Retrofit2 return null Unit in kotlin for 204 No Content response

Question

I'm using Retrofit 2.0.2 to post data to server using the interface below:

@POST("/path/to/post")
fun postData(/*args*/): Observable<Unit>

And received success response from server.

05-09 21:20:21.770 23117 23224 D OkHttp  : --> POST https://myserver.com/api/notifications/123/markRead http/1.1
05-09 21:20:21.770 23117 23224 D OkHttp  : Content-Length: 0
05-09 21:20:21.770 23117 23224 D OkHttp  : User-Agent: MyUserAgent
05-09 21:20:21.770 23117 23224 D OkHttp  : Authorization: Bearer xxxxx
05-09 21:20:21.770 23117 23224 D OkHttp  : --> END POST (0-byte body)
05-09 21:20:22.473 23117 23224 D OkHttp  : <-- 204 No Content https://myserver.com/api/notifications/123/markRead (701ms)

But i got exception below, seems retrofit pass null Unit to onNext. Is this expected behaviour?

05-09 21:20:22.496 23117 23117 W System.err: java.lang.IllegalArgumentException: Parameter specified as non-null is null: method b.d.b.k.b, parameter result
05-09 21:20:22.496 23117 23117 W System.err:    at com.myandroid.notification.dialog.c$a.a(NotificationListPresenter.kt)
05-09 21:20:22.496 23117 23117 W System.err:    at com.myandroid.notification.dialog.c$a.onNext(NotificationListPresenter.kt:57)
05-09 21:20:22.496 23117 23117 W System.err:    at e.g.b.onNext(SafeSubscriber.java:139)
05-09 21:20:22.496 23117 23117 W System.err:    at e.d.a.g$2$1.onNext(OnSubscribeRedo.java:249)
05-09 21:20:22.496 23117 23117 W System.err:    at e.d.a.p$a.call(OperatorObserveOn.java:215)
05-09 21:20:22.496 23117 23117 W System.err:    at e.a.b.b$b.run(LooperScheduler.java:107)
05-09 21:20:22.496 23117 23117 W System.err:    at android.os.Handler.handleCallback(Handler.java:730)
05-09 21:20:22.504 23117 23117 W System.err:    at android.os.Handler.dispatchMessage(Handler.java:92)
05-09 21:20:22.504 23117 23117 W System.err:    at android.os.Looper.loop(Looper.java:137)
05-09 21:20:22.504 23117 23117 W System.err:    at android.app.ActivityThread.main(ActivityThread.java:5103)
05-09 21:20:22.504 23117 23117 W System.err:    at java.lang.reflect.Method.invokeNative(Native Method)
05-09 21:20:22.504 23117 23117 W System.err:    at java.lang.reflect.Method.invoke(Method.java:525)
05-09 21:20:22.504 23117 23117 W System.err:    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
05-09 21:20:22.504 23117 23117 W System.err:    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
05-09 21:20:22.504 23117 23117 W System.err:    at dalvik.system.NativeStart.main(Native Method)

Thank you.

Answer 1

The right way of solving this problem is to use Completable like this:

@POST("/path/to/post")
fun postData(/*args*/): Completable 

Then you can subscribe like this:

service.postData(/*args*/)
    .subscribe({ view.dataPostedSuccesfully() }, { handleError(it) })

Work like a charm for me with Retrofit 2.2.0, RxJava 2.1.0.

Answer 2

The instance passed into onNext is the (parsed) body of the response. Since the response has no body, it is null. Therefore, null is being passed.

You should be able to use

fun postData(/*args*/): Observable<Unit?>