Develop Reference

suspend IO function never return

I have difficulties writing an UDP message receive loop for Android.
In the following code, in receiveLoop, the call to receiveMessages never returns and I therefore never enter the message treatment loop.
Note that I am still able to receive packets, but it stops when the channel buffer is full.
I would expect receiveMessages to return immediately, while the blocking IO loop inside it would still run forever.
class MySocketUDP(private val params: SocketParams) {
private val rcvSocket: DatagramSocket by lazy {
val sock = DatagramSocket(params.rcvPort)
sock.reuseAddress = true
sock.soTimeout = 1000
sock
}
suspend fun receiveMessages(channel: SendChannel<Message>) {
withContext(Dispatchers.IO) {
val buf = ByteArray(MAX_MSG_SIZE)
while (true) {
val pkt = DatagramPacket(buf, buf.size)
try {
if (channel.isClosedForSend) {
break
}
rcvSocket.receive(pkt)
val msg = packetToMessage(buf, 0, pkt.length)
Log.d("SOCKET", "filling channel with $msg")
channel.send(msg)
} catch (ex: SocketTimeoutException) {
} catch (ex: CancellationException) {
break
}
}
}
}
}
class MyModel {
private suspend fun receiveLoop(socket: MySocketUDP) {
withContext(Dispatchers.Main) {
val channel = Channel<Message>(16)
socket.receiveMessages(channel)
Log.d("MODEL", "Entering msg loop")
for (msg in channel) {
dispatchRcvMessage(msg)
}
}
}
}
Why does receiveMessages never return while it is running in the IO dispatcher and called from the Main dispatcher?
Do I need to actually spawn a thread to such producer/consumer work?
Can you show how to achieve such long blocking code nicely in a "coroutine-friendly" manner?
Thank you

receiveMessages() is a suspend function which calls another suspend function withContext(), which in turn has an infinite loop. So calling socket.receiveMessages(channel) will suspend code execution while the loop is not finished.
You need to launch separate coroutines for consumer and producer, e.g. using launch function.
Some example of using coroutines:
val someScope = CoroutineScope(Dispatchers.Main)
private suspend fun receiveLoop(socket: MySocketUDP) = someScope.launch {
val channel = Channel<Message>(16)
socket.receiveMessages(channel)
Log.d("MODEL", "Entering msg loop")
for (msg in channel) {
dispatchRcvMessage(msg)
}
}
// In MySocketUDP
suspend fun receiveMessages(channel: SendChannel<Message>) {
someAnotherScope.launch { // or can use coroutineScope builder function
val buf = ByteArray(MAX_MSG_SIZE)
while (true) {
val pkt = DatagramPacket(buf, buf.size)
try {
if (channel.isClosedForSend) {
break
}
rcvSocket.receive(pkt)
val msg = packetToMessage(buf, 0, pkt.length)
Log.d("SOCKET", "filling channel with $msg")
channel.send(msg)
} catch (ex: SocketTimeoutException) {
} catch (ex: CancellationException) {
break
}
}
}
}

No exception/error when no internet coroutine + retrofit

I have the following setup
Service
// ItunesService
suspend fun searchItunesPodcast(#Query("term") term: String): Response<PodcastResponse>
Repository
// ItunesRepo
override suspend fun searchByTerm(term: String) = withContext(ioDispatcher) {
return#withContext itunesService.searchItunesPodcast(term)
}
ViewModel
fun searchPodcasts(term: String) {
viewModelScope.launch {
_res.value = Result.loading()
try {
val response = itunesRepo.searchByTerm(term)
if (response.isSuccessful) { // Nothing from here when no internet
_res.value = Result.success(response.body())
} else {
_res.value = Result.error(response.errorBody().toString())
}
} catch (e: Exception) {
_res.value = Result.exception(e)
}
}
}
Everything works great until i turn off mobile data/internet on my testing device. _res value stuck on Loading state. I have tried adding break point at if (response.isSuccessful) when there is no internet and it seams like val response = itunesRepo.searchByTerm(term) never returns how can I fix this

I switched to using Flow api on my Repository
override suspend fun searchPodcasts(term: String) = flow {
emit(Result.Loading)
try {
val res = itunesService.searchItunesPodcast(term)
if (res.isSuccessful)
emit(Result.Success(res.body()))
else
emit(Result.Error("Generic error: ${res.code()}"))
} catch (e: Exception) {
emit(Result.Error("Unexpected error", e))
}
}.flowOn(ioDispatcher)
Then collect the results on my ViewModels

how to handle network exception using coroutine?

I try handling exception using coroutine. I wrote code like this, but didn't work. I can't see any log except for using try-catch. I do not want to use try catch at all function, but want to make clean code handling exception. what should I do for this?
viewmodel
private val handler = CoroutineExceptionHandler { _, exception ->
when (exception) {
is UnknownHostException -> {
showLog("login UnknownHostException : " +exception.message)
}
else -> {
}
}
}
fun login(mobile:String){
viewModelScope.launch(handler) {
try{
var login = apiRepository.login(mobile)
_isLogin.value = login
}catch(e:Exception){
}
}
}
repository
override suspend fun login(mobile: String): LoginResultData {
var result =LoginResultData()
withContext(ioDispatcher){
val request = apiServerModel.login(mobile)
val response = request.await()
result = response
}
return result
}

fun login(mobile:String){
viewModelScope.launch(handler) {
val login = apiRepository.login(mobile)
_isLogin.value = login
}
}

how can i wrap this (AdsWizz) Kotlin callback function in a couroutine?

I'm new to coroutines and having a hard time figuring out how to correctly wrap an existing callback in a coroutine.
My goal is to be able to do the following:
lifecycleScope.launch {
withContext(Dispatchers.Main) {
val theResult = getPreRollAd() //1. call this suspending func and wait for result
doSomethingWithResult(theResult) //2. now that the result is returned from AdsWizz API (below), do something with it
}
}
Here is the AdsWizz API call that I'd like to "wrap":
val adReqInterface: AdRequestHandlerInterface = object :
AdRequestHandlerInterface {
override fun onResponseError(error: AdswizzSDKError) {
Timber.e("onResponseError $error")
}
override fun onResponseReady(adResponse: AdResponse) {
Timber.d( "onResponseReadySingleAd")
//this contains the url to the ad, title, etc..
!!!*** I WANT TO RETURN THE adResponse.mediaFile?.source string back to "theResult" variable above (in lifecycleScope.launch {.... )
}
}
try {
AdswizzSDK.getAdsLoader().requestAd(adReqParams, adReqInterface)
} catch (e: IllegalArgumentException) {
Timber.d( "IllegalArgumentException")
} catch (e: SecurityException) {
Timber.d( "SecurityException")
} catch (e: Exception) {
Timber.d( "other exception")
e.printStackTrace()
}
I've tried using suspendCoroutine {... to wrap but nothing is working. Really appreciate someones help re the right way to achieve this.

the right way to do it is to use suspendCancellableCoroutine. It can return a result or can be cancelled with an exception.
suspend fun getPreRollAd(): AdResponse {
return suspendCancellableCoroutine {
...
val adReqInterface: AdRequestHandlerInterface = object : AdRequestHandlerInterface {
override fun onResponseError(error: AdswizzSDKError) {
Timber.e("onResponseError $error")
it.cancel(error)
}
override fun onResponseReady(adResponse: AdResponse) {
Timber.d( "onResponseReadySingleAd")
it.resume(adResponse)
}
}
AdswizzSDK.getAdsLoader().requestAd(adReqParams, adReqInterface)
}
}
viewModelScope.launch {
val result = try {
getPreRollAd()
} catch(e: Throwable) {
null
}
...
}

Handle no internet connection error of retrofit 2.6 with kotlin coroutines

I'm using retrofit 2.6 with kotlin coroutines to make API call without block the UI thread, I got it work but the app crashes when I switch off the internet connection. The logcat error is: E/AndroidRuntime: FATAL EXCEPTION: DefaultDispatcher-worker-1
Here is my code:
private fun handleIntent(slug: String) {
val service = UtilityMethods.migrationTimeService()
UtilityMethods.showView(loading_view)
UtilityMethods.hideView(network_error_msg)
CoroutineScope(Dispatchers.IO).launch {
val res = service.getPostBySlug(slug)
try {
withContext(Dispatchers.Main) {
//Do something with response e.g show to the UI.
val post = res.body()!!.first()
UtilityMethods.hideView(loading_view)
val title = post.title?.rendered
val content = post.content?.rendered
val imageUrl = post.jetPackFeaturedMediaUrl
title_txtView.text = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N)
Html.fromHtml(title, Html.FROM_HTML_MODE_COMPACT).toString()
else
Html.fromHtml(title).toString()
content_txtView.loadData(content.toString(), "text/html", "UTF-8")
Picasso.get().load(imageUrl).fit().centerCrop().into(thumbnail_imgview)
}
} catch (e: HttpException) {
UtilityMethods.showView(network_error_msg)
} catch (e: Throwable) {
Toast.makeText(this#PostContentActivity, "Ooops: Something else went wrong", Toast.LENGTH_LONG)
}
}
}

I've got the code working, the new code is:
private fun handleIntent(slug: String) = GlobalScope.launch(Dispatchers.Main) {
val service = UtilityMethods.migrationTimeService()
UtilityMethods.showView(loading_view)
UtilityMethods.hideView(network_error_msg)
try {
val res = withContext(Dispatchers.IO) {
service.getPostBySlug(slug)
}
//Do something with response e.g show to the UI.
val post = res.body()!!.first()
UtilityMethods.hideView(loading_view)
val title = post.title?.rendered
val content = post.content?.rendered
val imageUrl = post.jetPackFeaturedMediaUrl
title_txtView.text = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N)
Html.fromHtml(title, Html.FROM_HTML_MODE_COMPACT).toString()
else
Html.fromHtml(title).toString()
content_txtView.loadData(content.toString(), "text/html", "UTF-8")
Picasso.get().load(imageUrl).fit().centerCrop().into(thumbnail_imgview)
}
catch (e: HttpException) {
Toast.makeText(this#PostContentActivity, "Exception ${e.message}", Toast.LENGTH_LONG).show()
}catch (e: IOException) {
UtilityMethods.hideView(loading_view)
UtilityMethods.showView(network_error_msg)
} catch (e: Throwable) {
Toast.makeText(this#PostContentActivity, "Ooops: Something else went wrong ${e.message}", Toast.LENGTH_LONG).show()
}
}

So while looking into stacktrace I found that ConnectException is thrown when network is unavailable
And that's how I do it in kotlin and it works for me,
suspend fun<T: Any> safeAPICall(call: suspend () -> Response<T>) : T{
val response = try {
call.invoke()
}
catch (e:java.lang.Exception){
e.printStackTrace()
val message = if( e is ConnectException) "Connection Error" else "Something went wrong. Please try again."
throw IOException(ResponseError(message, 500).convertToJsonString())
}
// When connection is OK
if(response.isSuccessful){
return response.body()!!
}else{
val error = response.errorBody()?.string()
error?.let{
val message = JSONObject(it).optString("message", "Something went wrong")
val responseError = ResponseError(message, response.code())
throw IOException(responseError.convertToJsonString())
}
throw IOException(ResponseError("Something went wrong. Please try again.", 500).convertToJsonString())
}
}
The data class that I use
data class ResponseError(val message:String, val errorCode:Int)
Usage:
try {
val response = safeAPICall {APIClient.planner.viewSites(view.context.authToken)}
}
catch (e:Exception){
view.snack(e.message?.toModel<ResponseError>()?.message?: unspecified_error)
}
Bonus:
inline fun <reified T> JSONObject.toModel(): T? = this.run {
try {
Gson().fromJson<T>(this.toString(), T::class.java)
}
catch (e:java.lang.Exception){ e.printStackTrace(); null }
}
inline fun <reified T> String.toModel(): T? = this.run {
try {
JSONObject(this).toModel<T>()
}
catch (e:java.lang.Exception){ null }
}

Instead of this:
CoroutineScope(Dispatchers.IO).launch {
val res = service.getPostBySlug(slug)
try {
withContext(Dispatchers.Main) {
Try this one:
CoroutineScope(Dispatchers.Main).launch {
val res = service.getPostBySlug(slug)
withContext(Dispatchers.IO) {
try {
wrap your 'try and catch' block code within Dispatchers.IO instead of wraping your Dispatchers.IO with in yout try block