Modify the BtServiceNode to include an on_success call. (#2481)

* Modify the BtServiceNode to include an on_success call.

* PR: Fix linter error by removing trailing whitespaces.

* PR: Rename on_success() to on_completion() to improve understandability.

nav2_behavior_tree/include/nav2_behavior_tree/bt_service_node.hpp

@@ -142,6 +142,16 @@ class BtServiceNode : public BT::ActionNodeBase
   {
   }
 
+  /**
+   * @brief Function to perform some user-defined operation upon successful
+   * completion of the service. Could put a value on the blackboard.
+   * @return BT::NodeStatus Returns SUCCESS by default, user may override to return another value
+   */
+  virtual BT::NodeStatus on_completion()
+  {
+    return BT::NodeStatus::SUCCESS;
+  }
+
   /**
    * @brief Check the future and decide the status of BT
    * @return BT::NodeStatus SUCCESS if future complete before timeout, FAILURE otherwise
@@ -158,7 +168,8 @@ class BtServiceNode : public BT::ActionNodeBase
       rc = callback_group_executor_.spin_until_future_complete(future_result_, server_timeout_);
       if (rc == rclcpp::FutureReturnCode::SUCCESS) {
         request_sent_ = false;
-        return BT::NodeStatus::SUCCESS;
+        BT::NodeStatus status = on_completion();
+        return status;
       }
 
       if (rc == rclcpp::FutureReturnCode::TIMEOUT) {