Add type adaption example (#300)

* Add type masquerading examples

Signed-off-by: Audrow Nash <[email protected]>

* Update examples

Signed-off-by: Audrow Nash <[email protected]>

* Apply feedback

Signed-off-by: Audrow Nash <[email protected]>

* Update publisher's comment

Signed-off-by: Audrow Nash <[email protected]>

Author: audrow

rclcpp/topics/minimal_publisher/CMakeLists.txt

@@ -20,6 +20,9 @@ ament_target_dependencies(publisher_lambda rclcpp std_msgs)
 add_executable(publisher_member_function member_function.cpp)
 ament_target_dependencies(publisher_member_function rclcpp std_msgs)
 
+add_executable(publisher_member_function_with_type_adapter member_function_with_type_adapter.cpp)
+ament_target_dependencies(publisher_member_function_with_type_adapter rclcpp std_msgs)
+
 add_executable(publisher_member_function_with_unique_network_flow_endpoints member_function_with_unique_network_flow_endpoints.cpp)
 ament_target_dependencies(publisher_member_function_with_unique_network_flow_endpoints rclcpp std_msgs)
 
@@ -29,6 +32,7 @@ ament_target_dependencies(publisher_not_composable rclcpp std_msgs)
 install(TARGETS
   publisher_lambda
   publisher_member_function
+  publisher_member_function_with_type_adapter
   publisher_member_function_with_unique_network_flow_endpoints
   publisher_not_composable
   DESTINATION lib/${PROJECT_NAME}

rclcpp/topics/minimal_publisher/member_function_with_type_adapter.cpp

@@ -0,0 +1,94 @@
+// 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.
+
+#include <chrono>
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "rclcpp/type_adapter.hpp"
+#include "rclcpp/rclcpp.hpp"
+
+#include "std_msgs/msg/string.hpp"
+
+using namespace std::chrono_literals;
+
+/* Normally a TypeAdapter specialization like this would go in a header
+ * and be reused by the publisher and subscriber rather than copy-pasted
+ * like this. We chose to include this here because it makes this example
+ * more "self-contained". */
+
+template<>
+struct rclcpp::TypeAdapter<std::string, std_msgs::msg::String>
+{
+  using is_specialized = std::true_type;
+  using custom_type = std::string;
+  using ros_message_type = std_msgs::msg::String;
+
+  static
+  void
+  convert_to_ros_message(
+    const custom_type & source,
+    ros_message_type & destination)
+  {
+    destination.data = source;
+  }
+
+  static
+  void
+  convert_to_custom(
+    const ros_message_type & source,
+    custom_type & destination)
+  {
+    destination = source.data;
+  }
+};
+
+/* In this example, a publisher uses a type adapter to use a `std::string`
+ * in place of a `std_msgs::msg::String` in the argument expected by
+ * the publish method. Note that publish will also work with a
+ * `std_msgs::msg::String` argument. */
+
+class MinimalPublisher : public rclcpp::Node
+{
+  using MyAdaptedType = rclcpp::TypeAdapter<std::string, std_msgs::msg::String>;
+
+public:
+  MinimalPublisher()
+  : Node("minimal_publisher"), count_(0)
+  {
+    publisher_ = this->create_publisher<MyAdaptedType>("topic", 10);
+    timer_ = this->create_wall_timer(
+      500ms, std::bind(&MinimalPublisher::timer_callback, this));
+  }
+
+private:
+  void timer_callback()
+  {
+    std::string message = "Hello, world! " + std::to_string(count_++);
+    RCLCPP_INFO(this->get_logger(), "Publishing: '%s'", message.c_str());
+    publisher_->publish(message);
+  }
+  rclcpp::TimerBase::SharedPtr timer_;
+  rclcpp::Publisher<MyAdaptedType>::SharedPtr publisher_;
+  size_t count_;
+};
+
+int main(int argc, char * argv[])
+{
+  rclcpp::init(argc, argv);
+  rclcpp::spin(std::make_shared<MinimalPublisher>());
+  rclcpp::shutdown();
+  return 0;
+}

rclcpp/topics/minimal_subscriber/CMakeLists.txt

@@ -23,6 +23,9 @@ ament_target_dependencies(subscriber_member_function rclcpp std_msgs)
 add_executable(subscriber_member_function_with_topic_statistics member_function_with_topic_statistics.cpp)
 ament_target_dependencies(subscriber_member_function_with_topic_statistics rclcpp std_msgs)
 
+add_executable(subscriber_member_function_with_type_adapter member_function_with_type_adapter.cpp)
+ament_target_dependencies(subscriber_member_function_with_type_adapter rclcpp std_msgs)
+
 add_executable(subscriber_member_function_with_unique_network_flow_endpoints member_function_with_unique_network_flow_endpoints.cpp)
 ament_target_dependencies(subscriber_member_function_with_unique_network_flow_endpoints rclcpp std_msgs)
 
@@ -33,6 +36,7 @@ install(TARGETS
   subscriber_lambda
   subscriber_member_function
   subscriber_member_function_with_topic_statistics
+  subscriber_member_function_with_type_adapter
   subscriber_member_function_with_unique_network_flow_endpoints
   subscriber_not_composable
   DESTINATION lib/${PROJECT_NAME})

rclcpp/topics/minimal_subscriber/member_function_with_type_adapter.cpp

@@ -0,0 +1,86 @@
+// 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.
+
+#include <functional>
+#include <memory>
+#include <string>
+
+#include "rclcpp/type_adapter.hpp"
+#include "rclcpp/rclcpp.hpp"
+
+#include "std_msgs/msg/string.hpp"
+
+using std::placeholders::_1;
+
+/* Normally a TypeAdapter specialization like this would go in a header
+ * and be reused by the publisher and subscriber rather than copy-pasted
+ * like this. We chose to include this here because it makes this example
+ * more "self-contained". */
+
+template<>
+struct rclcpp::TypeAdapter<std::string, std_msgs::msg::String>
+{
+  using is_specialized = std::true_type;
+  using custom_type = std::string;
+  using ros_message_type = std_msgs::msg::String;
+
+  static
+  void
+  convert_to_ros_message(
+    const custom_type & source,
+    ros_message_type & destination)
+  {
+    destination.data = source;
+  }
+
+  static
+  void
+  convert_to_custom(
+    const ros_message_type & source,
+    custom_type & destination)
+  {
+    destination = source.data;
+  }
+};
+
+/* In this example, a subscriber uses a type adapter to use a `std::string`
+ * in place of a `std_msgs::msg::String` in the subscription's callback. */
+
+class MinimalSubscriber : public rclcpp::Node
+{
+  using MyAdaptedType = rclcpp::TypeAdapter<std::string, std_msgs::msg::String>;
+
+public:
+  MinimalSubscriber()
+  : Node("minimal_subscriber")
+  {
+    subscription_ = this->create_subscription<MyAdaptedType>(
+      "topic", 10, std::bind(&MinimalSubscriber::topic_callback, this, _1));
+  }
+
+private:
+  void topic_callback(const std::string & msg) const
+  {
+    RCLCPP_INFO(this->get_logger(), "I heard: '%s'", msg.c_str());
+  }
+  rclcpp::Subscription<MyAdaptedType>::SharedPtr subscription_;
+};
+
+int main(int argc, char * argv[])
+{
+  rclcpp::init(argc, argv);
+  rclcpp::spin(std::make_shared<MinimalSubscriber>());
+  rclcpp::shutdown();
+  return 0;
+}