Add dynamic message functionality

Author: nnmm

examples/dynamic_pub_sub/Cargo.toml

@@ -0,0 +1,10 @@
+[package]
+name = "dynamic_pub_sub"
+version = "0.1.0"
+edition = "2021"
+
+# See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
+
+[dependencies]
+rclrs = { version = "0.2", features = ["dyn_msg"] }
+anyhow = {version = "1", features = ["backtrace"]}

examples/dynamic_pub_sub/src/main.rs

@@ -0,0 +1,22 @@
+use anyhow::{Error, Result};
+use std::env;
+
+fn main() -> Result<(), Error> {
+    let context = rclrs::Context::new(env::args())?;
+
+    let mut node = rclrs::create_node(&context, "dynamic_subscriber")?;
+
+    let mut num_messages: usize = 0;
+
+    let _subscription = node.create_dynamic_subscription(
+        "topic",
+        "rclrs_example_msgs/msg/VariousTypes",
+        rclrs::QOS_PROFILE_DEFAULT,
+        move |msg| {
+            num_messages += 1;
+            println!("I heard: '{:#?}'", msg.structure());
+        },
+    )?;
+
+    rclrs::spin(&node).map_err(|err| err.into())
+}

rclrs/Cargo.toml

@@ -13,12 +13,16 @@ path = "src/lib.rs"
 # Please keep the list of dependencies alphabetically sorted,
 # and also state why each dependency is needed.
 [dependencies]
+# Needed for dynamically finding type support libraries 
+ament_rs = { version = "0.2", optional = true }
+# Needed for clients
+futures = "0.3"
+# Needed for dynamic messages
+libloading = { version = "0.7", optional = true }
 # Needed for FFI
 libc = "0.2.43"
 # Needed for the Message trait, among others
 rosidl_runtime_rs = "0.2.0"
-# Needed for clients
-futures = "0.3"
 
 [dev-dependencies]
 # Needed for e.g. writing yaml files in tests
@@ -27,3 +31,7 @@ tempfile = "3.3.0"
 [build-dependencies]
 # Needed for FFI
 bindgen = "0.59.1"
+
+[features]
+default = ["dyn_msg"]
+dyn_msg = ["ament_rs", "libloading"]

rclrs/build.rs

@@ -27,12 +27,15 @@ fn main() {
         .allowlist_type("rcl_.*")
         .allowlist_type("rmw_.*")
         .allowlist_type("rcutils_.*")
+        .allowlist_type("rosidl_.*")
         .allowlist_function("rcl_.*")
         .allowlist_function("rmw_.*")
         .allowlist_function("rcutils_.*")
+        .allowlist_function("rosidl_.*")
         .allowlist_var("rcl_.*")
         .allowlist_var("rmw_.*")
         .allowlist_var("rcutils_.*")
+        .allowlist_var("rosidl_.*")
         .layout_tests(false)
         .size_t_is_usize(true)
         .default_enum_style(bindgen::EnumVariation::Rust {