Add example for dynamic messages

rclrs/dynamic_pub_sub/Cargo.toml

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+[package]
+name = "examples_rclrs_dynamic_pub_sub"
+version = "0.4.1"
+authors = ["Luca Della Vedova <[email protected]>"]
+edition = "2021"
+
+[[bin]]
+name = "dynamic_subscriber"
+path = "src/dynamic_subscriber.rs"
+
+[[bin]]
+name = "dynamic_publisher"
+path = "src/dynamic_publisher.rs"
+
+[dependencies]
+# TODO(luca) change this to the correct version once dynamic message support is released on crates.io
+rclrs = { git = "https://github.com/luca-della-vedova/ros2_rust", branch = "dynamic_messages" }
+anyhow = {version = "1", features = ["backtrace"]}

rclrs/dynamic_pub_sub/package.xml

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+<?xml version="1.0"?>
+<?xml-model
+   href="http://download.ros.org/schema/package_format3.xsd"
+   schematypens="http://www.w3.org/2001/XMLSchema"?>
+<package format="3">
+  <name>examples_rclrs_dynamic_pub_sub</name>
+  <version>0.4.1</version>
+  <description>Package containing an example of dynamic messages publishers and subscribtions in rclrs.</description>
+  <maintainer email="[email protected]">Luca Della Vedova</maintainer>
+  <license>Apache License 2.0</license>
+
+  <build_depend>rclrs</build_depend>
+
+  <!-- Messages are only loaded at execution time so they are only exec dependencies -->
+  <exec_depend>example_interfaces</exec_depend>
+  <exec_depend>rclrs</exec_depend>
+
+  <export>
+    <build_type>ament_cargo</build_type>
+  </export>
+</package>
+

rclrs/dynamic_pub_sub/src/dynamic_publisher.rs

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+use anyhow::{Error, Result};
+use rclrs::*;
+
+fn main() -> Result<(), Error> {
+    let context = Context::default_from_env()?;
+    let executor = context.create_basic_executor();
+
+    let node = executor.create_node("minimal_publisher")?;
+
+    let message_type = MessageTypeName {
+        package_name: "example_interfaces".to_owned(),
+        type_name: "String".to_owned(),
+    };
+
+    let publisher = node.create_dynamic_publisher(message_type.clone(), "topic")?;
+
+    let message_metadata = DynamicMessageMetadata::new(message_type)?;
+
+    let mut publish_count: u32 = 1;
+
+    while context.ok() {
+        let mut message = message_metadata.create()?;
+
+        let Some(ValueMut::Simple(data)) = message.get_mut("data") else {
+            panic!("Unexpected value type, expected Simple value");
+        };
+
+        let SimpleValueMut::String(data) = data else {
+            panic!("Unexpected value type, expected String");
+        };
+        *data = format!("Hello, world! {}", publish_count).into();
+        println!("Publishing: [{}]", data);
+        publisher.publish(message)?;
+        publish_count += 1;
+        std::thread::sleep(std::time::Duration::from_millis(500));
+    }
+    Ok(())
+}

rclrs/dynamic_pub_sub/src/dynamic_subscriber.rs

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+use anyhow::{Error, Result};
+use rclrs::*;
+
+fn main() -> Result<(), Error> {
+    let context = Context::default_from_env()?;
+    let mut executor = context.create_basic_executor();
+
+    let node = executor.create_node("dynamic_subscriber")?;
+
+    let worker = node.create_worker::<usize>(0);
+    let _subscription = worker.create_dynamic_subscription(
+        "example_interfaces/msg/String".try_into()?,
+        "topic",
+        move |num_messages: &mut usize, msg, _msg_info| {
+            *num_messages += 1;
+            println!("#{} | I heard: '{:#?}'", *num_messages, msg.structure());
+        },
+    )?;
+
+    println!("Waiting for messages...");
+    executor.spin(SpinOptions::default()).first_error()?;
+    Ok(())
+}