add more comments to chat example (#1665)

examples/chat/src/main.rs

@@ -26,7 +26,9 @@ use tracing_subscriber::{layer::SubscriberExt, util::SubscriberInitExt};
 
 // Our shared state
 struct AppState {
+    // We require unique usernames. This tracks which usernames have been taken.
     user_set: Mutex<HashSet<String>>,
+    // Channel used to send messages to all connected clients.
     tx: broadcast::Sender<String>,
 }
 
@@ -40,6 +42,7 @@ async fn main() {
         .with(tracing_subscriber::fmt::layer())
         .init();
 
+    // Set up application state for use with with_state().
     let user_set = Mutex::new(HashSet::new());
     let (tx, _rx) = broadcast::channel(100);
 
@@ -65,8 +68,11 @@ async fn websocket_handler(
     ws.on_upgrade(|socket| websocket(socket, state))
 }
 
+// This function deals with a single websocket connection, i.e., a single
+// connected client / user, for which we will spawn two independent tasks (for
+// receiving / sending chat messages).
 async fn websocket(stream: WebSocket, state: Arc<AppState>) {
-    // By splitting we can send and receive at the same time.
+    // By splitting, we can send and receive at the same time.
     let (mut sender, mut receiver) = stream.split();
 
     // Username gets set in the receive loop, if it's valid.
@@ -91,15 +97,17 @@ async fn websocket(stream: WebSocket, state: Arc<AppState>) {
         }
     }
 
-    // Subscribe before sending joined message.
+    // We subscribe *before* sending the "joined" message, so that we will also
+    // display it to our client.
     let mut rx = state.tx.subscribe();
 
-    // Send joined message to all subscribers.
+    // Now send the "joined" message to all subscribers.
     let msg = format!("{} joined.", username);
     tracing::debug!("{}", msg);
     let _ = state.tx.send(msg);
 
-    // This task will receive broadcast messages and send text message to our client.
+    // Spawn the first task that will receive broadcast messages and send text
+    // messages over the websocket to our client.
     let mut send_task = tokio::spawn(async move {
         while let Ok(msg) = rx.recv().await {
             // In any websocket error, break loop.
@@ -109,11 +117,12 @@ async fn websocket(stream: WebSocket, state: Arc<AppState>) {
         }
     });
 
-    // Clone things we want to pass to the receiving task.
+    // Clone things we want to pass (move) to the receiving task.
     let tx = state.tx.clone();
     let name = username.clone();
 
-    // This task will receive messages from client and send them to broadcast subscribers.
+    // Spawn a task that takes messages from the websocket, prepends the user
+    // name, and sends them to all broadcast subscribers.
     let mut recv_task = tokio::spawn(async move {
         while let Some(Ok(Message::Text(text))) = receiver.next().await {
             // Add username before message.
@@ -121,17 +130,18 @@ async fn websocket(stream: WebSocket, state: Arc<AppState>) {
         }
     });
 
-    // If any one of the tasks exit, abort the other.
+    // If any one of the tasks run to completion, we abort the other.
     tokio::select! {
         _ = (&mut send_task) => recv_task.abort(),
         _ = (&mut recv_task) => send_task.abort(),
     };
 
-    // Send user left message.
+    // Send "user left" message (similar to "joined" above).
     let msg = format!("{} left.", username);
     tracing::debug!("{}", msg);
     let _ = state.tx.send(msg);
-    // Remove username from map so new clients can take it.
+
+    // Remove username from map so new clients can take it again.
     state.user_set.lock().unwrap().remove(&username);
 }