How to detect when the WebSockets session is closed

[Stexxe]

This guide shows you how to run code when a WebSockets session closes, on both the client and server sides.

---

Client-side

When using an HttpClient with the WebSockets plugin:

val client = HttpClient {
    install(WebSockets) {}
}

You have two main ways to detect a session close:

1. Using try/finally block: Code inside the finally block runs just before the session closes.

   client.webSocket("wss://ws.postman-echo.com/raw") {
       try {
           // ...
       } finally {
           println("Session is closed")
       }
   }

2. Using outgoing.invokeOnClose: Register a handler to be called when the outgoing channel closes, which happens after the Close frame is sent.

   client.webSocket("wss://ws.postman-echo.com/raw") {
       outgoing.invokeOnClose {
           println("Session is closed")
       }
       // ...
   }

This invokeOnClose method also works when using webSocketSession directly:

val session = client.webSocketSession("wss://ws.postman-echo.com/raw")
session.outgoing.invokeOnClose {
    println("Session is closed")
}

// ...

session.close()

---

Server-side

Similarly, there are two ways to detect a WebSockets session close on the server:

1. Using try/finally block: The finally block's code is executed when the session closes.

   routing {
       webSocket("/ws") {
           try {
               // ...
           } finally {
               println("Session is closed")
           }
       }
   }

2. Using coroutineContext[Job]?.invokeOnCompletion: Register a handler that runs when the session's underlying Job completes, which occurs after the Close frame is sent.

   routing {
       webSocket("/ws") {
           coroutineContext[Job]?.invokeOnCompletion {
               println("Session is closed")
           }
   
           // ...
       }
   }