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How to handle error response return from server api in kotlin+retrofit+coroutine+mvvm

I have two different response from the same endpoint. One being the actual success result data model and one being an error response model. Both json structure like this:
SuccessResponse:
{
"result":{
"id":1,
"name_en":"Stack Over Flow",
"summary":"Stack Overflow is the largest, most trusted online community for developers to learn, share​ ​their programming ​knowledge, and build their careers."
}
}
ErrorResponse:
{
"message": "Login Failed"
}
I can handle the success response but I can't show the error message what I get from the server. I have tried many ways but I can't do this.
Here my I share my some aspect what I did
MainViewModel.kt
var job: Job? = null
val myDataResponse: MutableLiveData<HandleResource<DataResponse>> =MutableLiveData()
fun myData() {
job = CoroutineScope(Dispatchers.IO).launch {
val myDataList = mainRepository.myData()
withContext(Dispatchers.Main) {
myDataResponse.postValue(handleMyDataResponse(myDataList))
}
}
}
private fun handleMyDataResponse(myDataResponse: Response<DataResponse>): HandleResource<DataResponse>? {
if (myDataResponse.isSuccessful) {
myDataResponse.body()?.let { myDataData ->
return HandleResource.Success(myDataData)
}
}
return HandleResource.Error(myDataResponse.message())
}
I need a solution while server give me error message I want to show same error message on my front side. How can I achieve this?

private fun handleMyDataResponse(myDataResponse: Response<DataResponse>): HandleResource<DataResponse>? {
myDataResponse.body()?.let { myDataData ->
if (myDataResponse.code() == 200) {
return HandleResource.Success(myDataData )
} else {
val rawResponse = myDataData.string()
return HandleResource.Error(getErrorMessage(rawResponse))
}
}
}
fun getErrorMessage(raw: String): String{
val object = JSONObject(raw);
return object.getString("message");
}
The body of the response (be it success or failure) is response.body(). And if you want to get it as a String, then call response.body().string(). Since you want to read message object from the response you need to convert it into Json.

If you are a following MVVM pattern then I suggest to create a sealed class for the API calls.
To handle api success and failure or network issue. Resource class is going to be generic because it will handle all kind of api response
sealed class Resource<out T> {
data class Success<out T>(val value: T): Resource<T>()
data class Failure(
val isNetworkErro: Boolean?,
val errorCode: Int?,
val errorBody: ResponseBody?
): Resource<Nothing>()
}
on the base repository while calling the API, you can return the resource whether it is success or failure.
abstract class BaseRepository {
suspend fun <T> safeApiCall(
apiCall: suspend () -> T
): Resource<T>{
return withContext(Dispatchers.IO){
try {
Resource.Success(apiCall.invoke())
} catch (throwable: Throwable){
when (throwable){
is HttpException -> {
Resource.Failure(false,throwable.code(), throwable.response()?.errorBody())
}
else ->{
Resource.Failure(true, null, null)
}
}
}
}
}
}
If you follow this pattern you'll be able to handle all the failure and success response, I hope this will help.

Kotlin coroutines crash with no helpful stacktrace

My Android app crashes and I see this stack trace in Logcat. It doesn't tell me which line of code is causing the problem.
2021-05-05 09:13:33.143 1069-1069/com.mycompany.app E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.mycompany.app, PID: 1069
retrofit2.HttpException: HTTP 403
at retrofit2.KotlinExtensions$await$2$2.onResponse(KotlinExtensions.kt:53)
at retrofit2.OkHttpCall$1.onResponse(OkHttpCall.java:161)
at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
at java.lang.Thread.run(Thread.java:919)
Is there a way to map this back to my code, to see which call to retrofit is causing it? I have a repository with code like this:
suspend fun getSomeData(): Stuff {
return withContext(Dispatchers.IO) {
val body = myRetroApi.getStuff()
...
Do I need to wrap every withContext body to make sure no Throwables escape? I thought that if something threw an exception there, it would log an error, not crash the entire app.
Edit
I messed up when asking this question and put the emphasis on wrong things. So I'm removing the "retrofit" tag. It turns out the withContext(Dispatchers.IO) call does re-throw the Exception as expected, but when the exception gets back up to viewModelScope.launch, if that block does not catch it, the app crashes.

If the exception is not handled the app will crash of course.
You can add a try catch to avoid this:
suspend fun getSomeData() {
withContext(Dispatchers.IO) {
try{
val body = myRetroApi.getStuff()
...
} catch (e : Exception){
//your code
}
...

Retrofit is giving you a 403 Unauthorized HTTP exception. It may be that the server isn't passing any additional error message or that you need to catch HttpException and check for the message. In either case, this isn't a Retrofit issue hence it's just passing the error it's getting from the server you're calling.
It's best to create a network result wrapper and a wrapper function for API calls to handle exceptions.
You can do something like this. Keep in mind, the actual implementation is completely up to you. I would however suggest using runCatching when it comes to couroutines as it handles cancellation exceptions.
sealed class NetworkResult<out T> {
data class Success<T>(val data: T) : NetworkResult<T>()
data class Error(val exception: Throwable, val message: String?) : NetworkResult<Nothing>()
}
suspend fun networkCall(): String = ""
suspend fun <T> safeApiCall(block: suspend () -> T): NetworkResult<T> {
return runCatching {
withContext(Dispatchers.IO) {
block()
}
}.fold({
NetworkResult.Success(it)
}, {
when (it) {
is HttpException -> NetworkResult.Error(it, "Network error")
else -> NetworkResult.Error(it, "Some other message...")
// else -> throw it
}
})
}
suspend fun getData() {
val result: NetworkResult<String> = safeApiCall {
networkCall()
}
when (result) {
is NetworkResult.Success -> {
//Handle success
}
is NetworkResult.Error -> { //Handle error
}
}
}
runCatching uses Kotlin's built-in Result class and there are several ways of handling the result. These are just a few.
runCatching {
//.....
}.getOrElse { throwable ->
//handle exception
}
runCatching {
//.....
}.getOrThrow()
runCatching {
}.onSuccess {
}.onFailure {
}

How to get API error body using Kotlin coroutines

I am trying to do POST request with retrofit and coroutines,
and whenever the HTTP response status is not 200, it just throws an exception. My API has a response body that looks like this:
{
"auth_token": null,
"error": "Email can't be blank. Password can't be blank. First name can't be blank. Last name can't
be blank. Date of birth can't be blank"
}
I want to catch this error message WITHOUT CALLBACKS, is that possible ?
Api service class
#GET("flowers")
suspend fun getAllFlowers(#Query("page") page: Int): Flowers
#POST("users/register")
suspend fun registerUser(#Body user: User) : NetworkResponse
Network response class
class NetworkResponse(
val auth: String,
val error: String)
ViewModel class
fun registerUser(user: User) {
coroutineScope.launch {
try {
val registerUser = FlowerApi.retrofitService.registerUser(user)
_token.value = registerUser.auth
} catch (cause: Throwable) {
//CATCH EXCEPTION
}
}
}
Thanks in advance

Inside your catch block you need to check if the throwable is an HttpException (from Retrofit package) and then you can find the error body from it by calling cause.response()?.errorBody()?.string():
catch (cause: Throwable) {
when (cause) {
is HttpException -> {
cause.response()?.errorBody()?.string() //retruns the error body
}
else -> {
//Other errors like Network ...
}
}
}

Making synchronous calls to Cloud Firestore when running off the main thread

I am building an app based off of the Android Clean Architecture Kotlin version (https://github.com/android10/Android-CleanArchitecture-Kotlin).
Using this architecture, each time you want to invoke a use case, a Kotlin coroutine is launched and the result is posted in the main thread. This is achieved by this code:
abstract class UseCase<out Type, in Params> where Type : Any {
abstract suspend fun run(params: Params): Either<Failure, Type>
fun execute(onResult: (Either<Failure, Type>) -> Unit, params: Params) {
val job = async(CommonPool) { run(params) }
launch(UI) { onResult.invoke(job.await()) }
}
In his example architecture, Mr. Android10 uses Retrofit to make a synchronous api call inside the kotlin couroutine. For example:
override fun movies(): Either<Failure, List<Movie>> {
return when (networkHandler.isConnected) {
true -> request(service.movies(), { it.map { it.toMovie() } }, emptyList())
false, null -> Left(NetworkConnection())
}
}
private fun <T, R> request(call: Call<T>, transform: (T) -> R, default: T): Either<Failure, R> {
return try {
val response = call.execute()
when (response.isSuccessful) {
true -> Right(transform((response.body() ?: default)))
false -> Left(ServerError())
}
} catch (exception: Throwable) {
Left(ServerError())
}
}
'Either' represents a disjoint type, meaning the result will either be a Failure or the object of type T you want.
His service.movies() method is implemented like so (using retrofit)
#GET(MOVIES) fun movies(): Call<List<MovieEntity>>
Now here is my question. I am replacing retrofit with Google Cloud Firestore. I know that currently, Firebase/Firestore is an all async library. I want to know if anyone knows of a method more elegant way of making a synchronous API call to Firebase.
I implemented my own version of Call:
interface Call<T: Any> {
fun execute(): Response<T>
data class Response<T>(var isSuccessful: Boolean, var body: T?, var failure: Failure?)
}
and my API call is implemented here
override fun movieList(): Call<List<MovieEntity>> = object : Call<List<MovieEntity>> {
override fun execute(): Call.Response<List<MovieEntity>> {
return movieListResponse()
}
}
private fun movieListResponse(): Call.Response<List<MovieEntity>> {
var response: Call.Response<List<MovieEntity>>? = null
FirebaseFirestore.getInstance().collection(DataConfig.databasePath + MOVIES_PATH).get().addOnCompleteListener { task ->
response = when {
!task.isSuccessful -> Call.Response(false, null, Failure.ServerError())
task.result.isEmpty -> Call.Response(false, null, MovieFailure.ListNotAvailable())
else -> Call.Response(true, task.result.mapTo(ArrayList()) { MovieEntity.fromSnapshot(it) }, null)
}
}
while (response == null)
Thread.sleep(50)
return response as Call.Response<List<MovieEntity>>
}
Of course, the while loop at the end bothers me. Is there any other, more elegant ways, to wait for the response to be assigned before returning from the movieListResponse method?
I tried calling await() on the Task that is returned from the Firebase get() method, but the movieListResponse method would return immediately anyway. Thanks for the help!

So I found what I was looking for in the Google Tasks API: "If your program is already executing in a background thread you can block a task to get the result synchronously and avoid callbacks" https://developers.google.com/android/guides/tasks#blocking
So my previous problematic code becomes:
private fun movieListResponse(): Call.Response<List<MovieEntity>> {
return try {
val taskResult = Tasks.await(FirebaseFirestore.getInstance().
collection(DataConfig.databasePath + MOVIES_PATH).get(), 2, TimeUnit.SECONDS)
Call.Response(true, taskResult.mapTo(ArrayList()) { MovieEntity.fromSnapshot(it) }, null)
} catch (e: ExecutionException) {
Call.Response(false, null, Failure.ServerError())
} catch (e: InterruptedException) {
Call.Response(false, null, Failure.InterruptedError())
} catch (e: TimeoutException) {
Call.Response(false, null, Failure.TimeoutError())
}
}
Note I no longer need my Thread.sleep while loop.
This code should only be run in a background thread/kotlin coroutine.

This is overengineered, there are several layers trying to do the same thing. I suggest you go back a few steps, undo the abstractions and get into the mood of using coroutines directly. Implement a suspend fun according to this template. You don't need the crutches of Either, handle exceptions in the most natural way: a try-catch around a suspend fun call.
You should end up with a signature as follows:
suspend fun movieList(): List<MovieEntity>
Call site:
launch(UI) {
try {
val list = movieList()
...
} catch (e: FireException) {
// handle
}
}

That's is not the way how firebase works. Firebase is based on callback.
I recommend architecture component's livedata.
Please check the following example.
here is a link: https://android.jlelse.eu/android-architecture-components-with-firebase-907b7699f6a0

Retrofit and Moshi: how to handle com.squareup.moshi.JsonDataException

This scene takes place in an Android app using Retrofit2 and Moshi for JSON deserialization.
In a case where you don't have control over the server's implementation, and this said server have an inconsistent behavior in how it answers requests (also know as "a bad case"):
Is there a way to handle com.squareup.moshi.JsonDataException without crashing?
For example you expected a JSONArray, and here comes a JSONObject. Crash. Is there another way to handle this than having the app crashing?
Also in the case the server's implementation is updated, wouldn't it be better to display an error message to the user, instead of having it to crash / be totally out of service, even for one wrong request?

Make the call with Retrofit and use try and catch to handle exceptions, something similar to:
class NetworkCardDataSource(
private val networkApi: NetworkCardAPI,
private val mapper: CardResponseMapper,
private val networkExceptionMapper: RetrofitExceptionMapper,
private val parserExceptionMapper: MoshiExceptionMapper
) : RemoteCardDataSource {
override suspend fun getCard(id: String): Outcome<Card, Throwable> = withContext(Dispatchers.IO) {
val response: Response<CardResponseJson>
return#withContext try {
response = networkApi.getCard(id)
handleResponse(
response,
data = response.body(),
transform = { mapper.mapFromRemote(it.card) }
)
} catch (e: JsonDataException) {
// Moshi parsing error
Outcome.Failure(parserExceptionMapper.getException(e))
} catch (e: Exception) {
// Retrofit error
Outcome.Failure(networkExceptionMapper.getException(e))
}
}
private fun <Json, D, L> handleResponse(response: Response<Json>, data: D?, transform: (D) -> L): Outcome<L, Throwable> {
return if (response.isSuccessful) {
data?.let {
Outcome.Success(transform(it))
} ?: Outcome.Failure(RuntimeException("JSON cannot be deserialized"))
} else {
Outcome.Failure(
HTTPException(
response.message(),
Exception(response.raw().message),
response.code(),
response.body().toString()
)
)
}
}
}
where:
networkApi is your Retrofit object,
mapper is a class for mapping the received object to another one used in your app (if needed),
networkExceptionMapper and parserExceptionMapper map Retrofit and Moshi exceptions, respectively, to your own exceptions so that Retrofit and Moshi exceptions do not spread all over your app (if needed),
Outcome is just a iOS Result enum copy to return either a Success or a Failure result but not both,
HTTPException is a custom Runtime exception to return unsuccessful request.
This a snippet from a clean architecture example project.