Create command subscriber only when any command interface is present.…

… Update docs.

gpio_controllers/doc/userdoc.rst

@@ -23,8 +23,7 @@ This controller uses the `generate_parameter_library <https://github.com/PickNik
 .. generate_parameter_library_details::
   ../src/gpio_command_controller_parameters.yaml
 
-The controller expects at least one GPIO interface and the corresponding command interface names or state interface.
-Command and state interfaces are optional.
+The controller expects at least one GPIO interface and the corresponding command interface names or state interface. However, these Command and State interfaces are optional. The controller behaves as a broadcaster when no Command Interface is present, thereby publishing the configured GPIO state interfaces if set, else the one present in the URDF.
 
 .. note::
 

gpio_controllers/src/gpio_command_controller.cpp

@@ -81,9 +81,12 @@ try
     return CallbackReturn::ERROR;
   }
 
-  gpios_command_subscriber_ = get_node()->create_subscription<CmdType>(
-    "~/commands", rclcpp::SystemDefaultsQoS(),
-    [this](const CmdType::SharedPtr msg) { rt_command_ptr_.writeFromNonRT(msg); });
+  if (!command_interface_types_.empty())
+  {
+    gpios_command_subscriber_ = get_node()->create_subscription<CmdType>(
+      "~/commands", rclcpp::SystemDefaultsQoS(),
+      [this](const CmdType::SharedPtr msg) { rt_command_ptr_.writeFromNonRT(msg); });
+  }
 
   gpio_state_publisher_ =
     get_node()->create_publisher<StateType>("~/gpio_states", rclcpp::SystemDefaultsQoS());