Tutorial implement a custom feature (#131)

* Initial draft for implementing a custom feature
* Update tutorial name in front page
* Change headline names for consistency
* Revised some nitpicks
* Add folder structure for feature

Co-authored-by: anindex <an.thai.le97@gmail.com>
Co-authored-by: Claire Wang <22240514+claireyywang@users.noreply.github.com>

tutorials.md.in

@@ -14,6 +14,7 @@ Ignition @IGN_DESIGNATION_CAP@ library and how to use the library effectively.
 5. \subpage pluginloading "Loading a Physics Plugin"
 
 7. \subpage createphysicsplugin "Implement a physics plugin"
+8. \subpage createcustomfeature "Implement a custom feature"
 
 ## License
 

tutorials/08-implementing-a-custom-feature.md

@@ -0,0 +1,302 @@
+\page createcustomfeature "Implement a custom feature"
+
+## Prerequisites
+
+In the previous tutorial \ref installation "Installation", you have installed
+the Ignition Physics corresponding to the desired Ignition release. Note that
+the recommended Ignition release is Dome.
+
+## Implement a custom feature in DART plugin
+
+In the last \ref createphysicsplugin "Implement a physics plugin" tutorial, we
+know how to implement a dummy physics engine as a plugin and load it using
+\ref ignition::physics "Ignition Physics API". In this tutorial, we will look
+deeper into the structure of a physics engine plugin, for example, the available
+[DART](https://github.com/ignitionrobotics/ign-physics/tree/master/dartsim)
+physics engine in `ign-physics` repository and how to define a custom
+\ref ignition::physics::Feature "Feature" for the plugin.
+
+### Folder structure of the plugins
+
+Below is the general structure of the `ign-physics` repository:
+
+```
+ign-physics
+├── dartsim                   Files for dartsim plugin component.
+├── tpe                       Files for tpe plugin component.
+├── include/ignition/physics  Header files.
+├── mesh                      Files for mesh component.
+├── resources                 Model and mesh resource files used by tests.
+├── sdf                       Files for sdf component.
+├── src                       Source files and unit tests.
+├── test
+├── tutorials                 Tutorials, written in markdown.
+├── Changelog.md              Changelog.
+└── CMakeLists.txt            CMake build script.
+```
+
+As shown above, there are two physics engines available (more detail
+in \ref physicsplugin "Physics plugin tutorial"):
+- **DART**: `ignition-physics-dartsim-plugin`.
+- **TPE**: `ignition-physics-tpe-plugin`.
+
+and their plugin folders are placed just below the top level of `ign-physics`.
+
+Looking closer to a plugin folder, for example, the `dartsim` (DART) plugin:
+
+```
+dartsim
+├── worlds                            Example SDF files for testing dartsim plugin functionalities.
+├── src                               Main implementation files of the plugin features interfacing the physics engines API
+├── include/ignition/physics/dartsim  Header files for the plugin features.
+└── CMakeLists.txt                    CMake plugin build script.
+```
+
+Basically, new implementation of \ref ignition::physics::Feature "Feature" or
+\ref ignition::physics::FeatureList "FeatureList", which is corresponded to a
+functionality of the external physics engine can be defined as a header in
+`include/ignition/physics/<plugin_name>` folder. The custom feature could
+be added in a \ref ignition::physics::FeatureList "FeatureList"
+and implemented its functionalities in `src` folder.
+
+The `dartsim` plugin's \ref ignition::physics::FeatureList "FeatureList" could be
+found in \ref physicsplugin "Understanding the Physics Plugin" tutorial.
+
+### Plugin and feature requirements
+
+In general, the minimum set of features that any physics engine plugin must
+implement to be supported by Ignition Gazebo is as below:
+- \ref ignition::physics::FindFreeGroupFeature "FindFreeGroupFeature"
+- \ref ignition::physics::SetFreeGroupWorldPose "SetFreeGroupWorldPose"
+- \ref ignition::physics::FreeGroupFrameSemantics "FreeGroupFrameSemantics"
+- \ref ignition::physics::LinkFrameSemantics "LinkFrameSemantics"
+- \ref ignition::physics::ForwardStep "ForwardStep"
+- \ref ignition::physics::RemoveEntities "RemoveEntities"
+- \ref ignition::physics::sdf::ConstructSdfLink "ConstructSdfLink"
+- \ref ignition::physics::sdf::ConstructSdfModel "ConstructSdfModel"
+- \ref ignition::physics::sdf::ConstructSdfWorld "ConstructSdfWorld"
+
+This list defines the minimum requirements for the simulation capability of a
+physics engine plugin and also maintains backward compatibility with
+downstream physics plugins.
+
+For custom feature requirements, there are two main component classes
+in the general structure of a custom feature:
+- \ref ignition::physics::Entity "Entity" corresponds to the "proxy object" that
+the \ref ignition::physics::Feature "Feature" is implemented. These are the most
+common "proxy objects" that are inherited from `Entity` class:
+  - \ref ignition::physics::Feature::Engine "Engine": Placeholder class for the
+    Engine API. This class serves metadata for the physics engine (for example
+    the \ref ignition::physics::GetEngineInfo "GetEngineInfo" feature).
+    Every Engine feature **must** inherit this class.
+  - \ref ignition::physics::Feature::Joint "Joint": defines physics concept
+    `Joint` behaviors (for example the
+    \ref ignition::physics::GetBasicJointState "GetBasicJointState" feature).
+  - \ref ignition::physics::Feature::Link "Link": defines physics concept `Link`
+    structure.
+  - \ref ignition::physics::Feature::Model "Model": defines physics concept
+    `Model` structure (for example the
+    \ref ignition::physics::GetLinkFromModel "GetLinkFromModel" feature
+    including both `Link` and `Model` objects).
+  - \ref ignition::physics::Feature::Shape "Shape": defines physics concept
+    `Shape` structure (for example the
+    \ref ignition::physics::GetShapeKinematicProperties "GetShapeKinematicProperties"
+    feature).
+  - \ref ignition::physics::Feature::World "World": defines physics concept
+    `Shape` structure (for example
+    the \ref ignition::physics::dartsim::RetrieveWorld "RetrieveWorld" feature
+    in `dartsim` plugin).
+
+  Note that these object classes are not mutually exclusive and could be defined
+  in conjunction together to describe the `Feature`. There are also other
+  uncommon objects defined depending on feature functionality, for example, the
+  \ref ignition::physics::SetFreeGroupWorldPose::FreeGroup "FreeGroup"
+  object in `SetFreeGroupWorldPose` feature. For more information about the
+  physics concepts, please refer to
+  \ref physicsconcepts "Ignition Physics simulation concepts" tutorial.
+- \ref ignition::physics::Feature::Implementation "Implementation" interfaces
+the actual physics engines API for the custom feature. It has
+\ref ignition::physics::Feature::Implementation::InitiateEngine "InitiateEngine"
+to trigger physics engine initiation to provide the required functionalities.
+
+Moreover, we can define dependencies between custom `Features`:
+- By default, a blank feature will not require any other features.
+If the custom feature does require some other set of features,
+then it should be inherited from
+\ref ignition::physics::FeatureWithRequirements "FeatureWithRequirements" class,
+and provided a list of the `Features` required.
+- By default, a blank feature will not conflict with any other features. If
+the custom feature does conflict with some other set of features, then it should
+be inherited from
+\ref ignition::physics::FeatureWithConflicts "FeatureWithConflicts" class,
+and provided a list of the conflicting `Features`. The conflicting `Features`
+will not run at the same time when requested.
+
+### Define the custom feature
+
+With the requirements and restrictions above, we will implement an example
+custom `Feature` that retrieves a simulation world from `dartsim` physics engine.
+For example, we name it as [World.hh](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/include/ignition/physics/dartsim/World.hh)
+and the its content is as follow:
+
+```cpp
+#include <dart/simulation/World.hpp>
+#include <ignition/physics/FeatureList.hh>
+
+namespace ignition {
+namespace physics {
+namespace dartsim {
+
+/////////////////////////////////////////////////
+class RetrieveWorld : public virtual Feature
+{
+  public: template <typename PolicyT, typename FeaturesT>
+  class World : public virtual Feature::World<PolicyT, FeaturesT>
+  {
+    /// \brief Get the underlying dartsim world for this World object.
+    public: dart::simulation::WorldPtr GetDartsimWorld();
+  };
+
+  public: template <typename PolicyT>
+  class Implementation : public virtual Feature::Implementation<PolicyT>
+  {
+    public: virtual dart::simulation::WorldPtr GetDartsimWorld(
+        const Identity &_worldID) = 0;
+  };
+};
+
+/////////////////////////////////////////////////
+template <typename PolicyT, typename FeaturesT>
+dart::simulation::WorldPtr RetrieveWorld::World<PolicyT, FeaturesT>
+::GetDartsimWorld()
+{
+  return this->template Interface<RetrieveWorld>()
+      ->GetDartsimWorld(this->identity);
+}
+
+}
+}
+}
+```
+
+The new defined feature file is placed in `dartsim/include/ignition/physics/dartsim`:
+```
+dartsim
+├── worlds                           
+├── src                       
+├── include/ignition/physics/dartsim
+│    ├──  World.hh
+└── CMakeLists.txt                 
+```
+
+As seen above, after including the necessary library of `dartsim` and `ign-physics`,
+we define the `RetrieveWorld` custom feature inherited from the base
+\ref ignition::physics::Feature "Feature".
+
+As defined, the `RetrieveWorld` feature retrieves
+world pointer from physics engine, so it is natural to define `World` entity inherited
+from \ref ignition::physics::Feature::World "Feature::World" and declare the
+necessary member function `GetDartsimWorld`. Then we define the `Implementation`
+class having virtual member function for overriding in the actual implementation of
+the custom feature `RetrieveWorld` later.
+
+Finally, we implement the `World`
+entity's member function `GetDartsimWorld` to call the `Implementation`
+class's member function `GetDartsimWorld` via
+\ref ignition::physics::Feature::Entity::Interface "Entity::Interface"
+convenience function for querying the feature `Implementation` object.
+
+### Implement the custom feature
+
+After defining the custom feature, please look into where it is added to a
+\ref ignition::physics::FeatureList "FeatureList" in
+[CustomFeatures.hh](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/src/CustomFeatures.hh)
+and implemented in [CustomFeatures.cc](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/src/CustomFeatures.cc).
+These files are place as follows:
+```
+dartsim
+├── worlds                           
+├── src
+│    ├── CustomFeatures.hh
+│    ├── CustomFeatures.cc
+│    ├── ...                           
+├── include/ignition/physics/dartsim  
+└── CMakeLists.txt                 
+```
+
+We display them here for convenience:
+
+- `CustomFeatures.hh`:
+
+```cpp
+#include <ignition/physics/Implements.hh>
+#include <ignition/physics/dartsim/World.hh>
+#include "Base.hh"
+
+namespace ignition {
+namespace physics {
+namespace dartsim {
+
+using CustomFeatureList = FeatureList<
+  RetrieveWorld
+>;
+
+class CustomFeatures :
+    public virtual Base,
+    public virtual Implements3d<CustomFeatureList>
+{
+  public: dart::simulation::WorldPtr GetDartsimWorld(
+      const Identity &_worldID) override;
+};
+
+}
+}
+}
+
+```
+
+The custom feature `RetrieveWorld` is added to `CustomFeatureList`, other custom
+features could also be added here.
+The `CustomFeatures` "FeatureList" here inherits from:
+- [Base](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/src/Base.hh)
+class for foundation definitions of Models, Joints, Links, and Shapes objects
+of `dartsim` interfacing to Ignition Physics API.
+- \ref ignition::physics::Implements3d "Implements3d" for implementing the
+custom feature with \ref ignition::physics::FeaturePolicy3d "FeaturePolicy3d"
+("FeaturePolicy" of 3 dimensions and scalar type `double`).
+
+Based on the `CustomFeatureList`, we will override the corresponding member functions
+declared in each of the custom features. In this case, we have only the custom
+feature `RetrieveWorld` and its corresponding `Implementation::GetDartsimWorld`
+member function.
+
+- `CustomFeatures.cc`:
+
+```cpp
+#include "CustomFeatures.hh"
+
+namespace ignition {
+namespace physics {
+namespace dartsim {
+
+/////////////////////////////////////////////////
+dart::simulation::WorldPtr CustomFeatures::GetDartsimWorld(
+    const Identity &_worldID)
+{
+  return this->worlds.at(_worldID);
+}
+
+}
+}
+}
+```
+
+Here we simply implement the actual behavior of `GetDartsimWorld` to return the
+world pointer from `EntityStorage` object storing world pointers of `dartsim` in
+[Base](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/src/Base.hh) class.
+
+Finally, we add the implemented `CustomFeatures` "FeatureList" together with
+other \ref ignition::physics::FeatureList "FeatureList" to final `DartsimFeatures`
+"FeatureList" as in [dartsim/src/plugin.cc](https://github.com/ignitionrobotics/ign-physics/blob/main/dartsim/src/plugin.cc)
+(please see \ref createphysicsplugin "Implement a physics plugin" for
+registering the plugin to Ignition Physics).