05-handle-joins.md

Lesson 5 - Handling Chatroom Joins (15-20 minutes)

Goals

Let's start playing some more with concurrency!

In the previous lesson, we implemented ChatRoom.Join which placed a newly created ChatUser object on the joins channel.

What we want to do is to be able to track all of the users in the ChatRoom.users map, so we can tell the other users when new users have joined, and also make sure we can broadcast messages correctly.

We will do this by implementing the dispatcher in chatroom.ListenForMessages which is designed to handle the messages on the various chatroom channels, and act accordingly (e.g. add or remove users from internal data structures, broadcast connection, disconnection, and other messages to all users). This dispatcher will run in its own goroutine constantly looking for message on its queues.

So, let's go!

Steps

1. In chat.go, remember how the main function has some code that calls chatroom.ListenForMessages?

// This binds the socket to TCP port 6677
// on all interfaces.
listener, err := net.Listen("tcp", ":6677")

chatroom.ListenForMessages()

Well, the job of chatroom.ListenForMessages is to listen in a loop for any messages on the channels in the chatroom object, and then handle those messages accordingly.

Let's write some code in this function that will handle a new ChatUser object on the joins channel!

1. In ChatRoom.ListenForMessages, implement the following
   • 🌟 Create a for/select loop, and ensure this loop runs in a goroutine.
   • 🌟 In the loop, receive a ChatUser object on the cr.joins channel. Store it in the users map by its username.

[Stuck on any of the steps above? Ask your TA, or see the solution!](code/05-handle-joins/chat.go)

1. Great! now the object should be stored in the users map. However: it would be nice if the other people in the chatroom knew when new users joined the chatroom server.

To do this, we need to implement ChatRoom.Broadcast. This function will pass a message on each of the ChatUser.outgoing channels (using ChatUser.Send).

The idea is that each ChatUser will be listening on this channel for any outgoing messages via the ChatUser.WriteOutgoingMessages function. This function runs in a loop in a goroutine, and calls ChatUser.WriteString() to write that message to the socket when it sees a new message on the outgoing channel.

1. 🌟 Implement ChatUser.Send to place a message on the chatuser.outgoing channel.

   Stuck on any of the steps above? Ask your TA, or see the solution!

2. 🌟 Implement ChatUser.WriteOutgoingMessages by:

1. :star2: Creating a loop that constantly reads a msg from the `chatuser.outgoing` channel; 
1. :star2: Adding a newline to this msg;
1. :star2: Write the msg to the socket by calling `chatuser.WriteString`.

[Stuck on any of the steps above? Ask your TA, or see the solution!](code/05-handle-joins/chat.go)

4. 🌟 Let's start the outgoing message listener! At the end of ChatUser.Login, call cu.WriteOutgoingMessages loop and make sure it runs in a goroutine.

[Stuck on any of the steps above? Ask your TA, or see the solution!](code/05-handle-joins/chat.go)

1. Now that this is all setup, let's broadcast a message whenever the chatroom sees a new user login.

2. 🌟 Modify ChatRoom.ListenForMessages to broadcast a message whenever a new user logs in that says *** [name] joined the chatroom:

```go
  func (cr *ChatRoom) ListenForMessages() {
  	go func() {
  		for {
  			select {
  			case user := <-cr.joins:
  				cr.users[user.username] = user
  				cr.Broadcast("*** " + user.username + " just joined the chatroom")
  			}
  		}
  	}()
  }
```

1. Verify everything works as designed when you connect! You can try connecting with multiple simultaneous clients to make sure you see the join messages everywhere.

   $ telnet localhost 6677                                                                                                                      ~ 1 ↵
   Trying ::1...
   telnet: connect to address ::1: Connection refused
   Trying 127.0.0.1...
   Connected to localhost.
   Escape character is '^]'.
   Welcome to Jen's chat server!
   Please enter your username: funcuddles
   Welcome, funcuddles
   *** funcuddles just joined the chatroom

Finally! Let's chat! Proceed to Lesson 6