Added basic hook for parameter changes (#244)

* Added basic hook for parameter changes

* Rename hook and add docblock

rclcpp/include/rclcpp/node.hpp

@@ -353,6 +353,14 @@ public:
   size_t
   count_graph_users();
 
+  /// Register the callback for parameter changes
+  /**
+   * \param[in] User defined callback function, It is expected to atomically set parameters.
+   * \note Repeated invocations of this function will overwrite previous callbacks
+   */
+  template<typename CallbackT>
+  void register_param_change_callback(CallbackT && callback);
+
   std::atomic_bool has_executor;
 
 private:
@@ -400,6 +408,10 @@ private:
   /* graph_users_count_ is atomic so that it can be accessed without acquiring the graph_mutex_ */
   std::atomic_size_t graph_users_count_;
 
+  std::function<typename rcl_interfaces::msg::SetParametersResult(
+    const typename std::vector<rclcpp::parameter::ParameterVariant> &
+  )> parameters_callback_ = nullptr;
+
   std::map<std::string, rclcpp::parameter::ParameterVariant> parameters_;
 
   publisher::Publisher<rcl_interfaces::msg::ParameterEvent>::SharedPtr events_publisher_;

rclcpp/include/rclcpp/node_impl.hpp

@@ -362,6 +362,12 @@ Node::create_service(
   return serv;
 }
 
+template<typename CallbackT>
+void Node::register_param_change_callback(CallbackT && callback)
+{
+  this->parameters_callback_ = callback;
+}
+
 }  // namespace node
 }  // namespace rclcpp
 

rclcpp/src/rclcpp/node.cpp

@@ -189,6 +189,18 @@ Node::set_parameters_atomically(
   std::map<std::string, rclcpp::parameter::ParameterVariant> tmp_map;
   auto parameter_event = std::make_shared<rcl_interfaces::msg::ParameterEvent>();
 
+  // TODO(jacquelinekay): handle parameter constraints
+  rcl_interfaces::msg::SetParametersResult result;
+  if (parameters_callback_) {
+    result = parameters_callback_(parameters);
+  } else {
+    result.successful = true;
+  }
+
+  if (!result.successful) {
+    return result;
+  }
+
   for (auto p : parameters) {
     if (parameters_.find(p.get_name()) == parameters_.end()) {
       if (p.get_type() != rclcpp::parameter::ParameterType::PARAMETER_NOT_SET) {
@@ -206,10 +218,6 @@ Node::set_parameters_atomically(
   tmp_map.insert(parameters_.begin(), parameters_.end());
   std::swap(tmp_map, parameters_);
 
-  // TODO(jacquelinekay): handle parameter constraints
-  rcl_interfaces::msg::SetParametersResult result;
-  result.successful = true;
-
   events_publisher_->publish(parameter_event);
 
   return result;