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Switch from Main dispatcher to IO dispatcher from non lifecycle class

I m using coroutines in my android app, and i have this function that need to communicate with UI and Main thread.
private suspend fun init() : RequestProcessor<LocalData, ApiResult, ApiError>
{
#Suppress("LeakingThis")
_localData = listener.databaseCall()
withContext(Dispatchers.IO) {
if (_localData == null)
{
checkIfShouldFetch(null, null)
}
else
{
withContext(Dispatchers.Main) {
mediatorLiveData.addSource(_localData!!) { newLocalData ->
mediatorLiveData.removeSource(_localData!!)
// I want to call this from the IO thread.
checkIfShouldFetch(newLocalData, _localData)
}
}
}
}
return this
}
My question is, how to come back to the root context (IO) from the nested context (Main)?
when i call again withContext(Dispatchers.IO) this error is displayed : Suspension functions can be called only within coroutine body
I need to call the function checkIfShouldFetch(newLocalData, _localData) from the IO context and i didn't find how to do it.

You would need to launch a coroutine to call withContext in that place. What you can try to do without launching a coroutine is to use suspendCoroutine or suspendCancellableCoroutine to suspend execution until callback is fired:
withContext(Dispatchers.Main) {
val newLocalData = addSource()
checkIfShouldFetch(newLocalData, _localData)
}
suspend fun addSource(): LiveData<...> = suspendCoroutine { continuation ->
mediatorLiveData.addSource(_localData) { newLocalData ->
mediatorLiveData.removeSource(_localData)
continuation.resumeWith(newLocalData)
}
}
suspend fun checkIfShouldFetch(newLocalData: ..., _localData: ...) = withContext(Dispatchers.IO) {
// ...
}

Is it good idea to use Dispatcher.Main with delay( )?

fetchData() is suspendCoroutine function, so it is implemented on other thread.
viewModelScope is bound to Dispatchers.Main: this should be used only for interacting with the UI and performing quick work.
So should I have delay() in Dispatcher.Main or should I move out it?
fun loadData() {
viewModelScope.launch {
delay(START_DELAY)
when (val result = fetchData()) {
is Response.Success<IData> -> {}
is Response.Failure -> {}
}
}
}
fun fetchData(){
return suspendCoroutine { cont ->}
}

Answer is it can stay, delay will not hurt Main thread, it will not block it. This coroutine inside Main Dispatcher will be suspended, while other coroutines inside Main will continue to run.

AsyncTask as kotlin coroutine

Typical use for AsyncTask: I want to run a task in another thread and after that task is done, I want to perform some operation in my UI thread, namely hiding a progress bar.
The task is to be started in TextureView.SurfaceTextureListener.onSurfaceTextureAvailable and after it finished I want to hide the progress bar. Doing this synchronously does not work because it would block the thread building the UI, leaving the screen black, not even showing the progress bar I want to hide afterwards.
So far I use this:
inner class MyTask : AsyncTask<ProgressBar, Void, ProgressBar>() {
override fun doInBackground(vararg params: ProgressBar?) : ProgressBar {
// do async
return params[0]!!
}
override fun onPostExecute(result: ProgressBar?) {
super.onPostExecute(result)
result?.visibility = View.GONE
}
}
But these classes are beyond ugly so I'd like to get rid of them.
I'd like to do this with kotlin coroutines. I've tried some variants but none of them seem to work. The one I would most likely suspect to work is this:
runBlocking {
// do async
}
progressBar.visibility = View.GONE
But this does not work properly. As I understand it, the runBlockingdoes not start a new thread, as AsyncTask would, which is what I need it to do. But using the thread coroutine, I don't see a reasonable way to get notified when it finished. Also, I can't put progressBar.visibility = View.GONE in a new thread either, because only the UI thread is allowed to make such operations.
I'm new to coroutines so I don't quite understand what I'm missing here.

To use a coroutine you need a couple of things:
Implement CoroutineScope interface.
References to Job and CoroutineContext instances.
Use suspend function modifier to suspend a coroutine without blocking the Main Thread when calling function that runs code in Background Thread.
Use withContext(Dispatchers.IO) function to run code in background thread and launch function to start a coroutine.
Usually I use a separate class for that, e.g. "Presenter" or "ViewModel":
class Presenter : CoroutineScope {
private var job: Job = Job()
override val coroutineContext: CoroutineContext
get() = Dispatchers.Main + job // to run code in Main(UI) Thread
// call this method to cancel a coroutine when you don't need it anymore,
// e.g. when user closes the screen
fun cancel() {
job.cancel()
}
fun execute() = launch {
onPreExecute()
val result = doInBackground() // runs in background thread without blocking the Main Thread
onPostExecute(result)
}
private suspend fun doInBackground(): String = withContext(Dispatchers.IO) { // to run code in Background Thread
// do async work
delay(1000) // simulate async work
return#withContext "SomeResult"
}
// Runs on the Main(UI) Thread
private fun onPreExecute() {
// show progress
}
// Runs on the Main(UI) Thread
private fun onPostExecute(result: String) {
// hide progress
}
}
With ViewModel the code is more concise using viewModelScope:
class MyViewModel : ViewModel() {
fun execute() = viewModelScope.launch {
onPreExecute()
val result = doInBackground() // runs in background thread without blocking the Main Thread
onPostExecute(result)
}
private suspend fun doInBackground(): String = withContext(Dispatchers.IO) { // to run code in Background Thread
// do async work
delay(1000) // simulate async work
return#withContext "SomeResult"
}
// Runs on the Main(UI) Thread
private fun onPreExecute() {
// show progress
}
// Runs on the Main(UI) Thread
private fun onPostExecute(result: String) {
// hide progress
}
}
To use viewModelScope add next line to dependencies of the app's build.gradle file:
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$LIFECYCLE_VERSION"
At the time of writing final LIFECYCLE_VERSION = "2.3.0-alpha04"
Here is also implementation of Async Task using Kotlin coroutines and extension function on CoroutineScope.

Another approach is to create generic extension function on CoroutineScope:
fun <R> CoroutineScope.executeAsyncTask(
onPreExecute: () -> Unit,
doInBackground: () -> R,
onPostExecute: (R) -> Unit
) = launch {
onPreExecute()
val result = withContext(Dispatchers.IO) { // runs in background thread without blocking the Main Thread
doInBackground()
}
onPostExecute(result)
}
Now we can use it with any CoroutineScope:
In ViewModel:
class MyViewModel : ViewModel() {
fun someFun() {
viewModelScope.executeAsyncTask(onPreExecute = {
// ...
}, doInBackground = {
// ...
"Result" // send data to "onPostExecute"
}, onPostExecute = {
// ... here "it" is a data returned from "doInBackground"
})
}
}
In Activity or Fragment:
lifecycleScope.executeAsyncTask(onPreExecute = {
// ...
}, doInBackground = {
// ...
"Result" // send data to "onPostExecute"
}, onPostExecute = {
// ... here "it" is a data returned from "doInBackground"
})
To use viewModelScope or lifecycleScope add next line(s) to dependencies of the app's build.gradle file:
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$LIFECYCLE_VERSION" // for viewModelScope
implementation "androidx.lifecycle:lifecycle-runtime-ktx:$LIFECYCLE_VERSION" // for lifecycleScope
At the time of writing final LIFECYCLE_VERSION = "2.3.0-alpha05".

You can get ProgressBar to run on the UI Main Thread, while using coroutine to run your task asynchronously.
Inside your override fun onCreate() method,
GlobalScope.launch(Dispatchers.Main) { // Coroutine Dispatcher confined to Main UI Thread
yourTask() // your task implementation
}
You can initialize,
private var jobStart: Job? = null
In Kotlin, var declaration means the property is mutable. If you
declare it as val, it is immutable, read-only & cannot be reassigned.
Outside the onCreate() method, yourTask() can be implemented as a suspending function, which does not block main caller thread.
When the function is suspended while waiting for the result to be returned, its running thread is unblocked for other functions to execute.
private suspend fun yourTask() = withContext(Dispatchers.Default){ // with a given coroutine context
jobStart = launch {
try{
// your task implementation
} catch (e: Exception) {
throw RuntimeException("To catch any exception thrown for yourTask", e)
}
}
}
For your progress bar, you can create a button to show the progress bar when the button is clicked.
buttonRecognize!!.setOnClickListener {
trackProgress(false)
}
Outside of onCreate(),
private fun trackProgress(isCompleted:Boolean) {
buttonRecognize?.isEnabled = isCompleted // ?. safe call
buttonRecognize!!.isEnabled // !! non-null asserted call
if(isCompleted) {
loading_progress_bar.visibility = View.GONE
} else {
loading_progress_bar.visibility = View.VISIBLE
}
}
An additional tip is to check that your coroutine is indeed running on
another thread, eg. DefaultDispatcher-worker-1,
Log.e("yourTask", "Running on thread ${Thread.currentThread().name}")
Hope this is helpful.

First, you have to run coroutine with launch(context), not with runBlocking:
https://kotlinlang.org/docs/reference/coroutines/coroutine-context-and-dispatchers.html
Second, to get the effect of onPostExecute, you have to use
Activity.runOnUiThread(Runnable)
or View.post(Runnable).

This does not use coroutines, but it's a quick solution to have a task run in background and do something on UI after that.
I'm not sure about the pros and cons of this approach compared to the others, but it works and is super easy to understand:
Thread {
// do the async Stuff
runOnUIThread {
// do the UI stuff
}
// maybe do some more stuff
}.start()
With this solution, you can easily pass values and objects between the two entities. You can also nest this indefinitely.

The following approach might be able to suffice your needs. It requires less boilerplate code and works for 100% of usecases
GlobalScope.launch {
bitmap = BitmapFactory.decodeStream(url.openStream())
}.invokeOnCompletion {
createNotification()
}

private val TAG = MainActivity::class.simpleName.toString()
private var job = Job()
//coroutine Exception
val handler = CoroutineExceptionHandler { _, exception ->
Log.d(TAG, "$exception handled !")
}
//coroutine context
val coroutineContext: CoroutineContext get() = Dispatchers.Main + job + handler
//coroutine scope
private val coroutineScope = CoroutineScope(coroutineContext)
fun execute() = coroutineScope.launch {
onPreExecute()
val result = doInBackground() // runs in background thread without blocking the Main Thread
onPostExecute(result)
}
private suspend fun doInBackground(): String =
withContext(Dispatchers.IO) { // to run code in Background Thread
// do async work
//delay(5000) // simulate async work
loadFileFromStorage()
return#withContext "SomeResult"
}
// Runs on the Main(UI) Thread
private fun onPreExecute() {
LoadingScreen.displayLoadingWithText(this,"Loading Files",false)
}
// Runs on the Main(UI) Thread
private fun onPostExecute(result: String) {
//progressDialogDialog?.dismiss()
LoadingScreen.hideLoading()
// hide progress
}

I started migrating my AsyncTask stuff in my Android Project to using coroutines...and if you just really need to do something on the UI after completing the async task (i.e., you're just overriding doInBackGround and onPostExecute in AsyncTask)...something like this can be done (i tried this myself and it works):
val job = CoroutineScope(Dispatchers.IO).async {
val rc = ...
return#async rc
}
CoroutineScope(Dispatchers.Main).launch {
val job_rc = job.await() // whatever job returns is fed to job_rc
// do UI updates here
}
The job that you have doesn't need to use the I/O Dispatcher...you can just use the default if it's not I/O intensive.
however the the coroutine waiting for the job to complete needs to be in on the Main/UI thread so you can update UI.
Yes, there's some syntax sugar that can be used to make the above code look more cool but this is at least easier to grasp when one is just starting to migrate to using coroutines.

Switching to UI context in coroutines

I'm new to coroutines and I'm wondering if it's possible to switch from coroutineScope (GlobalScope) to UI scope for the code below. My problem is that the steps inside the coroutine launch body must be executed in a worker thread, otherwise the listener notification must be executed in the ui thread in order to avoid to call runOnUiThread in my activity code.
override suspend fun startRent(name: String, bikeMode: BikeMode, listener: StartRentListener) {
var bleDevice : RxBleDevice
val scanFilter: ScanFilter = ScanFilter.Builder().setDeviceName(name).build()
val scanSettings: ScanSettings = ScanSettings.Builder().build()
val job = GlobalScope.launch {
try {
bleDevice = rxBleClient.scanBleDevicesExt(rxBleClient, scanSettings, scanFilter)
val bleConnection = bleDevice.establishConnectionExt()
// write handshake
connectionManager.writeHandshake(bleDevice, bleConnection)
// open lock
openLock(bleDevice, bikeMode, bleConnection)
// getting user position
apiHelper.sendLockRequest(bleDevice.name, getPosition())
bleDevice.disconnect()
// this should be called on main thread once all the previous operations are finished
listener.onSuccess()
} catch (e: Exception) {
listener.onError(e)
}
}
job.join()
}
A snippet of my current activity code:
bikeAccessClient.startRent(bikeBLEName, BikeMode.HYBRID, object :
StartRentListener {
override fun onSuccess() {
runOnUiThread {
// UI update here
}
}

You may use withContext(Dispatchers.Main) {..} function to execute a part of your code with the other Coroutine Dispatcher.
kotlinx.coroutines.android contains the definition of the Dispatchers.Main function and it integrates correctly with Android UI.
Using explicit Dispatcher in your code is quite error-prone. Instead, I would recommend designing the code with fewer explicit requirements.
I would wrote something like that:
fun uiActionHandlerToStartTheProcess() {
launch(Dispatchers.Main) {
val result = startRent(...) // no callback here, suspend function
//UI Update Here
}
}
suspend fun CoroutineScope.startRent() : SomeResultOfWork {
//that function offloads the execution to a IO (aka brackground) thread
return withContext(Dispatchers.IO){
//here goes your code from `startRent`
//use `suspendCancellableCoroutine {cont -> .. }` if you need to handle callbacks from it
SomeResultOfWork()
}
The code in the launch(Dispatchers.Main){..} block is executed in the UI thread. The call to startRent suspend function suspends the execution in the UI thread. Once the startRent is ready with the reply (from a background thread) it resumes the execution (which is done by the Dispatchers.Main and equivalent to the runOnUiThread {...}) and executes the UI update from the right thread

Kotlin Coroutines the right way in Android

I'm trying to update a list inside the adapter using async, I can see there is too much boilerplate.
Is it the right way to use Kotlin Coroutines?
can this be optimized more?
fun loadListOfMediaInAsync() = async(CommonPool) {
try {
//Long running task
adapter.listOfMediaItems.addAll(resources.getAllTracks())
runOnUiThread {
adapter.notifyDataSetChanged()
progress.dismiss()
}
} catch (e: Exception) {
e.printStackTrace()
runOnUiThread {progress.dismiss()}
} catch (o: OutOfMemoryError) {
o.printStackTrace()
runOnUiThread {progress.dismiss()}
}
}

After struggling with this question for days, I think the most simple and clear async-await pattern for Android activities using Kotlin is:
override fun onCreate(savedInstanceState: Bundle?) {
//...
loadDataAsync(); //"Fire-and-forget"
}
fun loadDataAsync() = async(UI) {
try {
//Turn on busy indicator.
val job = async(CommonPool) {
//We're on a background thread here.
//Execute blocking calls, such as retrofit call.execute().body() + caching.
}
job.await();
//We're back on the main thread here.
//Update UI controls such as RecyclerView adapter data.
}
catch (e: Exception) {
}
finally {
//Turn off busy indicator.
}
}
The only Gradle dependencies for coroutines are: kotlin-stdlib-jre7, kotlinx-coroutines-android.
Note: Use job.await() instead of job.join() because await() rethrows exceptions, but join() does not. If you use join() you will need to check job.isCompletedExceptionally after the job completes.
To start concurrent retrofit calls, you can do this:
val jobA = async(CommonPool) { /* Blocking call A */ };
val jobB = async(CommonPool) { /* Blocking call B */ };
jobA.await();
jobB.await();
Or:
val jobs = arrayListOf<Deferred<Unit>>();
jobs += async(CommonPool) { /* Blocking call A */ };
jobs += async(CommonPool) { /* Blocking call B */ };
jobs.forEach { it.await(); };

How to launch a coroutine
In the kotlinx.coroutines library you can start new coroutine using either launch or async function.
Conceptually, async is just like launch. It starts a separate coroutine which is a light-weight thread that works concurrently with all the other coroutines.
The difference is that launch returns a Job and does not carry any resulting value, while async returns a Deferred - a light-weight non-blocking future that represents a promise to provide a result later. You can use .await() on a deferred value to get its eventual result, but Deferred is also a Job, so you can cancel it if needed.
Coroutine context
In Android we usually use two context:
uiContext to dispatch execution onto the Android main UI thread (for the parent coroutine).
bgContext to dispatch execution in background thread (for the child coroutines).
Example
//dispatches execution onto the Android main UI thread
private val uiContext: CoroutineContext = UI
//represents a common pool of shared threads as the coroutine dispatcher
private val bgContext: CoroutineContext = CommonPool
In following example we are going to use CommonPool for bgContext which limit the number of threads running in parallel to the value of Runtime.getRuntime.availableProcessors()-1. So if the coroutine task is scheduled, but all cores are occupied, it will be queued.
You may want to consider using newFixedThreadPoolContext or your own implementation of cached thread pool.
launch + async (execute task)
private fun loadData() = launch(uiContext) {
view.showLoading() // ui thread
val task = async(bgContext) { dataProvider.loadData("Task") }
val result = task.await() // non ui thread, suspend until finished
view.showData(result) // ui thread
}
launch + async + async (execute two tasks sequentially)
Note: task1 and task2 are executed sequentially.
private fun loadData() = launch(uiContext) {
view.showLoading() // ui thread
// non ui thread, suspend until task is finished
val result1 = async(bgContext) { dataProvider.loadData("Task 1") }.await()
// non ui thread, suspend until task is finished
val result2 = async(bgContext) { dataProvider.loadData("Task 2") }.await()
val result = "$result1 $result2" // ui thread
view.showData(result) // ui thread
}
launch + async + async (execute two tasks parallel)
Note: task1 and task2 are executed in parallel.
private fun loadData() = launch(uiContext) {
view.showLoading() // ui thread
val task1 = async(bgContext) { dataProvider.loadData("Task 1") }
val task2 = async(bgContext) { dataProvider.loadData("Task 2") }
val result = "${task1.await()} ${task2.await()}" // non ui thread, suspend until finished
view.showData(result) // ui thread
}
How to cancel a coroutine
The function loadData returns a Job object which may be cancelled. When the parent coroutine is cancelled, all its children are recursively cancelled, too.
If the stopPresenting function was called while dataProvider.loadData was still in progress, the function view.showData will never be called.
var job: Job? = null
fun startPresenting() {
job = loadData()
}
fun stopPresenting() {
job?.cancel()
}
private fun loadData() = launch(uiContext) {
view.showLoading() // ui thread
val task = async(bgContext) { dataProvider.loadData("Task") }
val result = task.await() // non ui thread, suspend until finished
view.showData(result) // ui thread
}
The complete answer is available in my article Android Coroutine Recipes

I think you can get rid of runOnUiThread { ... } by using UI context for Android applications instead of CommonPool.
The UI context is provided by the kotlinx-coroutines-android module.

We also have another option. if we use Anko library , then it looks like this
doAsync {
// Call all operation related to network or other ui blocking operations here.
uiThread {
// perform all ui related operation here
}
}
Add dependency for Anko in your app gradle like this.
implementation "org.jetbrains.anko:anko:0.10.5"

Like sdeff said, if you use the UI context, the code inside that coroutine will run on UI thread by default. And, if you need to run an instruction on another thread you can use run(CommonPool) {}
Furthermore, if you don't need to return nothing from the method, you can use the function launch(UI) instead of async(UI) (the former will return a Job and the latter a Deferred<Unit>).
An example could be:
fun loadListOfMediaInAsync() = launch(UI) {
try {
withContext(CommonPool) { //The coroutine is suspended until run() ends
adapter.listOfMediaItems.addAll(resources.getAllTracks())
}
adapter.notifyDataSetChanged()
} catch(e: Exception) {
e.printStackTrace()
} catch(o: OutOfMemoryError) {
o.printStackTrace()
} finally {
progress.dismiss()
}
}
If you need more help I recommend you to read the main guide of kotlinx.coroutines and, in addition, the guide of coroutines + UI

If you want to return some thing from background thread use async
launch(UI) {
val result = async(CommonPool) {
//do long running operation
}.await()
//do stuff on UI thread
view.setText(result)
}
If background thread is not returning anything
launch(UI) {
launch(CommonPool) {
//do long running operation
}.await()
//do stuff on UI thread
}

All the above answers are right, but I was having a hard time finding the right import for the UI from kotlinx.coroutines, it was conflicting with UI from Anko.
Its
import kotlinx.coroutines.experimental.android.UI

Here's the right way to use Kotlin Coroutines. Coroutine scope simply suspends the current coroutine until all child coroutines have finished their execution. This example explicitly shows us how child coroutine works within parent coroutine.
An example with explanations:
fun main() = blockingMethod { // coroutine scope
launch {
delay(2000L) // suspends the current coroutine for 2 seconds
println("Tasks from some blockingMethod")
}
coroutineScope { // creates a new coroutine scope
launch {
delay(3000L) // suspends this coroutine for 3 seconds
println("Task from nested launch")
}
delay(1000L)
println("Task from coroutine scope") // this line will be printed before nested launch
}
println("Coroutine scope is over") // but this line isn't printed until nested launch completes
}
Hope this helps.

Please find attached the implementation for a remote API call with Kotlin Coroutines & Retrofit library.
import android.view.View
import android.util.Log
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import com.test.nyt_most_viewed.NYTApp
import com.test.nyt_most_viewed.data.local.PreferenceHelper
import com.test.nyt_most_viewed.data.model.NytAPI
import com.test.nyt_most_viewed.data.model.response.reviews.ResultsItem
import kotlinx.coroutines.*
import javax.inject.Inject
class MoviesReviewViewModel #Inject constructor(
private val nytAPI: NytAPI,
private val nytApp: NYTApp,
appPreference: PreferenceHelper
) : ViewModel() {
val moviesReviewsResponse: MutableLiveData<List<ResultsItem>> = MutableLiveData()
val message: MutableLiveData<String> = MutableLiveData()
val loaderProgressVisibility: MutableLiveData<Int> = MutableLiveData()
val coroutineJobs = mutableListOf<Job>()
override fun onCleared() {
super.onCleared()
coroutineJobs.forEach {
it.cancel()
}
}
// You will call this method from your activity/Fragment
fun getMoviesReviewWithCoroutine() {
viewModelScope.launch(Dispatchers.Main + handler) {
// Update your UI
showLoadingUI()
val deferredResult = async(Dispatchers.IO) {
return#async nytAPI.getMoviesReviewWithCoroutine("full-time")
}
val moviesReviewsResponse = deferredResult.await()
this#MoviesReviewViewModel.moviesReviewsResponse.value = moviesReviewsResponse.results
// Update your UI
resetLoadingUI()
}
}
val handler = CoroutineExceptionHandler { _, exception ->
onMoviesReviewFailure(exception)
}
/*Handle failure case*/
private fun onMoviesReviewFailure(throwable: Throwable) {
resetLoadingUI()
Log.d("MOVIES-REVIEWS-ERROR", throwable.toString())
}
private fun showLoadingUI() {
setLoaderVisibility(View.VISIBLE)
setMessage(STATES.INITIALIZED)
}
private fun resetLoadingUI() {
setMessage(STATES.DONE)
setLoaderVisibility(View.GONE)
}
private fun setMessage(states: STATES) {
message.value = states.name
}
private fun setLoaderVisibility(visibility: Int) {
loaderProgressVisibility.value = visibility
}
enum class STATES {
INITIALIZED,
DONE
}
}