Parameters

This version of SlicerROS2 only supports parameter clients, i.e. retrieving parameters held by other ROS nodes (i.e. ROS processes running along Slicer).

The parameter MRML ROS node is slightly different from the other MRML ROS nodes implemented in SlicerROS2. A ROS parameter is fully identified by the ROS node n that hold the parameter and the parameter name p so we could have an implementation that would require one MRML ROS node for each parameter. In practice, this can lead to way too many MRML nodes. For example, if you have two parameters p1 and p2 held by the same ROS node n1, we would have to create two MRML ROS nodes, n1p1 and n1p2. Since the ROS 2 libraries provide a single message to retrieve all the parameters help by a single node, we decided to require one MRML ROS node per ROS node observed. This MRML ROS node can then observe all the parameters held by the ROS node.

To create a new parameter node, one should use the MRML ROS 2 node method vtkMRMLROS2NodeNode::CreateAndAddParameterNode. This method takes one parameter:

• the name of the ROS node that holds the parameters you wish to retrieve (std::string). This is an actual ROS node, running along Slicer.

Once the vtkMRMLROS2ParameterNode observing the ROS node n1 is added, one needs to specify which parameters to observe using the method vtkMRMLROS2ParameterNode::AddParameter.

Parameter nodes get updated when the ROS 2 node is spun. Users can set their own callback to act on newly received messages using an observer on the MRML ROS parameter node.

ROS supports a limited number of types to encode parameters (ROS 2 parameters). Most types are supported in SlicerROS2 and we provide methods to determine the type at runtime. The recommended steps are:

• check if the parameter exists using IsParameterSet(parameterName)

• get the parameter type using GetParameterType(parameterName)

• use the correct accessor based on the parameter’s type, for example GetParameterAsString(parameterName)

Parameter types and accessors

Type	Accessor
“bool”	GetParameterAsBool
“integer”	GetParameterAsInteger
“double”	GetParameterAsDouble
“string”	GetParameterAsString
“bool_array”	GetParameterAsVectorOfBools
“integer_array”	GetParameterAsVectorOfIntegers
“double_array”	GetParameterAsVectorOfDoubles
“string_array”	GetParameterAsVectorOfStrings

For convenience, we also provide the method vtkMRMLROS2ParameterNode::PrintParameter which will provide a human readable description of the parameter whatever the type is (using std::string).

PythonC++

rosLogic = slicer.util.getModuleLogic('ROS2')
rosNode = rosLogic.GetDefaultROS2Node()
# setup to get parameter robot_description from node state_publisher
parameterNode = rosNode.CreateAndAddParameterNode('state_publisher')
parameterNode.AddParameter('robot_description')
# check if parameter is set, C++ example is more detailed
if parameterNode.IsParameterSet('robot_description'):
  # then check the type
  if parameterNode.GetParameterType('robot_description') == 'string':
    robotDescription = parameterNode.GetParameterAsString('robot_description')

To remove the broadcaster node, use the method vtkMRMLROS2NodeNode::RemoveAndDeleteParameterNode. This method takes one parameter:

• the monitored node name (std::string) i.e. ‘robot_state_publisher’