AsyncTask as kotlin coroutine

Question

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.

Answer 1

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.

Answer 2

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".

Answer 3

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.

Answer 4

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).

Answer 5

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.

Answer 6

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()
            }

Answer 7

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
    }

Answer 8

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.