problem with use Mvvm and Retrofit and Rxjava live data in kotlin - kotlin

Question

I'm new in kotlin and I'm trying to use retrofit with Rxjava and live data in MVVM architecture.
I config retrofit, and also use observable and subscribe in ViewModel to make observable variable to use in activity binding layout.
I have a button in my view and when I click on it, the request method gets to start and subscription write a log of its own data. but my variable gets null at first and after seconds, when retrofit completed its task, my variable gets data but log value doesn't update.
this is my retrofit initialize class
class ApiService {
private val INSTANCE =
Retrofit.Builder()
.baseUrl("http://www.janbarar.ir/App/")
.addConverterFactory(GsonConverterFactory.create())
.build()
.create(IRetrofitMethods::class.java)
private fun <T> callBack(iDataTransfer: IDataTransfer<T>) =
object : Callback<T> {
override fun onResponse(call: Call<T>, response: Response<T>) {
val data = response.body()
if (data != null)
iDataTransfer.onSuccess(data)
else
try {
throw Exception("data is empty")
} catch (ex: Exception) {
iDataTransfer.onError(ex)
}
}
override fun onFailure(call: Call<T>, t: Throwable) {
iDataTransfer.onError(t)
}
}
fun getCategories(iDataTransfer: IDataTransfer<List<Category>>) =
INSTANCE.getCategories().enqueue(callBack(iDataTransfer))
this is an interface for retrofit
#GET("GetCategories")
fun getCategories(): Call<List<Category>>
this is my model class. I think the problem is here. because the observable send null data before retrofit finish its work.
fun getCategories(): Observable<ArrayList<Category>> {
val result = arrayListOf<Category>()
api.getCategories(object : IDataTransfer<List<Category>> {
override fun onSuccess(data: List<Category>) {
result.addAll(data)
}
override fun onError(throwable: Throwable) {
Log.e("Model", throwable.message!!)
}
})
return Observable.just(result)
}
and this is also my ViewModel class
class ProductViewModel(private val model: ProductModel) : ViewModel() {
var isLoading = ObservableField(false)
var categoryList = MutableLiveData<ArrayList<Category>>()
private var compositeDisposable = CompositeDisposable()
fun getCategories() {
isLoading.set(true)
compositeDisposable +=
model.getCategories()
.subscribeOn(Schedulers.io())
.observeOn(AndroidSchedulers.mainThread())
.subscribe({
categoryList.value = it
}, {
Log.e("ViewModel", it.message.toString())
})
isLoading.set(false)
}
finally, it's my activity
lateinit var binding: ActivityMainBinding
private val vm: ProductViewModel by viewModel()
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
binding = DataBindingUtil.setContentView(this, R.layout.activity_main)
binding.vm = vm
vm.categoryList.observe(this, Observer {
if (it != null)
Log.e("activity", it.toString())
})
}

Answer 1

As ExpensiveBelly mentioned in a comment, Retrofit provides a call adapter for RxJava, so you can let your API return Observable<List<Category>> directly. To do this, you will need to add the RxJava call adapter dependency to your app module's build.gradle:
implementation 'com.squareup.retrofit2:adapter-rxjava2:(version)'
Add the call adapter factory when constructing your Retrofit instance:
private val INSTANCE =
Retrofit.Builder()
.baseUrl("http://www.janbarar.ir/App/")
.addCallAdapterFactory(RxJava2CallAdapterFactory.create()) // add this
.addConverterFactory(GsonConverterFactory.create())
.build()
.create(IRetrofitMethods::class.java)
And then just let your service return an Observable directly:
#GET("GetCategories")
fun getCategories(): Observable<List<Category>>
If ApiService needs to do some handling before the rest of the app gets the response, you can use RxJava operators like map.
It would be illustrative to see why your code doesn't work and how to fix it. When you call api.getCategories(someCallback), one of your callback methods will be called at some point in the future. In the meantime, the model.getCategories() method will return immediately.
When you subscribe to the returned Observable, it emits the result variable, which is currently an empty list. result will eventually have some data in it, but your code will not be informed of this at all.
What you really want to do is emit the list of categories when it becomes available. The standard way to get an Observable from a callback API is with Observable.create:
fun getCategories(): Observable<ArrayList<Category>> {
return Observable.create { emitter ->
api.getCategories(object : IDataTransfer<List<Category>> {
override fun onSuccess(data: List<Category>) {
emitter.onNext(data)
emitter.onComplete()
}
override fun onError(throwable: Throwable) {
emitter.onError(throwable)
}
})
}
}
Of course, it's better to just use RxJava2CallAdapterFactory if possible, since this work has already been done there.