OpenAI API Integration Patterns for Android

/** The app depends on this, not on any provider SDK. */
interface AiChatService {
    fun stream(conversationId: String, userMessage: String): Flow<ChatEvent>
}

sealed interface ChatEvent {
    data class Delta(val text: String) : ChatEvent
    data class Done(val messageId: String) : ChatEvent
    data class Error(val kind: ErrorKind) : ChatEvent
}
enum class ErrorKind { RATE_LIMITED, QUOTA_EXCEEDED, UNAVAILABLE, BLOCKED, NETWORK }

class BackendChatService(
    private val client: OkHttpClient,              // with an auth interceptor adding the user's token
    private val baseUrl: String,
    private val json: Json,
) : AiChatService {

    override fun stream(conversationId: String, userMessage: String): Flow<ChatEvent> = callbackFlow {
        val body = json.encodeToString(ChatRequest(conversationId, userMessage))
            .toRequestBody("application/json".toMediaType())
        val request = Request.Builder().url("$baseUrl/v1/chat/stream").post(body).build()

        val source = EventSources.createFactory(client).newEventSource(request, object : EventSourceListener() {
            override fun onEvent(es: EventSource, id: String?, type: String?, data: String) {
                when (type) {
                    "delta" -> trySend(ChatEvent.Delta(json.decodeFromString<Delta>(data).text))
                    "done" -> { trySend(ChatEvent.Done(data)); close() }
                }
            }
            override fun onFailure(es: EventSource, t: Throwable?, response: Response?) {
                trySend(ChatEvent.Error(response?.code.toErrorKind()))
                close()
            }
        })
        awaitClose { source.cancel() }               // cancels the HTTP call when the collector stops
    }
}

private fun Int?.toErrorKind() = when (this) {
    429 -> ErrorKind.RATE_LIMITED
    402, 403 -> ErrorKind.QUOTA_EXCEEDED
    null -> ErrorKind.NETWORK
    else -> ErrorKind.UNAVAILABLE
}