Example for parameter_bridge (#330)

* Add example for using the parameter_bridge

Signed-off-by: Loy van Beek <[email protected]>
Signed-off-by: Chris Lalancette <[email protected]>

README.md

@@ -346,3 +346,76 @@ Launch AddTwoInts client:
 . <ros2-install-dir>/setup.bash
 ros2 run demo_nodes_cpp add_two_ints_client
 ```
+
+## Example 4: bridge only selected topics and services
+This example expands on example 3 by selecting a subset of topics and services to be bridged.
+This is handy when, for example, you have a system that runs most of it's stuff in either ROS 1 or ROS 2 but needs a few nodes from the 'opposite' version of ROS.
+Where the `dynamic_bridge` bridges all topics and service, the `parameter_bridge` uses the ROS 1 parameter server to choose which topics and services are bridged.
+For example, to bridge only eg. the `/chatter` topic and the `/add_two_ints service` from ROS1 to ROS2, create this configuration file, `bridge.yaml`:
+
+```yaml
+topics:
+  -
+    topic: /chatter  # ROS1 topic name
+    type: std_msgs/msg/String  # ROS2 type name
+    queue_size: 1  # For the publisher back to ROS1
+services_1_to_2:
+  -
+    service: /add_two_ints  # ROS1 service name
+    type: example_interfaces/srv/AddTwoInts  # The ROS2 type name
+```
+
+Start a ROS 1 roscore:
+
+```bash
+# Shell A (ROS 1 only):
+. /opt/ros/melodic/setup.bash
+# Or, on OSX, something like:
+# . ~/ros_catkin_ws/install_isolated/setup.bash
+roscore
+```
+
+Then load the bridge.yaml config file and start the talker to publish on the `/chatter` topic:
+
+```bash
+Shell B: (ROS1 only):
+. /opt/ros/melodic/setup.bash
+# Or, on OSX, something like:
+# . ~/ros_catkin_ws/install_isolated/setup.bash
+rosparam load bridge.yaml
+
+rosrun rospy_tutorials talker
+```
+
+```bash
+Shell C: (ROS1 only):
+. /opt/ros/melodic/setup.bash
+# Or, on OSX, something like:
+# . ~/ros_catkin_ws/install_isolated/setup.bash
+
+rosrun roscpp_tutorials add_two_ints_server
+```
+
+Then, in a few ROS 2 terminals:
+
+```bash
+# Shell D:
+. <install-space-with-ros2>/setup.bash
+ros2 run ros1_bridge parameter_bridge
+```
+
+If all is well, the logging shows it is creating bridges for the topic and service and you should be able to call the service and listen to the ROS 1 talker from ROS 2:
+
+```bash
+# Shell E:
+. <install-space-with-ros2>/setup.bash
+ros2 run demo_nodes_cpp listener
+```
+This should start printing text like `I heard: [hello world ...]` with a timestamp.
+
+```bash
+# Shell F:
+. <install-space-with-ros2>/setup.bash
+ros2 service call /add_two_ints example_interfaces/srv/AddTwoInts "{a: 1, b: 2}"
+```
+If all is well, the output should contain `example_interfaces.srv.AddTwoInts_Response(sum=3)`