Migration from Gazebo Classic: Plugins

Gazebo Classic supports 6 different C++ plugin types, each providing access to different parts of the API, like physics, rendering, sensors, GUI, etc. Due to Ignition Gazebo's architecture based on an ECS , plugin interfaces are somewhat different, but more varied and in many cases much more powerful. Some plugins in Ignition are systems within Ignition Gazebo, while others are specific plugin types from other Ignition libraries.

Note

Plugin types other than systems may be added to Ignition Gazebo in the future.

For example, plugins which get and set properties of simulation entities would be Ignition Gazebo systems. On the other hand, there are now plugin interfaces which didn't exist in Gazebo Classic, such as integrating a new physics or rendering engine, and these can be also used in projects outside of Ignition Gazebo.

Take a look at the comparison below:

---

Classic plugin	Features	Ignition equivalent	Differences
World	Get/set properties of the world and its children	Gazebo system	Get/set components.
Model	Get/set properties of the model and its children	Gazebo system	Get/set components.
Visual	Get/set properties of the visual and its children	Gazebo system	Get/set components.
Sensor	Get/set sensor properties and readings	Gazebo system	Get/set components.
World / Model	Access physics-engine-specific features	Physics plugin	Loaded by ign-physics
Visual	Access rendering-engine-specific features	Rendering plugin	Loaded by ign-rendering
Sensor	Connect to callbacks	Standalone program	Subscribe to Ignition Transport messages.
All	Connect to simulation events	Gazebo system	Use PreUpdate, Update and PostUpdate callbacks for the update loop, and the event manager for other events.
GUI	Add an overlay UI	GUI plugin	Support for overlays and docked widgets
GUI / System	Change the default UI	GUI plugin / SDF	All GUI elements can be removed or configured through SDF
System	Access command line arguments	TBD	-

---

Another key difference is that systems will be able to access all entity properties at once, despite the entity type. So while in Gazebo Classic you may need 3 different plugins to interact with physics, rendering and sensors, on Ignition you could, for example, set a camera's visual color, its velocity and frame rate, all from a single plugin.

Plugin interfaces

Let's take a look at a typical Gazebo Classic plugin which accesses a property from an entity and does something with it. In this case, it will print the current pose of a link.

In general, the plugin will need to:

1. Get the link and store it.
2. Register a callback that is called at every simulation update.
3. At the callback, query the link's pose and print it.

On classic Gazebo, that would look something like this:

#include <gazebo/common/Plugin.hh>

// Inherit from ModelPlugin
class MyPlugin : public ModelPlugin
{
  // Implement Load callback, provided by ModelPlugin
  // and called once at startup.
  virtual void MyPlugin::Load(physics::ModelPtr _model, sdf::ElementPtr _sdf)
  {
    // Read property from SDF
    auto linkName = _sdf->Get<std::string>("link_name");

    // Store link pointer
    this->link = _model->GetLink(linkName);

    // Register callback to be called at every iteration
    this->updateConnection = event::Events::ConnectWorldUpdateBegin(
            std::bind(&MyPlugin::OnUpdate, this));
  }

  // Custom callback called at every iteration
  void MyPlugin::OnUpdate()
  {
    // Get link pose and print it
    std::cout << this->link->WorldPose() << std::endl;
  }

  // Must keep the connection
  private: event::ConnectionPtr updateConnection;

  // Keep pointer to link
  private: physics::LinkPtr link;
};

// Register plugin
GZ_REGISTER_MODEL_PLUGIN(MyPlugin)

On Ignition Gazebo, that would be implemented as follows:

#include <ignition/gazebo/Model.hh>
#include <ignition/gazebo/Util.hh>
#include <ignition/gazebo/System.hh>
#include <ignition/plugin/Register.hh>

using namespace ignition;
using namespace gazebo;
using namespace systems;

// Inherit from System and 2 extra interfaces:
// ISystemConfigure and ISystemPostUpdate
class MyPlugin
      : public System,
        public ISystemConfigure,
        public ISystemPostUpdate
{
  // Implement Configure callback, provided by ISystemConfigure
  // and called once at startup.
  virtual void Configure(const Entity &_entity,
                         const std::shared_ptr<const sdf::Element> &_sdf,
                         EntityComponentManager &_ecm,
                         EventManager &/*_eventMgr*/) override
  {
    // Read property from SDF
    auto linkName = _sdf->Get<std::string>("link_name");

    // Create model object to access convenient functions
    auto model = Model(_entity);

    // Get link entity
    this->linkEntity = model.LinkByName(_ecm, linkName);
  }

  // Implement PostUpdate callback, provided by ISystemPostUpdate
  // and called at every iteration, after physics is done
  virtual void PostUpdate(const UpdateInfo &/*_info*/,
                          const EntityComponentManager &_ecm) override
  {
    // Get link pose and print it
    std::cout << worldPose(this->linkEntity, _ecm) << std::endl;
  }

  // ID of link entity
  private: Entity linkEntity;
};

// Register plugin
IGNITION_ADD_PLUGIN(MyPlugin,
                    ignition::gazebo::System,
                    MyPlugin::ISystemConfigure,
                    MyPlugin::ISystemPostUpdate)

// Add plugin alias so that we can refer to the plugin without the version
// namespace
IGNITION_ADD_PLUGIN_ALIAS(MyPlugin, "ignition::gazebo::systems::MyPlugin")

The example above uses headers like Model.hh and Util.hh, which offer convenient APIs for some tasks that are common during simulation. However, the real power of the ECS architecture is the direct access to all entities and components. Let's take a look at how to do the same just using the ECM's API:

#include <ignition/gazebo/EntityComponentManager.hh>
#include <ignition/gazebo/System.hh>
#include <ignition/gazebo/components/Link.hh>
#include <ignition/gazebo/components/Name.hh>
#include <ignition/gazebo/components/ParentEntity.hh>
#include <ignition/gazebo/components/Pose.hh>
#include <ignition/plugin/Register.hh>

using namespace ignition;
using namespace gazebo;
using namespace systems;

// Inherit from System and 2 extra interfaces:
// ISystemConfigure and ISystemPostUpdate
class MyPlugin
      : public System,
        public ISystemConfigure,
        public ISystemPostUpdate
{
  virtual void Configure(const Entity &_entity,
                         const std::shared_ptr<const sdf::Element> &_sdf,
                         EntityComponentManager &_ecm,
                         EventManager &/*_eventMgr*/) override
  {
    // Read property from SDF
    auto linkName = _sdf->Get<std::string>("link_name");

    // Get link entity by querying for an entity that has a specific set of
    // components
    this->linkEntity = _ecm.EntityByComponents(
        components::ParentEntity(_entity),
        components::Name(linkName), components::Link());
  }

  virtual void PostUpdate(const UpdateInfo &/*_info*/,
                          const EntityComponentManager &_ecm) override
  {
    // Get link's local pose
    auto pose = _ecm.Component<components::Pose>(this->linkEntity)->Data();

    // Get link's parent entity
    auto parent = _ecm.Component<components::ParentEntity>(this->linkEntity);

    // Iterate over parents until world is reached
    while (parent)
    {
      // Get parent entity's pose
      auto parentPose = _ecm.Component<components::Pose>(parent->Data());
      if (!parentPose)
        break;

      // Add pose
      pose = parentPose->Data() * pose;

      // keep going up the tree
      parent = _ecm.Component<components::ParentEntity>(parent->Data());
    }

    std::cout << pose << std::endl;
  }

  // ID of link entity
  private: Entity linkEntity;
};

IGNITION_ADD_PLUGIN(MyPlugin,
                    ignition::gazebo::System,
                    MyPlugin::ISystemConfigure,
                    MyPlugin::ISystemPostUpdate)

IGNITION_ADD_PLUGIN_ALIAS(MyPlugin, "ignition::gazebo::systems::MyPlugin")

In summary, the key differences between Gazebo Classic and Ignition Gazebo are:

• Plugins must inherit from the ISystemConfigure class to be able to override the Configure callback that gives access to many things, such as the SDF pointer. This used to be done through the Load callback.
• Plugins no longer need to manage connections to loop-related events. Instead, they implement an interface such as ISystemPostUpdate.
• Plugins don't have direct access to physics objects such as physics::Model. Instead, they can either deal directly with entities and their components by calling functions in the ECM, or using convenient objects such as ignition::gazebo::Model which wrap the ECM interface.

All these changes are meant to give plugin developers more flexibility to only use the features they need, and several layers of abstraction which can be chosen according to the developer's experience and specific use-case.