Create System Plugins

In Ignition Gazebo, all systems are loaded as plugins at runtime. To create a system plugin for use in the simulation environment, follow the steps below.

Decide on interfaces to implement

The first step of implementing a system plugin is to determine the subset of available interfaces to implement. Aside from the base System object, there are currently three additional available interfaces:

1. ISystemPreUpdate
   1. Has read-write access to world entities and components.
   2. Executed with simulation time at (t0).
   3. Can be used to modify state before physics runs, for example for applying control signals or performing network synchronization.
2. ISystemUpdate
   1. Has read-write access to world entities and components.
   2. Responsible for propagating time from (t0) to (t0 + dt).
   3. Used for physics simulation step.
3. ISystemPostUpdate
   1. Has read-only access to world entities and components.
   2. Executed with simulation time at (t0 + dt).
   3. Used to read out results at the end of a simulation step to be used for sensor or controller updates.

Systems that are only used to read the current state of the world (sensors, graphics, and rendering) should implement ISystemPostUpdate.

Generally, systems that do not manage physics stepping will not need to implement the ISystemUpdate interface.

Controllers and systems that provide feedback based on the state of the world will need to implement ISystemPostUpdate to read the state at the end of an update frame, as well as ISystemPreUpdate to provide feedback at the beginning of the next frame.

Implement Header

The header should include the System header:

Your System object should inherit from the System object as well as from any interfaces that it provides. It should then implement the corresponding methods from the inherited interfaces.

#include <ignition/gazebo/System.hh>

namespace sample_system
{
  class SampleSystem:
    // This class is a system.
    public ignition::gazebo::System,
    // This class also implements the ISystemPostUpdate interface.
    public ignition::gazebo::ISystemPostUpdate
  {
    public: SampleSystem();

    public: ~SampleSystem() override;

    public: void PostUpdate(const ignition::gazebo::UpdateInfo &_info,
                const ignition::gazebo::EntityComponentManager &_ecm) override;
  };

  class SampleSystem2:
    // This class is a system.
    public ignition::gazebo::System,
    // This class also implements the ISystemPreUpdate, ISystemUpdate,
    // and ISystemPostUpdate interfaces.
    public ignition::gazebo::ISystemPreUpdate,
    public ignition::gazebo::ISystemUpdate,
    public ignition::gazebo::ISystemPostUpdate
  {
    public: SampleSystem2();

    public: ~SampleSystem2() override;

    public: void PreUpdate(const ignition::gazebo::UpdateInfo &_info,
                ignition::gazebo::EntityComponentManager &_ecm) override;

    public: void Update(const ignition::gazebo::UpdateInfo &_info,
                ignition::gazebo::EntityComponentManager &_ecm) override;

    public: void PostUpdate(const ignition::gazebo::UpdateInfo &_info,
                const ignition::gazebo::EntityComponentManager &_ecm) override;
  };
}

Register Plugin

If the library will only contain one plugin:

#include <ignition/plugin/Register.hh>

// Include a line in your source file for each interface implemented.
IGNITION_ADD_PLUGIN(
    sample_system::SampleSystem,
    ignition::gazebo::System,
    sample_system::SampleSystem::ISystemPostUpdate)

If the library will contain multiple plugins, in one implementation do as above, and then for each successive implementation use:

#include <ignition/plugin/RegisterMore.hh>

IGNITION_ADD_PLUGIN(
    sample_system::SampleSystem2,
    ignition::gazebo::System,
    sample_system::SampleSystem2::ISystemPreUpdate,
    sample_system::SampleSystem2::ISystemUpdate,
    sample_system::SampleSystem2::ISystemPostUpdate)

Implement Source

Implement the system class as usual, for example:

using namespace sample_system;

SampleSystem::SampleSystem()
{
}

SampleSystem::~SampleSystem()
{
}

void SampleSystem::PostUpdate(const ignition::gazebo::UpdateInfo &_info,
    const ignition::gazebo::EntityComponentManager &_ecm)
{
  ignmsg << "SampleSystem::PostUpdate" << std::endl;
}

Setup the build

In your CMakeLists.txt add the following

cmake_minimum_required(VERSION 3.10.2 FATAL_ERROR)

find_package(ignition-cmake2 REQUIRED)

project(SampleSystem)

find_package(ignition-plugin1 REQUIRED COMPONENTS register)
set(IGN_PLUGIN_VER ${ignition-plugin1_VERSION_MAJOR})

find_package(ignition-gazebo3 REQUIRED)
add_library(SampleSystem SHARED SampleSystem.cc SampleSystem2.cc)
set_property(TARGET SampleSystem PROPERTY CXX_STANDARD 17)
target_link_libraries(SampleSystem
  PRIVATE ignition-plugin${IGN_PLUGIN_VER}::ignition-plugin${IGN_PLUGIN_VER}
  PRIVATE ignition-gazebo3::ignition-gazebo3)

Loading your plugin

In the SDF file representing your simulation, add the plugin to the world section:

<sdf version="1.6">
  <world name="default">
    ...
    <plugin
      filename="libSampleSystem.so"
      name="sample_system::SampleSystem">
    </plugin>
    <plugin
      filename="libSampleSystem.so"
      name="sample_system::SampleSystem2">
    </plugin>
    ...