How to write a custom streamer plugin

This tutorial will guide you through the steps to create a simple custom streamer plugin for the web_video_server package in ROS 2. The example plugin will log messages when it is created, started, and when frames are restreamed.

Create you local workspace if you don’t have one:
```
mkdir -p ~/ros_ws/src
cd ~/ros_ws/src
```

Create a new package for your custom streamer plugin:

ros2 pkg create --build-type ament_cmake test_streamer_plugin --dependencies web_video_server pluginlib --library-name test_streamer_plugin
cd test_streamer_plugin

Add TestStreamer and TestStreamerFactory classes to include/test_streamer_plugin/test_streamer_plugin.hpp header file:

#ifndef TEST_STREAMER_PLUGIN__TEST_STREAMER_PLUGIN_HPP_
#define TEST_STREAMER_PLUGIN__TEST_STREAMER_PLUGIN_HPP_

#include "test_streamer_plugin/visibility_control.h"

#include "web_video_server/streamer.hpp"

namespace test_streamer_plugin
{

class TestStreamer : public web_video_server::StreamerBase
{
public:
  TestStreamer(
    const async_web_server_cpp::HttpRequest & request,
    async_web_server_cpp::HttpConnectionPtr connection,
    rclcpp::Node::WeakPtr node);

  virtual ~TestStreamer();

  void start() override;
  void restream_frame(std::chrono::duration<double> max_age) override;
};

class TestStreamerFactory : public web_video_server::StreamerFactoryInterface
{
public:
  std::string get_type() override {return "test";}

  std::shared_ptr<web_video_server::StreamerInterface> create_streamer(
    const async_web_server_cpp::HttpRequest & request,
    async_web_server_cpp::HttpConnectionPtr connection,
    rclcpp::Node::WeakPtr node) override;
};

}  // namespace test_streamer_plugin

#endif  // TEST_STREAMER_PLUGIN__TEST_STREAMER_PLUGIN_HPP_

Implement the TestStreamer and TestStreamerFactory classes in src/test_streamer_plugin.cpp:

#include "test_streamer_plugin/test_streamer_plugin.hpp"

namespace test_streamer_plugin
{

TestStreamer::TestStreamer(
  const async_web_server_cpp::HttpRequest & request,
  async_web_server_cpp::HttpConnectionPtr connection,
  rclcpp::Node::WeakPtr node)
: web_video_server::StreamerBase(request, connection, node, "test_streamer")
{
  RCLCPP_INFO(logger_, "TestStreamer created for topic: %s", topic_.c_str());
}

TestStreamer::~TestStreamer()
{
  RCLCPP_INFO(logger_, "TestStreamer destroyed for topic: %s", topic_.c_str());
}

void TestStreamer::start()
{
  RCLCPP_INFO(logger_, "TestStreamer started for topic: %s", topic_.c_str());
}

void TestStreamer::restream_frame(std::chrono::duration<double> max_age)
{
  RCLCPP_INFO(logger_, "TestStreamer restream_frame called for topic: %s", topic_.c_str());
}

std::shared_ptr<web_video_server::StreamerInterface> TestStreamerFactory::create_streamer(
  const async_web_server_cpp::HttpRequest & request,
  async_web_server_cpp::HttpConnectionPtr connection,
  rclcpp::Node::WeakPtr node)
{
  return std::make_shared<TestStreamer>(request, connection, node);
}

}  // namespace test_streamer_plugin

#include "pluginlib/class_list_macros.hpp"

PLUGINLIB_EXPORT_CLASS(
  test_streamer_plugin::TestStreamerFactory,
  web_video_server::StreamerFactoryInterface)

Add plugins.xml file with plugin description:

<library path="test_streamer_plugin">
  <class name="test_streamer_plugin/stream/test"
    type="test_streamer_plugin::TestStreamerFactory"
    base_class_type="web_video_server::StreamerFactoryInterface">
    <description>Test streamer implementation</description>
  </class>
</library>

Update CMakeLists.txt to export the plugin description file (Add this anywhere after find_package section):
```
pluginlib_export_plugin_description_file(web_video_server plugins.xml)
```

Build your package:

cd ~/ros_ws
colcon build --packages-select test_streamer_plugin
source install/setup.bash

Run the web_video_server node and test your custom streamer plugin by accessing a topic with the test format:
```
ros2 run web_video_server web_video_server
```
Then open your web browser and navigate to:
```
http://localhost:8080/stream?topic=/your_image_topic&format=test
```

Implementation hints

You can access query parameters from the HTTP request in your streamer constructor using request.get_query_param_value_or_default method.
Use logger_ member variable from the base StreamerBase class for logging.
Inherit from web_video_server::streamers::ImageTransportStreamerBase instead of web_video_server::StreamerBase if you want to use image transport functionality without writing boilerplate code.
By default, in the topic list view, your custom streamer will not be shown for any topic. To change it, overwrite get_available_topics method in your StreamerFactory class or inherit from ImageTransportStreamerFactoryBase to make your format available for all topics using sensor_msgs/msg/Image message type.

Link specific targets in CMakeLists.txt. For example, replace:

target_link_libraries(
  test_streamer_plugin PUBLIC
  ${web_video_server_TARGETS}
  ${pluginlib_TARGETS}
)

with:

target_link_libraries(
  test_streamer_plugin
  web_video_server::web_video_server
  pluginlib::pluginlib
)

Add web_video_server::web_video_server_streamers if you inherit from ImageTransportStreamerBase.