Develop Reference

Kotlin Multiplatform Mobile: Ktor - how to cancel active coroutine (network request, background work) in Kotlin Native (iOS)?

In my project I write View and ViewModel natively and share Repository, Db, networking.
When user navigates from one screen to another, I want to cancel all network requests or other heavy background operations that are currently running in the first screen.
Example function in Repository class:
#Throws(Throwable::class)
suspend fun fetchData(): List<String>
In Android's ViewModel I can use viewModelScope to automatically cancel all active coroutines. But how to cancel those tasks in iOS app?

Lets suppose that the object session is a URLSession instance, you can cancel it by:
session.invalidateAndCancel()

I didn't find any first party information about this or any good solution, so I came up with my own. Shortly, it will require turning repository suspend functions to regular functions with return type of custom interface that has cancel() member function. Function will take action lambda as parameter. On implementation side, coroutine will be launched and reference for Job will be kept so later when it is required to stop background work interface cancel() function will cancel job.
In addition, because it is very hard to read type of error (in case it happens) from NSError, I wrapped return data with custom class which will hold error message and type. Earlier I asked related question but got no good answer for my case where ViewModel is written natively in each platform.
If you find any problems with this approach or have any ideas please share.
Custom return data wrapper:
class Result<T>(
val status: Status,
val value: T? = null,
val error: KError? = null
)
enum class Status {
SUCCESS, FAIL
}
data class KError(
val type: ErrorType,
val message: String? = null,
)
enum class ErrorType {
UNAUTHORIZED, CANCELED, OTHER
}
Custom interface
interface Cancelable {
fun cancel()
}
Repository interface:
//Convert this code inside of Repository interface:
#Throws(Throwable::class)
suspend fun fetchData(): List<String>
//To this:
fun fetchData(action: (Result<List<String>>) -> Unit): Cancelable
Repository implementation:
override fun fetchData(action: (Result<List<String>>) -> Unit): Cancelable = runInsideOfCancelableCoroutine {
val result = executeAndHandleExceptions {
val data = networkExample()
// do mapping, db operations, etc.
data
}
action.invoke(result)
}
// example of doing heavy background work
private suspend fun networkExample(): List<String> {
// delay, thread sleep
return listOf("data 1", "data 2", "data 3")
}
// generic function for reuse
private fun runInsideOfCancelableCoroutine(task: suspend () -> Unit): Cancelable {
val job = Job()
CoroutineScope(Dispatchers.Main + job).launch {
ensureActive()
task.invoke()
}
return object : Cancelable {
override fun cancel() {
job.cancel()
}
}
}
// generic function for reuse
private suspend fun <T> executeAndHandleExceptions(action: suspend () -> T?): Result<T> {
return try {
val data = action.invoke()
Result(status = Status.SUCCESS, value = data, error = null)
} catch (t: Throwable) {
Result(status = Status.FAIL, value = null, error = ErrorHandler.getError(t))
}
}
ErrorHandler:
object ErrorHandler {
fun getError(t: Throwable): KError {
when (t) {
is ClientRequestException -> {
try {
when (t.response.status.value) {
401 -> return KError(ErrorType.UNAUTHORIZED)
}
} catch (t: Throwable) {
}
}
is CancellationException -> {
return KError(ErrorType.CANCELED)
}
}
return KError(ErrorType.OTHER, t.stackTraceToString())
}
}

You probably have 3 options:
If you're using a some sort of reactive set up iOS side (e.g. MVVM) you could just choose to ignore cancellation. Cancellation will only save a minimal amount of work.
Wrap your iOS calls to shared code in an iOS reactive framework (e.g. combine) and handle cancellation using the iOS framework. The shared work would still be done, but the view won't be updated as your iOS framework is handling cancellation when leaving the screen.
Use Flow with this closable helper

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.

Testing a RxJava call with Retrofit in MVP get Wanted but not invoked

I'm trying to test my server call with retrofit and rxJava. I'm using a MVP pattern with koin, and I'm having some problems when I try to test the method that do the call to get the data from the server.
I have a prenter that call the interactor to retrieve the data. Interactor DI is did with koin.
I've done some research here and in google and all the examples that I've been watching don't work for me.
The error that I have is this:
Wanted but not invoked:
callback.onResponseSearchFilm(
[Film(uid=1, id=1724, title=The incredible Hulk, tagline=You'll like him when he's angry., overview=Scientist Bruce Banner scours the planet for an antidote to the unbridled force of rage within..., popularity=22.619048, rating=6.1, ratingCount=4283, runtime=114, releaseDate=2008-06-12, revenue=163712074, budget=150000000, posterPath=/bleR2qj9UluYl7x0Js7VXuLhV3s.jpg, originalLanguage=en, genres=null, cast=null, poster=null, favourite=false), Film(uid=2, id=1724, title=The incredible Hulk, tagline=You'll like him when he's angry., overview=Scientist Bruce Banner scours the planet for an antidote to the unbridled force of rage within..., popularity=22.619048, rating=8.0, ratingCount=4283, runtime=114, releaseDate=2008-06-12, revenue=163712074, budget=150000000, posterPath=/bleR2qj9UluYl7x0Js7VXuLhV3s.jpg, originalLanguage=en, genres=null, cast=null, poster=null, favourite=false), Film(uid=3, id=1724, title=The incredible Hulk, tagline=You'll like him when he's angry., overview=Scientist Bruce Banner scours the planet for an antidote to the unbridled force of rage within..., popularity=22.619048, rating=8.5, ratingCount=4283, runtime=114, releaseDate=2008-06-12, revenue=163712074, budget=150000000, posterPath=/bleR2qj9UluYl7x0Js7VXuLhV3s.jpg, originalLanguage=en, genres=null, cast=null, poster=null, favourite=false)]
);
-> at com.filmfy.SearchImplTest.loadItems_WhenDataIsAvailable(SearchImplTest.kt:30)
Actually, there were zero interactions with this mock.
This is my test
class SearchImplTest: KoinTest {
private val searchImpl: SearchImpl = mock()
private val callback: SearchContract.Callback? = mock()
private val api: RetrofitAdapter = mock()
#Test
fun loadItems_WhenDataIsAvailable() {
`when`(api.getFilms()).thenReturn(Observable.just(filmRequestFacke()))
searchImpl.getfilms(callback)
verify(callback)?.onResponseSearchFilm(fackeFilms())
}
}
My interactor code:
class SearchImpl : AbstractInteractor() {
private val voucherApiServe by lazy {
RetrofitAdapter.create()
}
fun getfilms(callback: SearchContract.Callback?){
disposable = voucherApiServe.getFilms()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ result -> processFilmSearch(result.data, callback)},
{ error -> processError(error) }
)
}
fun processFilmSearch(filmList : ArrayList<Film>?, callback: SearchContract.Callback?){
callback?.onResponseSearchFilm(filmList)
}
.
.
.
My module with koin:
factory<SearchContract.Presenter> { (view: SearchContract.View) -> SearchPresenter(view, mSearchImpl = get()) }
Api call
#GET(Api.ENDPOINT.FILMS)
fun getFilms(): Observable<FilmRequest>

It is because during unit tests system call your method
searchImpl.getfilms(callback)
and before it will finish immediately call
verify(callback)?.onResponseSearchFilm(fackeFilms())
so getfilms() method not invoked and your test fail.
To wait until your rx code will finish you should inject and replace your Schedulers during unit test.
Change code:
fun getfilms(callback: SearchContract.Callback?){
disposable = voucherApiServe.getFilms()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe(
{ result -> processFilmSearch(result.data, callback)},
{ error -> processError(error) }
)
}
to:
fun getfilms(callback: SearchContract.Callback?){
disposable = voucherApiServe.getFilms()
.subscribeOn(ioScheduler) //injected scheduler
.observeOn(mainScheduler) //injected scheduler
.subscribe(
{ result -> processFilmSearch(result.data, callback)},
{ error -> processError(error) }
)
}
create Dagger module like:
#Module
class SchedulersModule {
#Provides
#Named(Names.MAIN)
fun main(): Scheduler {
return AndroidSchedulers.mainThread()
}
#Provides
#Named(Names.IO)
fun io(): Scheduler {
return Schedulers.io()
}
#Provides
#Named(Names.COMPUTATION)
fun computation(): Scheduler {
return Schedulers.computation()
}
}
where Names is just a file with string constants (which ofcource as we know have to be different)
and in your SearchImpl class inject this schedulers in constructor.
When you will create your SearchImpl class under test use TestScheduler to replace schedulers inside your voucherApiServe.getFilms() chain.
So. The last part is to force rxjava's schedulers to finish work before you will verify result.
your test should look like this:
import io.reactivex.schedulers.TestScheduler
val testScheduler = TestScheduler()
#Before
fun before() {
//you create your SearchImpl class here and use testScheduler to replace real schedulers inside it
}
#Test
fun loadItems_WhenDataIsAvailable() {
`when`(api.getFilms()).thenReturn(Observable.just(filmRequestFacke()))
searchImpl.getfilms(callback)
testScheduler.triggerActions() //Triggers any actions that have not yet been triggered and that are scheduled to be triggered at or before this Scheduler's present time.
verify(callback)?.onResponseSearchFilm(fackeFilms())
}
So this test will work. This also will help you during UI tests (to remove all delays in Observable.timer for example).
Hope it'll help :)

How do you call a suspend function inside a SAM?

I'm trying to create a Flow that needs to emit values from a callback but I can't call the emit function since the SAM is a normal function
Here's the class with the SAM from a library that I can't really modify it the way I need it to be.
class ValueClass {
fun registerListener(listener: Listener) {
...
}
interface Listener {
fun onNewValue(): String
}
}
And here's my take on creating the Flow object
class MyClass(private val valueClass: ValueClass) {
fun listenToValue = flow<String> {
valueClass.registerListener { value ->
emit(value) // Suspension functions can only be called on coroutine body
}
}
}
I guess it would've been simple if I could change the ValueClass but in this case, I can't. I've been wrapping my head around this and trying to look for implementations.
At least from what I know so far, one solution would be to use GlobalScope like this
class MyClass(private val valueClass: ValueClass) {
fun listenToValue = flow<String> {
valueClass.registerListener { value ->
GlobalScope.launch {
emit(value)
}
}
}
}
Now, this works but I don't want to use GlobalScope since I'll be using viewModelScope to tie it to my app's lifecycle.
Is there any way to work around this?
Thanks in advance. Any help would be greatly appreciated!

You can use callbackFlow to create a Flow from the callback. It will look something like:
fun listenToValue(): Flow<String> = callbackFlow {
valueClass.registerListener { value ->
trySend(value)
channel.close() // close channel if no more values are expected
}
awaitClose { /*unregister listener*/ }
}
Or if only one value is expected from the callback, you can use suspendCoroutine or suspendCancellableCoroutine. It this case listenToValue() function must be suspend and later called from a coroutine(e.g. someScope.launch):
suspend fun listenToValue(): String = suspendCoroutine { continuation ->
valueClass.registerListener { value ->
continuation.resumeWith(value)
}
}

Flakiness in tests on Android using LiveData, RxJava/RxKotlin and Spek

Setup:
In our project (at work - I cannot post real code), we have implemented clean MVVM. Views communicate with ViewModels via LiveData. ViewModel hosts two kinds of use cases: 'action use cases' to do something, and 'state updater use cases'. Backward communication is asynchronous (in terms of action reaction). It's not like an API call where you get the result from the call. It's BLE, so after writing the characteristic there will be a notification characteristic we listen to. So we use a lot of Rx to update the state. It's in Kotlin.
ViewModel:
#PerFragment
class SomeViewModel #Inject constructor(private val someActionUseCase: SomeActionUseCase,
someUpdateStateUseCase: SomeUpdateStateUseCase) : ViewModel() {
private val someState = MutableLiveData<SomeState>()
private val stateSubscription: Disposable
// region Lifecycle
init {
stateSubscription = someUpdateStateUseCase.state()
.subscribeIoObserveMain() // extension function
.subscribe { newState ->
someState.value = newState
})
}
override fun onCleared() {
stateSubscription.dispose()
super.onCleared()
}
// endregion
// region Public Functions
fun someState() = someState
fun someAction(someValue: Boolean) {
val someNewValue = if (someValue) "This" else "That"
someActionUseCase.someAction(someNewValue)
}
// endregion
}
Update state use case:
#Singleton
class UpdateSomeStateUseCase #Inject constructor(
private var state: SomeState = initialState) {
private val statePublisher: PublishProcessor<SomeState> =
PublishProcessor.create()
fun update(state: SomeState) {
this.state = state
statePublisher.onNext(state)
}
fun state(): Observable<SomeState> = statePublisher.toObservable()
.startWith(state)
}
We are using Spek for unit tests.
#RunWith(JUnitPlatform::class)
class SomeViewModelTest : SubjectSpek<SomeViewModel>({
setRxSchedulersTrampolineOnMain()
var mockSomeActionUseCase = mock<SomeActionUseCase>()
var mockSomeUpdateStateUseCase = mock<SomeUpdateStateUseCase>()
var liveState = MutableLiveData<SomeState>()
val initialState = SomeState(initialValue)
val newState = SomeState(newValue)
val behaviorSubject = BehaviorSubject.createDefault(initialState)
subject {
mockSomeActionUseCase = mock()
mockSomeUpdateStateUseCase = mock()
whenever(mockSomeUpdateStateUseCase.state()).thenReturn(behaviorSubject)
SomeViewModel(mockSomeActionUseCase, mockSomeUpdateStateUseCase).apply {
liveState = state() as MutableLiveData<SomeState>
}
}
beforeGroup { setTestRxAndLiveData() }
afterGroup { resetTestRxAndLiveData() }
context("some screen") {
given("the action to open the screen") {
on("screen opened") {
subject
behaviorSubject.startWith(initialState)
it("displays the initial state") {
assertEquals(liveState.value, initialState)
}
}
}
given("some setup") {
on("some action") {
it("does something") {
subject.doSomething(someValue)
verify(mockSomeUpdateStateUseCase).someAction(someOtherValue)
}
}
on("action updating the state") {
it("displays new state") {
behaviorSubject.onNext(newState)
assertEquals(liveState.value, newState)
}
}
}
}
}
At first we were using an Observable instead of the BehaviorSubject:
var observable = Observable.just(initialState)
...
whenever(mockSomeUpdateStateUseCase.state()).thenReturn(observable)
...
observable = Observable.just(newState)
assertEquals(liveState.value, newState)
instead of the:
val behaviorSubject = BehaviorSubject.createDefault(initialState)
...
whenever(mockSomeUpdateStateUseCase.state()).thenReturn(behaviorSubject)
...
behaviorSubject.onNext(newState)
assertEquals(liveState.value, newState)
but the unit test were being flaky. Mostly they would pass (always when ran in isolation), but sometime they would fail when running the whole suit. Thinking it is to do with asynchronous nature of the Rx we moved to BehaviourSubject to be able to control when the onNext() happens. Test are now passing when we run them from AndroidStudio on the local machine, but they are still flaky on the build machine. Restarting the build often makes them pass.
The tests which fail are always the ones where we assert the value of LiveData. So the suspects are LiveData, Rx, Spek or their combination.
Question: Did anyone have similar experiences writing unit tests with LiveData, using Spek or maybe Rx, and did you find ways to write them which solve these flakiness issues?
....................
Helper and extension functions used:
fun instantTaskExecutorRuleStart() =
ArchTaskExecutor.getInstance().setDelegate(object : TaskExecutor() {
override fun executeOnDiskIO(runnable: Runnable) {
runnable.run()
}
override fun isMainThread(): Boolean {
return true
}
override fun postToMainThread(runnable: Runnable) {
runnable.run()
}
})
fun instantTaskExecutorRuleFinish() = ArchTaskExecutor.getInstance().setDelegate(null)
fun setRxSchedulersTrampolineOnMain() = RxAndroidPlugins.setInitMainThreadSchedulerHandler { Schedulers.trampoline() }
fun setTestRxAndLiveData() {
setRxSchedulersTrampolineOnMain()
instantTaskExecutorRuleStart()
}
fun resetTestRxAndLiveData() {
RxAndroidPlugins.reset()
instantTaskExecutorRuleFinish()
}
fun <T> Observable<T>.subscribeIoObserveMain(): Observable<T> =
subscribeOnIoThread().observeOnMainThread()
fun <T> Observable<T>.subscribeOnIoThread(): Observable<T> = subscribeOn(Schedulers.io())
fun <T> Observable<T>.observeOnMainThread(): Observable<T> =
observeOn(AndroidSchedulers.mainThread())

I didn't used Speck for unit-testing. I've used java unit-test platform and it works perfect with Rx & LiveData, but you have to keep in mind one thing. Rx & LiveData are async and you can't do something like someObserver.subscribe{}, someObserver.doSmth{}, assert{} this will work sometimes but it's not the correct way to do it.
For Rx there's TestObservers for observing Rx events. Something like:
#Test
public void testMethod() {
TestObserver<SomeObject> observer = new TestObserver()
someClass.doSomethingThatReturnsObserver().subscribe(observer)
observer.assertError(...)
// or
observer.awaitTerminalEvent(1, TimeUnit.SECONDS)
observer.assertValue(somethingReturnedForOnNext)
}
For LiveData also, you'll have to use CountDownLatch to wait for LiveData execution. Something like this:
#Test
public void someLiveDataTest() {
CountDownLatch latch = new CountDownLatch(1); // if you want to check one time exec
somethingTahtReturnsLiveData.observeForever(params -> {
/// you can take the params value here
latch.countDown();
}
//trigger live data here
....
latch.await(1, TimeUnit.SECONDS)
assert(...)
}
Using this approach your test should run ok in any order on any machine. Also the wait time for latch & terminal event should be as low as possible, the tests should run fast.
Note1: The code is in JAVA but you can change it easily in kotlin.
Note2: Singleton are the biggest enemy of unit-testing ;). (With static methods by their side).

The issue is not with LiveData; it is the more common problem - singletons. Here the Update...StateUseCases had to be singletons; otherwise if observers got a different instance they would have a different PublishProcessor and would not get what was published.
There is a test for each Update...StateUseCases and there is a test for each ViewModel into which Update...StateUseCases is injected (well indirectly via the ...StateObserver).
The state exists within the Update...StateUseCases, and since it is a singleton, it gets changed in both tests and they use the same instance becoming dependent on each other.
Firstly try to avoid using singletons if possible.
If not, reset the state after each test group.