Add loaned message listener. #526
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| // Copyright 2021 Open Source Robotics Foundation, Inc. | ||
| // | ||
| // Licensed under the Apache License, Version 2.0 (the "License"); | ||
| // you may not use this file except in compliance with the License. | ||
| // You may obtain a copy of the License at | ||
| // | ||
| // http://www.apache.org/licenses/LICENSE-2.0 | ||
| // | ||
| // Unless required by applicable law or agreed to in writing, software | ||
| // distributed under the License is distributed on an "AS IS" BASIS, | ||
| // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. | ||
| // See the License for the specific language governing permissions and | ||
| // limitations under the License. | ||
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| #include "rclcpp/rclcpp.hpp" | ||
| #include "rclcpp_components/register_node_macro.hpp" | ||
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| #include "std_msgs/msg/float64.hpp" | ||
| #include "demo_nodes_cpp/visibility_control.h" | ||
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| namespace demo_nodes_cpp | ||
| { | ||
| // Create a Listener class that subclasses the generic rclcpp::Node base class. | ||
| // The main function below will instantiate the class as a ROS node. | ||
| class LoanedMessageListener : public rclcpp::Node | ||
| { | ||
| public: | ||
| DEMO_NODES_CPP_PUBLIC | ||
| explicit LoanedMessageListener(const rclcpp::NodeOptions & options) | ||
| : Node("listener_loaned_message", options) | ||
| { | ||
| // Create a callback function for when messages are received. | ||
| // Variations of this function also exist using, for example UniquePtr for zero-copy transport. | ||
| setvbuf(stdout, NULL, _IONBF, BUFSIZ); | ||
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There was a problem hiding this comment. I know we have this in our other examples, but it actually isn't necessary anymore. Since Foxy, we print out all |
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| auto callback = | ||
| [this](const std_msgs::msg::Float64::SharedPtr msg) -> void | ||
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There was a problem hiding this comment. I think it is import to have a comment that shows that this can only be zero-copy for fixed-sized types. There was a problem hiding this comment. You could print some information about the listener in the constructor of this class and print whether the middleware supports zero-copy. That way we can demonstrate both, a loaned message where the middleware allocates all messages and a zero-copy transport in case the middleware supports it. |
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| { | ||
| RCLCPP_INFO(this->get_logger(), "I heard: [%f]", msg->data); | ||
| }; | ||
| // 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. | ||
| sub_ = create_subscription<std_msgs::msg::Float64>("chatter_pod", 10, callback); | ||
| } | ||
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| private: | ||
| rclcpp::Subscription<std_msgs::msg::Float64>::SharedPtr sub_; | ||
| }; | ||
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| } // namespace demo_nodes_cpp | ||
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| RCLCPP_COMPONENTS_REGISTER_NODE(demo_nodes_cpp::LoanedMessageListener) | ||
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Personally, I would rename this class. The fact that it is a loaned message under the hood isn't super-interesting to the user. What's more interesting is that it is zero-copy, so I'd rename this to
ZeroCopyListeneror whatever.There was a problem hiding this comment.
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I kind of disagree with this. We have the equivalent talker which is called
LoanedMessageTalkerand I believe this pattern makes sense, nonetheless for consistency - we also have a pair ofSerializedMessage[Talker|Listener].The idiom of a loaned message is needed for zero-copy, but you can perfectly fine use a loaned message without the middleware supporting zero-copy.
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OK, that makes sense. I think the thing I'm stuck on here is how the user knows that loaning is happening, and what they should expect different when loaning is used.