WebSockets vs REST APIs in Android

sealed interface SocketEvent {
    data object Connected : SocketEvent
    data class Message(val text: String) : SocketEvent
    data class Closed(val reason: String) : SocketEvent
}

fun OkHttpClient.webSocketFlow(url: String): Flow<SocketEvent> = callbackFlow {
    val socket = newWebSocket(Request.Builder().url(url).build(), object : WebSocketListener() {
        override fun onOpen(webSocket: WebSocket, response: Response) { trySend(SocketEvent.Connected) }
        override fun onMessage(webSocket: WebSocket, text: String) { trySend(SocketEvent.Message(text)) }
        override fun onClosed(webSocket: WebSocket, code: Int, reason: String) {
            trySend(SocketEvent.Closed(reason)); close()
        }
        override fun onFailure(webSocket: WebSocket, t: Throwable, response: Response?) { close(t) }
    })
    awaitClose { socket.close(1000, "Collector cancelled") }   // closes when the screen stops collecting
}

/** Reconnect with capped exponential backoff while collected. */
fun OkHttpClient.resilientSocket(url: String): Flow<SocketEvent> =
    webSocketFlow(url).retryWhen { cause, attempt ->
        if (cause is IOException) { delay(minOf(30_000L, 1_000L shl attempt.toInt().coerceAtMost(5))); true }
        else false
    }

class ChatViewModel(client: OkHttpClient, private val repo: ChatRepository) : ViewModel() {
    val messages: StateFlow<List<ChatMessage>> = client.resilientSocket("wss://chat.example.com/rooms/42")
        .onEach { event ->
            when (event) {
                SocketEvent.Connected -> repo.syncMissed()                 // REST catch-up after (re)connect
                is SocketEvent.Message -> repo.saveIncoming(event.text)    // persist; UI observes Room
                is SocketEvent.Closed -> Unit
            }
        }
        .flatMapLatest { repo.observeMessages() }
        .stateIn(viewModelScope, SharingStarted.WhileSubscribed(5_000), emptyList())
}