Add simple P2P listener example

examples/listener.rs

@@ -0,0 +1,41 @@
+extern crate bitcoin;
+extern crate electrs;
+extern crate error_chain;
+
+use bitcoin::network::constants::Network;
+use bitcoin::network::message::NetworkMessage;
+use bitcoin::network::socket::Socket;
+
+use electrs::errors::*;
+
+fn run() -> Result<()> {
+    // Open socket
+    let mut sock = Socket::new(Network::Bitcoin);
+    sock.connect("127.0.0.1", 8333)
+        .chain_err(|| "failed to connect to node")?;
+
+    let mut outgoing = vec![sock.version_message(0).unwrap()];
+    loop {
+        for msg in outgoing.split_off(0) {
+            eprintln!("send {:?}", msg);
+            sock.send_message(msg.clone())
+                .chain_err(|| format!("failed to send {:?}", msg))?;
+        }
+        // Receive new message
+        let msg = sock
+            .receive_message()
+            .chain_err(|| "failed to receive p2p message")?;
+
+        match msg {
+            NetworkMessage::Alert(_) => continue, // deprecated
+            NetworkMessage::Version(_) => outgoing.push(NetworkMessage::Verack),
+            NetworkMessage::Ping(nonce) => outgoing.push(NetworkMessage::Pong(nonce)),
+            _ => (),
+        };
+        eprintln!("recv {:?}", msg);
+    }
+}
+
+fn main() {
+    run().expect("p2p listener failed");
+}