Enhance NetworkBehaviour docs (#1362)

* add docs about deriving NetworkBehaviour

* add commentary to the chat example

also minor stuff like reordering to match the struct elements, note why the listening address is
reported in poll.

* fix remove the warning on the example ignored field

* Apply suggestions from code review

Thanks for suggestions!

Co-Authored-By: Max Inden <mail@max-inden.de>

* chore cleanup added docs after suggestions

* fix print all listening addresses in examples/chat

not that there should be more than one but it makes sense to repeat the full example here.

* chore remove confusing writing on printing the listening addrs

* fix use intra-docs, spelling

Co-Authored-By: Roman S. Borschel <roman@parity.io>

Co-authored-by: Max Inden <mail@max-inden.de>
Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

examples/chat.rs

@@ -79,13 +79,30 @@ fn main() -> Result<(), Box<dyn Error>> {
 
     // We create a custom network behaviour that combines floodsub and mDNS.
     // In the future, we want to improve libp2p to make this easier to do.
+    // Use the derive to generate delegating NetworkBehaviour impl and require the
+    // NetworkBehaviourEventProcess implementations below.
     #[derive(NetworkBehaviour)]
     struct MyBehaviour<TSubstream: AsyncRead + AsyncWrite> {
         floodsub: Floodsub<TSubstream>,
         mdns: Mdns<TSubstream>,
+
+        // Struct fields which do not implement NetworkBehaviour need to be ignored
+        #[behaviour(ignore)]
+        #[allow(dead_code)]
+        ignored_member: bool,
+    }
+
+    impl<TSubstream: AsyncRead + AsyncWrite> NetworkBehaviourEventProcess<FloodsubEvent> for MyBehaviour<TSubstream> {
+        // Called when `floodsub` produces an event.
+        fn inject_event(&mut self, message: FloodsubEvent) {
+            if let FloodsubEvent::Message(message) = message {
+                println!("Received: '{:?}' from {:?}", String::from_utf8_lossy(&message.data), message.source);
+            }
+        }
     }
 
     impl<TSubstream: AsyncRead + AsyncWrite> NetworkBehaviourEventProcess<MdnsEvent> for MyBehaviour<TSubstream> {
+        // Called when `mdns` produces an event.
         fn inject_event(&mut self, event: MdnsEvent) {
             match event {
                 MdnsEvent::Discovered(list) =>
@@ -102,21 +119,13 @@ fn main() -> Result<(), Box<dyn Error>> {
         }
     }
 
-    impl<TSubstream: AsyncRead + AsyncWrite> NetworkBehaviourEventProcess<FloodsubEvent> for MyBehaviour<TSubstream> {
-        // Called when `floodsub` produces an event.
-        fn inject_event(&mut self, message: FloodsubEvent) {
-            if let FloodsubEvent::Message(message) = message {
-                println!("Received: '{:?}' from {:?}", String::from_utf8_lossy(&message.data), message.source);
-            }
-        }
-    }
-
     // Create a Swarm to manage peers and events
     let mut swarm = {
         let mdns = task::block_on(Mdns::new())?;
         let mut behaviour = MyBehaviour {
             floodsub: Floodsub::new(local_peer_id.clone()),
-            mdns
+            mdns,
+            ignored_member: false,
         };
 
         behaviour.floodsub.subscribe(floodsub_topic.clone());
@@ -152,8 +161,8 @@ fn main() -> Result<(), Box<dyn Error>> {
                 Poll::Ready(None) => return Poll::Ready(Ok(())),
                 Poll::Pending => {
                     if !listening {
-                        if let Some(a) = Swarm::listeners(&swarm).next() {
+                        for addr in Swarm::listeners(&swarm) {
-                            println!("Listening on {:?}", a);
+                            println!("Listening on {:?}", addr);
                             listening = true;
                         }
                     }

misc/core-derive/src/lib.rs

@@ -26,7 +26,8 @@ use quote::quote;
 use proc_macro::TokenStream;
 use syn::{parse_macro_input, DeriveInput, Data, DataStruct, Ident};
 
-/// The interface that satisfies Rust.
+/// Generates a delegating `NetworkBehaviour` implementation for the struct this is used for. See
+/// the trait documentation for better description.
 #[proc_macro_derive(NetworkBehaviour, attributes(behaviour))]
 pub fn hello_macro_derive(input: TokenStream) -> TokenStream {
     let ast = parse_macro_input!(input as DeriveInput);

swarm/src/behaviour.rs

@@ -26,6 +26,23 @@ use std::{error, task::Context, task::Poll};
 ///
 /// This trait has been designed to be composable. Multiple implementations can be combined into
 /// one that handles all the behaviours at once.
+///
+/// # Deriving `NetworkBehaviour`
+///
+/// Crate users can implement this trait by using the the `#[derive(NetworkBehaviour)]`
+/// proc macro re-exported by the `libp2p` crate. The macro generates a delegating `trait`
+/// implementation for the `struct`, which delegates method calls to all trait members. Any events
+/// generated by struct members are delegated to [`NetworkBehaviourEventProcess`] implementations
+/// which are expected to be provided by the user.
+///
+/// Optionally one can implement a custom `poll` function, which needs to be tagged with the
+/// `#[behaviour(poll_method = "poll")]` attribute, and would be called last with no parameters.
+///
+/// By default the derive sets the `NetworkBehaviour::OutEvent` as `()` but this can be overriden
+/// with `#[behaviour(out_event = "AnotherType")]`.
+///
+/// `#[behaviour(ignore)]` can be added on a struct field to disable generation of delegation to
+/// the fields which do not implement `NetworkBehaviour`.
 pub trait NetworkBehaviour {
     /// Handler for all the protocols the network behaviour supports.
     type ProtocolsHandler: IntoProtocolsHandler;
@@ -163,9 +180,8 @@ pub trait PollParameters {
     fn local_peer_id(&self) -> &PeerId;
 }
 
-/// Used when deriving `NetworkBehaviour`. When deriving `NetworkBehaviour`, must be implemented
+/// When deriving [`NetworkBehaviour`] this trait must be implemented for all the possible event types
-/// for all the possible event types generated by the various fields.
+/// generated by the inner behaviours.
-// TODO: document how the custom behaviour works and link this here
 pub trait NetworkBehaviourEventProcess<TEvent> {
     /// Called when one of the fields of the type you're deriving `NetworkBehaviour` on generates
     /// an event.