A SOFA component is a class or a class template that inherits, directly or not, from sofa::core::objectmodel::BaseObject. A component must also respect some conventions:

• the declaration must begin with the SOFA_CLASS macro. It will expand to some code required by SOFA, that will enable some kind of reflection mechanisms;
• a component must be registered in the ObjectFactory, using the RegisterObject mechanism (see example below), otherwise SOFA won’t be aware of it.

Here is a minimal example, for a component that does nothing:

#include <sofa/core/objectmodel/BaseObject.h>

class MyComponent : public sofa::core::objectmodel::BaseObject
{
public:
SOFA_CLASS(MyComponent, sofa::core::objectmodel::BaseObject);

    MyComponent ();
    virtual ~MyComponent ();
};

#include "MyComponent.h"
#include <sofa/core/ObjectFactory.h>

MyComponent::MyComponent()
{
}

MyComponent::~MyComponent()
{
}

int MyComponentClass = sofa::core::RegisterObject("This component does nothing.").add<MyComponent>();

Once the plugin is compiled, this component can be used in an XML scene file. You should also add a RequiredPlugin element in the root node in order to load MyPlugin when the scene is loaded.

<?xml version="1.0"?>
<Node name="Root">
    <RequiredPlugin pluginName="MyPlugin"/>
    <MyComponent>
</Node>

This is a basic example; most components don’t inherit directly from BaseObject, but from a subclass which represent a specific aspect of a simulation. For example, force field components inherit from sofa::core::BaseForcefield, or one of its subclasses.

Add Data to your component

Almost every component has parameters, inputs, or outputs: they are referred to as Data; in a scene file, those are the XML attributes of the component. Each XML attribute of a component is actually a member of the class, declared in a special way: it is encapsulated in a Data. For example, in order to add a float Data named myparam to our component, we will add this member to MyComponent:

Data<float> m_myparam;

Then we must register and initialise it in each Constructor of MyComponent:

MyComponent::MyComponent(): d_myparam(initData(&d_myparam, 0.42, "myparam", "Here should be a short description of myparam."))
{
}

This allows us to use this Data in scene files; it can be assigned a value like so:

<?xml version="1.0"?>
<Node name="Root">
    <RequiredPlugin pluginName="MyPlugin"/>
    <MyComponent myparam="3.7">
</Node>