src: Move introduction to new tutorial.rs (#2018)

This commit extends the ping example in `src/tutorial.rs, by walking a
newcomer through the implementation of a simple ping node step-by-step,
introducing all the core libp2p concepts along the way.

With the ping tutorial in place, there is no need for the lengthy libp2p
crate level introduction, which is thus removed with this commit.

Co-authored-by: Roman Borschel <romanb@users.noreply.github.com>

examples/ping.rs

@@ -18,7 +18,9 @@
 // FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
-//! A basic example demonstrating some core APIs and concepts of libp2p.
+//! Ping example
+//!
+//! See ../src/tutorial.rs for a step-by-step guide building the example below.
 //!
 //! In the first terminal window, run:
 //!
@@ -38,22 +40,20 @@
 //! The two nodes establish a connection, negotiate the ping protocol
 //! and begin pinging each other.
 
-use async_std::task;
-use futures::{future, prelude::*};
-use libp2p::{identity, PeerId, ping::{Ping, PingConfig}, Swarm};
-use std::{error::Error, task::{Context, Poll}};
+use futures::executor::block_on;
+use futures::prelude::*;
+use libp2p::ping::{Ping, PingConfig};
+use libp2p::swarm::Swarm;
+use libp2p::{identity, PeerId};
+use std::error::Error;
+use std::task::Poll;
 
-#[async_std::main]
-async fn main() -> Result<(), Box<dyn Error>> {
-    env_logger::init();
+fn main() -> Result<(), Box<dyn Error>> {
+    let local_key = identity::Keypair::generate_ed25519();
+    let local_peer_id = PeerId::from(local_key.public());
+    println!("Local peer id: {:?}", local_peer_id);
 
-    // Create a random PeerId.
-    let id_keys = identity::Keypair::generate_ed25519();
-    let peer_id = PeerId::from(id_keys.public());
-    println!("Local peer id: {:?}", peer_id);
-
-    // Create a transport.
-    let transport = libp2p::development_transport(id_keys).await?;
+    let transport = block_on(libp2p::development_transport(local_key))?;
 
     // Create a ping network behaviour.
     //
@@ -62,9 +62,11 @@ async fn main() -> Result<(), Box<dyn Error>> {
     // can be observed.
     let behaviour = Ping::new(PingConfig::new().with_keep_alive(true));
 
-    // Create a Swarm that establishes connections through the given transport
-    // and applies the ping behaviour on each connection.
-    let mut swarm = Swarm::new(transport, behaviour, peer_id);
+    let mut swarm = Swarm::new(transport, behaviour, local_peer_id);
+
+    // Tell the swarm to listen on all interfaces and a random, OS-assigned
+    // port.
+    swarm.listen_on("/ip4/0.0.0.0/tcp/0".parse()?)?;
 
     // Dial the peer identified by the multi-address given as the second
     // command-line argument, if any.
@@ -74,24 +76,19 @@ async fn main() -> Result<(), Box<dyn Error>> {
         println!("Dialed {}", addr)
     }
 
-    // Tell the swarm to listen on all interfaces and a random, OS-assigned port.
-    swarm.listen_on("/ip4/0.0.0.0/tcp/0".parse()?)?;
-
     let mut listening = false;
-    task::block_on(future::poll_fn(move |cx: &mut Context<'_>| {
-        loop {
-            match swarm.poll_next_unpin(cx) {
-                Poll::Ready(Some(event)) => println!("{:?}", event),
-                Poll::Ready(None) => return Poll::Ready(()),
-                Poll::Pending => {
-                    if !listening {
-                        for addr in Swarm::listeners(&swarm) {
-                            println!("Listening on {}", addr);
-                            listening = true;
-                        }
+    block_on(future::poll_fn(move |cx| loop {
+        match swarm.poll_next_unpin(cx) {
+            Poll::Ready(Some(event)) => println!("{:?}", event),
+            Poll::Ready(None) => return Poll::Ready(()),
+            Poll::Pending => {
+                if !listening {
+                    for addr in Swarm::listeners(&swarm) {
+                        println!("Listening on {}", addr);
+                        listening = true;
                     }
-                    return Poll::Pending
                 }
+                return Poll::Pending;
             }
         }
     }));