Tutorial implement a physics plugin

tutorials.md.in

@@ -12,6 +12,9 @@ Ignition @IGN_DESIGNATION_CAP@ library and how to use the library effectively.
 3. \subpage physicsplugin "Understanding the Physics Plugin"
 4. \subpage switchphysicsengines "Switching physics engines"
 
+
+7. \subpage createphysicsplugin "Implement a physics plugin"
+
 ## License
 
 The code associated with this documentation is licensed under an [Apache 2.0 License](https://www.apache.org/licenses/LICENSE-2.0).

tutorials/07-implementing-a-physics-plugin.md

@@ -0,0 +1,174 @@
+\page createphysicsplugin "Implement a physics plugin"
+
+This tutorial guides how to develop a simple plugin that implements a 3D policy
+and includes a sample list of Features.
+
+## 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.
+
+## Write a simple physics plugin
+
+Please create a folder for the plugin first:
+
+```bash
+cd ~
+mkdir -p ~/simple_plugin/build
+cd simple_plugin
+```
+
+Then download the example loader into your current directory by:
+
+```bash
+wget https://raw.githubusercontent.com/ignitionrobotics/ign-physics/main/examples/hello_world_plugin/HelloWorldPlugin.cc
+```
+
+### Examine the code
+
+We first include these:
+- \ref ignition::physics::FeatureList "FeatureList" header: to define a list of
+\ref ignition::physics::Feature "Feature" that will be implemented for the
+plugin, e.g. \ref ignition::physics::GetEngineInfo "GetEngineInfo".
+- \ref ignition::physics::FeaturePolicy "FeaturePolicy" header: to specify the
+metadata about the coordinate system (e.g. 2 or 3 dimensions) and numeric system
+(e.g. float, int) that the `Features` will be implemented accordingly.
+- `GetEntities.hh` header: to retrieve the pre-defined list of features in
+Ignition Physics. Please refer to this [API Reference](https://ignitionrobotics.org/api/physics/3.0/GetEntities_8hh.html)
+for a list of features defined in `GetEntities.hh` and \ref physicsplugin
+"Understanding the Physics Plugin" tutorial for an example list of common features
+implemented with a plugin.
+- `Register.hh` header: to register a plugin to Ignition Physics.
+
+```cpp
+#include <ignition/physics/FeatureList.hh>
+#include <ignition/physics/FeaturePolicy.hh>
+#include <ignition/physics/GetEntities.hh>
+#include <ignition/physics/Register.hh>
+```
+
+Next, let us define a namespace `mock` and implement our plugin in it.
+We will add a pre-defined feature `GetEngineInfo` to our plugin's
+`HelloWorldFeatureList` as follow:
+
+```cpp
+struct HelloWorldFeatureList : ignition::physics::FeatureList<
+    ignition::physics::GetEngineInfo
+> { };
+```
+
+The plugin will be able to return its physics engine metadata.
+We will now implement our plugin class named `HelloWorldPlugin`
+using the defined `FeatureList` above. The class is inherited from
+\ref ignition::physics::Implements3d "Implements3d" to declare that our plugin's
+`HelloWorldFeatureList` will be in the 3D coordinate system.
+
+```cpp
+class HelloWorldPlugin
+    : public ignition::physics::Implements3d<HelloWorldFeatureList>
+{
+  using Identity = ignition::physics::Identity;
+
+  public: Identity InitiateEngine(std::size_t /*_engineID*/) override
+  {
+    this->engineName = "HelloWorld";
+
+    return this->GenerateIdentity(0);
+  }
+
+  public: std::size_t GetEngineIndex(const Identity &/*_id*/) const override
+  {
+    return 0;
+  }
+
+  public: const std::string &GetEngineName(const Identity &/*_id*/) const override
+  {
+    return this->engineName;
+  }
+
+  std::string engineName;
+};
+```
+
+For now, because we do not have any real physics engines, we will define a dummy
+physics engines inside member function `InitiateEngine` by simply setting
+```cpp
+this->engineName = "HelloWorld"
+```
+and returning the engine object using \ref ignition::physics::Identity. Then, we
+define the metadata getters `GetEngineIndex` and `GetEngineName` for our plugin
+and that's it.
+
+Finally, we only have to register our plugin in Ignition Physics as a physics
+engine by:
+
+```cpp
+IGN_PHYSICS_ADD_PLUGIN(
+    HelloWorldPlugin,
+    ignition::physics::FeaturePolicy3d,
+    HelloWorldFeatureList)
+```
+Note that:
+- The first argument is the name of the class that wraps the physics engine into
+a plugin.
+- The second argument is the `FeaturePolicy` for this plugin, i.e. `FeaturePolicy3d`
+- The third argument is the `FeatureList`, specifying all the features that this
+plugin provides, i.e. `HelloWorldFeatureList`
+
+### Setup CMakeLists.txt for building (Dome release)
+
+Now create a file named `CMakeLists.txt` with your favorite editor and add these
+lines for finding `ign-plugin` and `ign-physics` dependencies for Dome release:
+
+```cmake
+cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
+set(IGN_PLUGIN_VER 1)
+find_package(ignition-plugin${IGN_PLUGIN_VER} 1.1 REQUIRED COMPONENTS all)
+set(IGN_PHYSICS_VER 3)
+find_package(ignition-physics${IGN_PHYSICS_VER} REQUIRED)
+```
+
+After that, add the executable pointing to our plugin file and add linking
+library so that `cmake` can compile it:
+
+```cmake
+add_library(HelloWorldPlugin SHARED HelloWorldPlugin.cc)
+target_link_libraries(HelloWorldPlugin
+  PRIVATE
+    ignition-physics${IGN_PHYSICS_VER}::ignition-physics${IGN_PHYSICS_VER})
+```
+
+## Build and run
+
+### Compile the plugin
+
+Now you can build the plugin by:
+
+```bash
+cd build
+cmake ..
+make
+```
+
+This will generate the `HelloWorldPlugin` library under `build`.
+The exact name of the library file depends on the operating system
+such as `libHelloWorldPlugin.so` on Linux, `libHelloWorldPlugin.dylib` on macOS,
+and `HelloWorldPlugin.dll` on Windows.
+
+### Test loading the plugin on Linux
+
+Please first follow the \ref pluginloading "Loading a Physics Plugin" tutorial
+to create a simple loader. Then we test our plugin using the loader as follow:
+
+```bash
+cd ~
+./simple_loader/build/hello_world_loader simple_plugin/build/libHelloWorldPlugin.so
+```
+
+And you will see the engine info of our plugin:
+
+```bash
+Testing plugin: mock::HelloWorldPlugin
+  engine name: HelloWorld
+```