ChatSDK.java

// Just a small snippet of an SDK
public final class ChatSDK {
    private static final Handler sUIThreadHandler

    public static void connect(SDKConnectCallback handler) {

        // do some weird obfuscated internal SDK processing stuff here
        User user = connectAndGetUser();

        if (sUIThreadHandler != null) {
            sUIThreadHandler.post({
                handler.onConnected(user, null);
            })
        }
    }
  
    // Imagine a ConnectHandler interface here that has a onConnected method
}

ConnectHandler.kt

open class ConnectHandler(open val emitter: MaybeEmitter<User>) : ChatSDK.ConnectHandler {

    override fun onConnected(user: User?, error: Exception?) {
        when {
            error != null -> onError(error)
            user == null -> onMissingUser()
            else -> onSuccess(user)
        }
    }

    fun onSuccess(user: User) {
        if (!emitter.isDisposed) emitter.onSuccess(user)
    }

    fun onError(error: Exception) {
        if (!emitter.isDisposed) emitter.onError(error)
    }

    fun onMissingUser() {
        if (!emitter.isDisposed) emitter.onError(NullPointerException("Missing user"))
    }
}

SDKWrapper.kt

class SDKWrapper {

    fun connect(observer: MaybeObserver<User>) {
        Maybe<User>.create { ChatSDK.connect(ConnectHandler(it)) }
                .subscribeOn(Schedulers.io())
                .observeOn(Schedulers.io())
                .subscribe(observer)
    }
}