Make deriving the NetworkBehaviour more ergonomic (#782)

core/src/swarm.rs

@@ -336,6 +336,15 @@ pub trait NetworkBehaviour<TTopology> {
     fn poll(&mut self, topology: &mut PollParameters<TTopology>) -> Async<NetworkBehaviourAction<<Self::ProtocolsHandler as ProtocolsHandler>::InEvent, Self::OutEvent>>;
 }
 
+/// Used when deriving `NetworkBehaviour`. When deriving `NetworkBehaviour`, must be implemented
+/// for all the possible event types generated by the various fields.
+// 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.
+    fn inject_event(&mut self, event: TEvent);
+}
+
 /// Parameters passed to `poll()` that the `NetworkBehaviour` has access to.
 // TODO: #[derive(Debug)]
 pub struct PollParameters<'a, TTopology: 'a> {