Develop Reference

Kotlin recursive problem when type checking

I have the following code which i think is valid, because the recursion happens as a result of a callback. It's not called directly as a result of the function call. But the compiler seems to think there is a recursion issue
class Model(callBack: CallBack) {
interface CallBack {
fun onSomething()
}
}
class SomeClass {
fun createModel() = Model(callBack)
val callBack = object : Model.CallBack {
override fun onSomething() {
val anotherModel = createModel()
// Use model for something
}
}
}
Type checking has run into a recursive problem. Easiest workaround: specify types of your declarations explicitly
Is there a workaround for this?
EDIT
I also tried changing callBack to a function so that the same instance is not referenced by multiple models, but I get the same error

The recursive problem mentioned is not about function calls, it's about the compiler trying to find out the types of the declaration and it has stuck in a recursive type checking. It wants to find the output type of createModel which depends on the type of val callback and it depends on createModel again. As it says, declare their types to fix the issue.
class Model(callBack: CallBack)
{
interface CallBack {
fun onSomething()
}
}
class SomeClass {
fun createModel() : Model = Model(callBack)
val callBack : Model.CallBack = object : Model.CallBack {
override fun onSomething() {
val anotherModel : Model = createModel()
// Use model for something
}
}
}

Communication between view and ViewModel in MVVM with LiveData

What is a proper way to communicate between the ViewModel and the View, Google architecture components give use LiveData in which the view subscribes to the changes and update itself accordingly, but this communication not suitable for single events, for example show message, show progress, hide progress etc.
There are some hacks like SingleLiveEvent in Googles example but it work only for 1 observer.
Some developers using EventBus but i think it can quickly get out of control when the project grows.
Is there a convenience and correct way to implement it, how do you implement it?
(Java examples welcome too)

Yeah I agree, SingleLiveEvent is a hacky solution and EventBus (in my experience) always lead to trouble.
I found a class called ConsumableValue a while back when reading the Google CodeLabs for Kotlin Coroutines, and I found it to be a good, clean solution that has served me well (ConsumableValue.kt):
class ConsumableValue<T>(private val data: T) {
private var consumed = false
/**
* Process this event, will only be called once
*/
#UiThread
fun handle(block: ConsumableValue<T>.(T) -> Unit) {
val wasConsumed = consumed
consumed = true
if (!wasConsumed) {
this.block(data)
}
}
/**
* Inside a handle lambda, you may call this if you discover that you cannot handle
* the event right now. It will mark the event as available to be handled by another handler.
*/
#UiThread
fun ConsumableValue<T>.markUnhandled() {
consumed = false
}
}
class MyViewModel : ViewModel {
private val _oneShotEvent = MutableLiveData<ConsumableValue<String>>()
val oneShotEvent: LiveData<ConsumableValue<String>>() = _oneShotData
fun fireEvent(msg: String) {
_oneShotEvent.value = ConsumableValue(msg)
}
}
// In Fragment or Activity
viewModel.oneShotEvent.observe(this, Observer { value ->
value?.handle { Log("TAG", "Message:$it")}
})
In short, the handle {...} block will only be called once, so there's no need for clearing the value if you return to a screen.

What about using Kotlin Flow?
I do not believe they have the same behavior that LiveData has where it would alway give you the latest value. Its just a subscription similar to the workaround SingleLiveEvent for LiveData.
Here is a video explaining the difference that I think you will find interesting and answer your questions
https://youtu.be/B8ppnjGPAGE?t=535

try this:
/**
* Used as a wrapper for data that is exposed via a LiveData that represents an event.
*/
open class Event<out T>(private val content: T) {
var hasBeenHandled = false
private set // Allow external read but not write
/**
* Returns the content and prevents its use again.
*/
fun getContentIfNotHandled(): T? {
return if (hasBeenHandled) {
null
} else {
hasBeenHandled = true
content
}
}
/**
* Returns the content, even if it's already been handled.
*/
fun peekContent(): T = content
}
And wrapper it into LiveData
class ListViewModel : ViewModel {
private val _navigateToDetails = MutableLiveData<Event<String>>()
val navigateToDetails : LiveData<Event<String>>
get() = _navigateToDetails
fun userClicksOnButton(itemId: String) {
_navigateToDetails.value = Event(itemId) // Trigger the event by setting a new Event as a new value
}
}
And observe
myViewModel.navigateToDetails.observe(this, Observer {
it.getContentIfNotHandled()?.let { // Only proceed if the event has never been handled
startActivity(DetailsActivity...)
}
})
link reference: Use an Event wrapper

For showing/hiding progress dialogs and showing error messages from a failed network call on loading of the screen, you can use a wrapper that encapsulates the LiveData that the View is observing.
Details about this method are in the addendum to app architecture:
https://developer.android.com/jetpack/docs/guide#addendum
Define a Resource:
data class Resource<out T> constructor(
val state: ResourceState,
val data: T? = null,
val message: String? = null
)
And a ResourceState:
sealed class ResourceState {
object LOADING : ResourceState()
object SUCCESS : ResourceState()
object ERROR : ResourceState()
}
In the ViewModel, define your LiveData with the model wrapped in a Resource:
val exampleLiveData = MutableLiveData<Resource<ExampleModel>>()
Also in the ViewModel, define the method that makes the API call to load the data for the current screen:
fun loadDataForView() = compositeDisposable.add(
exampleUseCase.exampleApiCall()
.doOnSubscribe {
exampleLiveData.setLoading()
}
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{
exampleLiveData.setSuccess(it)
},
{
exampleLiveData.setError(it.message)
}
)
)
In the View, set up the Observer on creation:
viewModel.exampleLiveData.observe(this, Observer {
updateResponse(it)
})
Here is the example updateResponse() method, showing/hiding progress, and showing an error if appropriate:
private fun updateResponse(resource: Resource<ExampleModel>?) {
resource?.let {
when (it.state) {
ResourceState.LOADING -> {
showProgress()
}
ResourceState.SUCCESS -> {
hideProgress()
// Use data to populate data on screen
// it.data will have the data of type ExampleModel
}
ResourceState.ERROR -> {
hideProgress()
// Show error message
// it.message will have the error message
}
}
}
}

You can easily achieve this by not using LiveData, and instead using Event-Emitter library that I wrote specifically to solve this problem without relying on LiveData (which is an anti-pattern outlined by Google, and I am not aware of any other relevant alternatives).
allprojects {
repositories {
maven { url "https://jitpack.io" }
}
}
implementation 'com.github.Zhuinden:event-emitter:1.0.0'
If you also copy the LiveEvent class , then now you can do
private val emitter: EventEmitter<String> = EventEmitter()
val events: EventSource<String> get() = emitter
fun doSomething() {
emitter.emit("hello")
}
And
override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
viewModel = getViewModel<MyViewModel>()
viewModel.events.observe(viewLifecycleOwner) { event ->
// ...
}
}
// inline fun <reified T: ViewModel> Fragment.getViewModel(): T = ViewModelProviders.of(this).get(T::class.java)
For rationale, you can check out my article I wrote to explain why the alternatives aren't as valid approaches.
You can however nowadays also use a Channel(UNLIMITED) and expose it as a flow using asFlow(). That wasn't really applicable back in 2019.

Accessing Twitter REST API only works in mainActivity, but why?

I am really confused and I need your help! This is only my second App and my first time to work with REST API's. I am simply trying to display some User Information like name and profile picture. It is working perfectly fine when I am using the code in the main Activity, but as soon as I am using a different class for it the API call fails and the code is pretty similar, so I do not know any further. Since Twitter uses Retrofit in their own tutorial I am using it as well.
My Class extending TwitterApiClient, the file is also including the Interface for the custom service:
import android.util.Log
import com.twitter.sdk.android.core.*
import com.twitter.sdk.android.core.models.User
import retrofit2.Call
import retrofit2.http.GET
import retrofit2.http.Query
class MyTwitterApiClient(session: TwitterSession) : TwitterApiClient(session) {
fun getCustomService() : GetUsersShowAPICustomService {
return getService(GetUsersShowAPICustomService::class.java)
}
}
interface GetUsersShowAPICustomService {
#GET("/1.1/users/show.json")
fun show(#Query("user_id") userId: Long) : Call<User>
}
My Method in the MainActivity looks like this:
private fun loadTwitterAPI(userID: Long) {
MyTwitterApiClient(session).getCustomService().show(userID).enqueue(object : Callback<User>() {
override fun success(result: Result<User>?) {
text.text = (
"Name: "+result!!.data.name
+"\nLocation: "+result!!.data.location
+"\nFriends: "+result!!.data.friendsCount
)
Picasso.with(baseContext).load(result!!.data.profileImageUrl).resize(250, 250).into(imageView)
}
override fun failure(exception: TwitterException?) {
}
})
}
This works perfectly fine, but I do not want to have the call itself in my main activity and I created a companion Object in my Class extending the TwitterApi which should simply get called with the TwitterSession as parameter and it should return an object of the class User which contains all the important data.
The companion Object inside the MyTwitterApiClient class looks like this:
companion object {
fun start(session: TwitterSession): User {
val userID = session.userId
var data: User? = null
MyTwitterApiClient(session).getCustomService().show(userID).enqueue(object : Callback<User>() {
override fun success(result: Result<User>?) {
data = result!!.data
}
override fun failure(exception: TwitterException?) {
throw exception!!
}
})
return data!!
}
}
The new Method in the MainActivity looks like this:
private fun loadTwitterAPI(userID: Long) {
val t = MyTwitterApiClient.start(session)
text.text = (
"Name: "+t.name
+"\nLocation: "+t.location
+"\nFriends: "+t.friendsCount
)
Picasso.with(baseContext).load(t.profileImageUrl).resize(250, 250).into(imageView)
}
Through testing, I found out, that neither the success Method nor the failure Method gets called. And I do not understand at all why it does not call any Method and just fails.
If anyone here already worked with something like this or has a Tip for me it would be super helpful!
Greetings
Btw: The error that crashes my app in the end is a NullPointerException as the Success Method is not called and null gets returned in the end.
Pastebin to my files:
MainActivity: https://pastebin.com/hWByYUFT
MyTwitterApiClient: https://pastebin.com/85xH284K
activity_main.xml: https://pastebin.com/vkzbkL81
depencies in build.gradle: https://pastebin.com/CpX7cwkS

Ok, starting from your code:
fun start(session: TwitterSession): User {
val userID = session.userId
var data: User? = null
MyTwitterApiClient(session).getCustomService().show(userID).enqueue(object : Callback<User>() {
override fun success(result: Result<User>?) {
data = result!!.data
}
override fun failure(exception: TwitterException?) {
throw exception!!
}
})
return data!!
}
Here you are returning data as if it was assigned. You TwitterApiClient does asynchronous task and so the data from data = result!!.data wont be read correctly from
text.text = (
"Name: "+t.name
+"\nLocation: "+t.location
+"\nFriends: "+t.friendsCount
)
Because t is null then. Its data is not yet set. It will be, sometime in the futur, in the asynchronous callback success().
Your main issue seems to be with how to work with asynchronous tasks and how to notify results. Plenty of sources about it. LiveData, RxJava, EventBus might all be leads.
BTW, the reason why your code worked in MainActivity was because you were setting the text after the result came (in success()), so t was good to read.
Good luck and happy learning!

How to pass null to an Observable with nullable type in RxJava 2 and Kotlin

I initialize my variable like this:-
val user: BehaviorSubject<User?> user = BehaviorSubject.create()
But I can't do this. IDE throws an error:-
user.onNext(null)
And doing this, IDE says u will never be null:-
user.filter( u -> u!=null)

As Guenhter explained, this is not possible. However, instead of proposing the null-object pattern, I'd recommend an implementation of the Optional type:
data class Optional<T>(val value: T?)
fun <T> T?.asOptional() = Optional(this)
This makes your intent much clearer, and you can use a destructuring declaration in your functions:
Observable.just(Optional("Test"))
.map { (text: String?) -> text?.substring(1)?.asOptional() }
.subscribe()
Using the null-object pattern here can cause more bugs than it solves.

If you use rxkotlin/rxjava 2.0 (I assume so) than the answer is: you can't. The reason is explained here.
This is a break of the interface. Have a look at the Observable Interface
public interface Observer<T> {
/** ... */
void onSubscribe(#NonNull Disposable d);
/** ... */
void onNext(#NonNull T t);
/** ... */
void onError(#NonNull Throwable e);
/** ... */
void onSubscribe(#NonNull Disposable d);
/** ... */
void onNext(#NonNull T t);
/** ... */
void onError(#NonNull Throwable e);
...
The #NonNull will be considered by the Kotlin compiler and therefore you CAN'T pass null.
Even if you could, the onNext would immediately throw an error:
#Override
public void onNext(T t) {
if (t == null) {
onError(new NullPointerException("onNext called with null. Null values are generally not allowed in 2.x operators and sources."));
return;
}
...
}
If you really need such a thing as null you have to fake it. e.g. by creating a static object of User which represents your null-element.
e.g.
data class User(val username, val password) {
companion object {
val NULL_USER = User("", "")
}
}
...
val user = BehaviorSubject.create<User>()
...
user.onNext(User.NULL_USER)
...
user.filter { it !== User.NULL_USER }
But if is somehow possible, try to avoid the null concept and maybe think of another solution where this isn't needed.

Thank you very much for all your answers but I ultimately went with this solution:-
class UserEnvelope(val user:User?) {}
And using this in the observables.
This best suited my requirements.
I am new to Kotlin so I don't know how to use Optionals. But from what I understand, I would have to typecast it to User type everytime I need to observe the values right?

To implement the solution mentioned in the nhaarman's answer, you can use the util class Optional (doc) from the Android SDK which was added in API level 24.
If your app's minSdkVersion less than 24 then you still need to implement it by yourself.

Since RxJava 2 does not support null values, there are some other acceptable solutions you can use:
Work with a custom or third party wrapper library of Optionals like some of the posted answers suggest. When I got rid of Java in favour of Kotlin, Optionals went away in the same package since Kotlin per se supports nullability as part of its type System. Just by this change the code was much more clearer, and I personally don't want to get Optionals back in my code as long as I can avoid them.
Emit Any class instances with your subject type. For example you could create an Empty.INSTANCE enum class which would emulate the null value and then filter by the enum class.
The last one is the one I use and prefer being a variant of the previous solution and is based on specialisations. Our friends of JetBrains always emphasise that classes are very cheap in Kotlin, so this would be a quick example to distinguish logged users and not logged ones:
abstract class SessionUser
sealed class LoggedUser(val username: String, val password: String) : SessionUser()
sealed class LogoutUser : SessionUser()
private val user = BehaviorSubject.create<SessionUser>()
private val loggedUser =
user.filter { it is LoggedUser }.cast(LoggedUser::class.java)
fun login(username: String, password: String) {
user.onNext(LoggedUser(username, password))
}
fun logout() {
user.onNext(LogoutUser())
}

I've taken an approach similar to Optional<User> and UserEnvelope. I make a simple User class and a ReifiedUser class that inherits from it. The User class has a companion object that has a NONE instance. The BehaviorSubject is instantiated with the User.NONE instance. It looks something like this:
open class User {
companion object {
val NONE = User()
}
}
class ReifiedUser(
#field:JsonProperty(J.FirstName) val firstName: String,
#field:JsonProperty(J.LastName) val lastName: String
) : User()
My BehaviorSubject is instantiated like this:
val activeUser: BehaviorSubject<User> = BehaviorSubject.createDefault(User.NONE)
And wherever I need to use activeUser I either flatMap it to Observable.empty() if it's NONE or just figure out what it is and what to do in the subscriber.
I don't like mixing java Optional with kotlin nullable because mixing map and let gets really confusing and ugly. This way it's very obvious what's going on.

I think it makes more sense to write a container class such as Result. An example of that would be
data class Result<T>(value: T?, error: Throwable?)
Usage
Observable.create { observer ->
upstreamService.listen(object: UpstreamListener {
onSuccess(data: User) {
observer.onSuccess(Result(data))
}
onError(exception: Throwable) {
observer.onSuccess(Result(null, exception))
}
}
}

Mockito Wanted but not Invoked

I'm new to writing tests and using Mockito.
I've read the similar topics here on Stackoverflow and made the suggested changes, making sure that regarded classes / interfaces / methods are open.
I tried to follow this
Mocking the constructor injected dependencies
This is the test I came up with so far
class RegistrationPresenterTest {
#Test
fun testRegisterSuccess() {
val mockService = mock<IHerokuInteractor>()
val mockLocal = mock<ILocalStorageInteractor>()
val mockView = mock<RegisterView>()
val mockRegistrationResponse = HerokuRegisterResponse("hash")
val mockPair = ImeiPair("imei","hash")
val presenter = RegisterPresenterImpl(mockLocal,mockService)
whenever(mockService.register(any())).thenReturn(Observable.just(mockRegistrationResponse))
whenever(mockLocal.clearPreferences()).thenReturn(Observable.just(true))
whenever(mockLocal.putImeiPair(any())).thenReturn(Observable.just(true))
//whenever(presenter.writeImeiPairLocally(any())) How do I specify parameters since it uses a parameter from the register method?
presenter.bindView(mockView)
presenter.register("imei","male")
verify(mockService, times(1)).register(any())
verify(mockLocal,times(1)).clearPreferences()
verify(mockLocal,times(1)).putImeiPair(any())
verify(mockView,times(1)).moveToMain()
}
but the response I keep getting is
Wanted but not invoked:
registerPresenterImpl.writeImeiPairLocally(
<any com.company.appname.model.ImeiPair>
);
Actually, there were zero interactions with this mock.
I got this response even when I don't mention that method in the test.
This is my presenter register method. I've changed the classes / interfaces & methods involved to open (kotlin). I believe override methods are open by nature in kotlin.
open class RegisterPresenterImpl #Inject constructor(val localStorage : ILocalStorageInteractor, var herokuService : IHerokuInteractor)
override fun register(imei : String, gender : String){
subscription = herokuService.register(RegisterObject(imei,gender)).subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread()).subscribe(
{
registrationResult ->
Log.d(TAG,"${registrationResult}")
if(registrationResult.imei_hash != null){
writeImeiPairLocally(ImeiPair(imei,registrationResult.imei_hash))
}
else{
Log.e(TAG,"User already exists")
}
},
{
errorResponse -> Log.e(TAG,"Could not register user ${errorResponse.message}")
}
)
addSubscription(subscription)
}
and similarly the
open fun writeImeiPairLocally(pair : ImeiPair){
subscription = localStorage.clearPreferences().flatMap {
cleared -> localStorage.putImeiPair(pair)}.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread()).subscribe(
{
booleanResult -> view?.moveToMain()
},
{
errorResponse -> Log.e(TAG,"Could not write ImeiPair to SharedPreferences ${errorResponse.message}")
}
)
addSubscription(subscription)
}
Here is interfaces
open interface ILocalStorageInteractor : ILocalStorage{
fun getImeiPair() : Observable<ImeiPair>
fun putImeiPair(pair: ImeiPair) : Observable<Boolean>
}
open interface ILocalStorage {
fun clearPreferences() : Observable<Boolean>
}
All help is appreciated.

If you are using plain jUnit, then your AndroidSchedulers.mainThread() is null. That's why onNext is not called.
You need to override Schedulers in a setUp() method with:
RxAndroidPlugins.getInstance().registerSchedulersHook(new RxAndroidSchedulersHook() {
#Override
public Scheduler getMainThreadScheduler() {
return Schedulers.immediate(); // or .test()
}
});
To avoid concurrency in tests, I would recommend to override Schedulers.io() like this:
RxJavaHooks.setOnIOScheduler(scheduler1 -> Schedulers.immediate());
If you are going to use TestScheduler, don't forget to call TestScheduler.triggerActions() method.
Also don't forget to unregister Schedulers in tearDown() like this:
RxJavaHooks.reset();
RxAndroidPlugins.getInstance().reset();
AndroidSchedulers.reset();
Schedulers.reset();