Development/Tutorials/Adding_Options|the options tutorial]] for details on using options.

Latest revision as of 18:07, 28 May 2012

Writing a Sirikata Plugin

The primary way to extend Sirikata is to provide a new implementation of an interface in one of the libraries. These implementations generally live in plugins and only the plugins required to run the desired world are loaded. For example, the interface ObjectSegmentation defines the interface for the component that stores the mapping from objects to the servers managing them. However, we could implement it in different ways. Sirikata already has one implementation called LocalObjectSegmentation for spaces which will only ever run on a single server. This implementation is trivial. It also has another, CraqObjectSegmentation, which works in the distributed case by using a separate key-value store based on CRAQ.

Most components in the system can have different implementations. To make these options pluggable and not require all dependencies to build the system, we utilize a plugin-based architecture. This tutorial will explain how to create a new plugin to extend the system. Obviously this approach works for any interface, but this tutorial will focus on a new ObjectSegmentation implementation called TutorialObjectSegmentation.

Skeleton Code

The best way to start a new plugin is to base it on the skeleton plugin for libcore, found in libcore/plugins/skeleton. The rest of this tutorial will be based on this code, so you can start by copying it into place for your new plugin. It contains only a single file, PluginInterface.cpp, since it has no real interface implementation. You will add new files for the implementation later in this tutorial.

Adding the Plugin to the Build System

As with the skeleton code, there are some commands for adding the skeleton plugin to the build system. This is a good starting point, especially since the build system is complex. Translating the skeleton commands, we might end up adding something like this to build/cmake/CMakeLists.txt.

SET(LIBSPACE_PLUGIN_TUTORIAL_DIR ${LIBSPACE_PLUGIN_DIR}/tutorial)
SET(LIBSPACE_PLUGIN_TUTORIAL_SOURCES ${LIBSPACE_PLUGIN_TUTORIAL_DIR}/PluginInterface.cpp)

ADD_PLUGIN_TARGET(space-tutorial
                  SOURCES ${LIBSPACE_PLUGIN_TUTORIAL_SOURCES}
                  TARGET_LDFLAGS ${sirikata_LDFLAGS}
                  LIBRARIES ${SIRIKATA_CORE_LIB} ${SIRIKATA_SPACE_LIB}
                  TARGET_LIBRARIES ${SIRIKATA_CORE_LIB} ${SIRIKATA_SPACE_LIB})
SET(PLUGIN_INSTALL_LIST ${PLUGIN_INSTALL_LIST} space-tutorial)

First, note that we've labelled it appropriately as a space plugin by prefixing everything with LIBSPACE . Second, we've made sure all our variables have our plugin name in them, TUTORIAL . The utility macro ADD_PLUGIN_TARGET takes care of most of the complicated work of setting up a plugin to be built. You specify source files, any linker flags and target libraries, and the macro takes care of the rest. The final line adds the plugin for installation.

If this plugin has a non-standard dependency, then we should only add the plugin if that dependency is found. This would look like this for a dependency called tutdep

FIND_PACKAGE(tutdep)
IF(TUTDEP_FOUND)
  ADD_PLUGIN_TARGET(space-tutorial
    ...
  )
ENDIF(TUTDEP_FOUND)

If necessary, add the tutdep library to the LIBRARIES argument of ADD_PLUGIN_TARGET.

Implementing the Interface

How the interface is implemented obviously depends on the plugin you are writing. Here, we'll present the skeleton for `TutorialObjectSegmentation` , which is enough to explain the basic concepts.

Following the interface for ObjectSegmentation we create a file TutorialObjectSegmentation.hpp with the following contents

#ifndef _SIRIKATA_TUTORIAL_OBJECT_SEGMENTATION_HPP_
#define _SIRIKATA_TUTORIAL_OBJECT_SEGMENTATION_HPP_

#include <sirikata/space/ObjectSegmentation.hpp>

namespace Sirikata {

    class TutorialObjectSegmentation : public ObjectSegmentation {
    public:
     ...
    };

} // namespace Sirikata

#endif //_SIRIKATA_TUTORIAL_OBJECT_SEGMENTATION_HPP_

and a TutorialObjectSegmentation.cpp with the following contents

#include "TutorialObjectSegmentation.hpp"

namespace Sirikata {

    TutorialObjectSegmentation::TutorialObjectSegmentation() {
    }

    ...

} // namespace Sirikata

Recall that we had to specify our source files in the build system. Having added the implementation file, we need to add it to the build, changing our source file list:

SET(LIBSPACE_PLUGIN_TUTORIAL_SOURCES
      ${LIBSPACE_PLUGIN_TUTORIAL_DIR}/PluginInterface.cpp
      ${LIBSPACE_PLUGIN_TUTORIAL_DIR}/TutorialObjectSegmentation.cpp
   )

Exposing the Implemented Interface from the Plugin

The previous steps got the implementation into the dynamic library, but it isn't exposed to the rest of the system. Sirikata enables you to expose it as an option via a Factory. Usually, any interface which could be instantiated from a plugin has a Factory associated with it. The factory allows implementations to register a constructor function which can be invoked to create a new instance, and names that constructor with a string. In this way, the library, and its users, only needs to know about the generic interface, the factory, and what strings can be used to identify implementations.

Back in the PluginInterface.cpp we add (or replace) the following code:

namespace Sirikata {

static ObjectSegmentation* createTutorialOSeg(
   SpaceContext* ctx, Network::IOStrand* oseg_strand,
   CoordinateSegmentation* cseg, OSegCache* cache,
   const String& args)
{
    return new TutorialObjectSegmentation(ctx, oseg_strand, cseg, cache);
}

} // namespace Sirikata

SIRIKATA_PLUGIN_EXPORT_C void init() {
    using namespace Sirikata;
    if (space_tutorial_plugin_refcount==0) {
        using std::tr1::placeholders::_1;
        using std::tr1::placeholders::_2;
        using std::tr1::placeholders::_3;
        using std::tr1::placeholders::_4;
        OSegFactory::getSingleton()
            .registerConstructor("tutorial",
                std::tr1::bind(&createTutorialOSeg, _1, _2, _3, _4));
    }
    space_tutorial_plugin_refcount++;
}

SIRIKATA_PLUGIN_EXPORT_C void destroy() {
   using namespace Sirikata;
    if (space_tutorial_plugin_refcount==0) {
        OSegFactory::getSingleton().unregisterConstructor("tutorial");
    }
}

This registers our constructor (indirectly via a wrapper function) to the factory when the plugin is initialized, and unregisters it when the plugin is destroyed. In this case, you can ignore the parameters to the constructor --they are just the parameters all ObjectSegmentation implementations must take. While the createTutorialOSeg function must match this signature, the constructor could take additional parameters. In fact, a common pattern is to parse arguments from the last parameter and pass them directly into the TutorialObjectSegmentation constructor so the parsing of options is isolated from the implementation.

If you aren't familiar with it, you probably want to learn about boost::bind. It is a generalization of function pointers which is type-safe and allows you to curry arguments. In this case, it is used to turn createTutorialOSeg into a type-safe "constructor" which returns an ObjectSegmentation*.

That's all that's required -- no modifications to the core code and no need to export symbols from the plugin.

Conclusion

This tutorial should have enabled you to create a new plugin for Sirikata which adds and exposes a new interface implementation to Sirikata. Frequently, after a basic implementation works one of the first needs is to control its behavior with options. See the options tutorial for details on using options.