Merge pull request #125 from Carter12s/generic-messages-example

Working example of a generic subscriber

roslibrust/examples/generic_message.rs

@@ -0,0 +1,74 @@
+//! This example shows off creating a custom generic message, and leveraging serde_json's parsing resolution
+//! to decode to the right type.
+use log::*;
+use roslibrust::ClientHandle;
+use roslibrust_codegen::RosMessageType;
+
+/// We place the ros1 generate code into a module to prevent name collisions with the identically
+/// named ros2 types.
+mod ros1 {
+    roslibrust_codegen_macro::find_and_generate_ros_messages!(
+        "assets/ros1_common_interfaces/std_msgs"
+    );
+}
+
+mod ros2 {
+    roslibrust_codegen_macro::find_and_generate_ros_messages_without_ros_package_path!(
+        "assets/ros2_common_interfaces/std_msgs"
+    );
+}
+
+/// Here we manually define a custom type with the traits needed to send it as a message
+#[derive(serde::Serialize, serde::Deserialize, Clone, Debug)]
+/// This serde flag lets the parser know that the incoming type won't say which variant it is
+/// instead serde_json will try to deserialize as each variant in order and give whichever one first
+/// succeeds
+#[serde(untagged)]
+enum GenericHeader {
+    V1(ros1::std_msgs::Header),
+    V2(ros2::std_msgs::Header),
+}
+
+/// We need to manually implement this trait for our custom type, normally this is done
+/// for us by th code generation
+impl RosMessageType for GenericHeader {
+    /// Note: this trick only works (well) if the messages we're being generic over have
+    /// the same ROS type name.
+    const ROS_TYPE_NAME: &'static str = "std_msgs/Header";
+
+    // TODO these fields should be removed and not required for this example see
+    // https://github.com/Carter12s/roslibrust/issues/124
+    const MD5SUM: &'static str = "";
+    const DEFINITION: &'static str = "";
+}
+
+/// Sets up a subscriber that could get either of two versions of a message
+#[tokio::main]
+async fn main() -> Result<(), Box<dyn std::error::Error + Send + Sync>> {
+    simple_logger::SimpleLogger::new()
+        .with_level(log::LevelFilter::Debug)
+        .without_timestamps() // required for running in wsl2
+        .init()
+        .unwrap();
+
+    // An instance of rosbridge needs to be running at this address for the example to work
+    let client = ClientHandle::new("ws://localhost:9090").await?;
+    info!("ClientHandle connected");
+
+    let rx = client.subscribe::<GenericHeader>("talker").await?;
+    info!("Successfully subscribed to topic: talker");
+
+    // Once the code reaches here, we'll need someone to publish to trigger these prints
+    // In ros1 this would be `rostopic pub /talker <message data>`
+    loop {
+        let msg = rx.next().await;
+        match msg {
+            GenericHeader::V1(ros1_header) => {
+                info!("Got ros1: {ros1_header:?}");
+            }
+            GenericHeader::V2(ros2_header) => {
+                info!("Got ros2: {ros2_header:?}");
+            }
+        }
+    }
+}