Update comments in quality_of_service_demo_cpp message_lost_talker and message_lost_listener (#458)

Signed-off-by: Ivan Santiago Paunovic <[email protected]>

Author: ivanpauno

quality_of_service_demo/rclcpp/src/message_lost_listener.cpp

@@ -28,6 +28,7 @@ class MessageLostListener : public rclcpp::Node
   explicit MessageLostListener(const rclcpp::NodeOptions & options)
   : Node("MessageLostListener", options)
   {
+    // Callback that will be used when a message is received.
     auto callback =
       [this](const sensor_msgs::msg::Image::SharedPtr msg) -> void
       {
@@ -38,11 +39,9 @@ class MessageLostListener : public rclcpp::Node
           "I heard an image. Message single trip latency: [%f]",
           diff.seconds());
       };
-    // Create a subscription to the topic which can be matched with one or more compatible ROS
-    // publishers.
-    // Note that not all publishers on the same topic with the same type will be compatible:
-    // they must have compatible Quality of Service policies.
     rclcpp::SubscriptionOptions sub_opts;
+    // Update the subscription options, so an event handler that will report the number of lost
+    // messages is created together with the subscription.
     sub_opts.event_callbacks.message_lost_callback =
       [logger = this->get_logger()](rclcpp::QOSMessageLostInfo & info)
       {
@@ -52,6 +51,8 @@ class MessageLostListener : public rclcpp::Node
             info.total_count_change << " \n>\tTotal number of messages lost: " <<
             info.total_count);
       };
+    // Create the subscription. This will also create an event handler based on the above
+    // subscription options.
     sub_ = create_subscription<sensor_msgs::msg::Image>(
       "message_lost_chatter", 1, callback, sub_opts);
   }

quality_of_service_demo/rclcpp/src/message_lost_talker.cpp

@@ -82,6 +82,7 @@ class MessageLostTalker : public rclcpp::Node
         message_size_ *= 1024uL;
       }
     }
+    // Timer callback
     auto publish_message =
       [this]() -> void
       {
@@ -97,6 +98,7 @@ class MessageLostTalker : public rclcpp::Node
 
         pub_->publish(msg_);
       };
+    // Create a publisher
     pub_ = this->create_publisher<sensor_msgs::msg::Image>("message_lost_chatter", 1);
 
     // Use a timer to schedule periodic message publishing.