AsyncTask as kotlin coroutine
Solution 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:$LIFECYCLE_VERSION"
At the time of writing final LIFECYCLE_VERSION = "2.3.0-alpha04"
Solution 2
Another approach is to create generic extension function on CoroutineScope:
fun <R> CoroutineScope.executeAsyncTask(
onPreExecute: () -> Unit,
doInBackground: () -> R,
onPostExecute: (R) -> Unit
) = launch {
onPreExecute()
val result = withContext(Dispatchers.IO) { // runs in background thread without blocking the Main Thread
doInBackground()
}
onPostExecute(result)
}
Now we can use it with any CoroutineScope:
-
In
ViewModel:class MyViewModel : ViewModel() { fun someFun() { viewModelScope.executeAsyncTask(onPreExecute = { // ... }, doInBackground = { // ... "Result" // send data to "onPostExecute" }, onPostExecute = { // ... here "it" is a data returned from "doInBackground" }) } } -
In
ActivityorFragment:lifecycleScope.executeAsyncTask(onPreExecute = { // ... }, doInBackground = { // ... "Result" // send data to "onPostExecute" }, onPostExecute = { // ... here "it" is a data returned from "doInBackground" })
To use viewModelScope or lifecycleScope add next line(s) to dependencies of the app's build.gradle file:
implementation "androidx.lifecycle:lifecycle-viewmodel-ktx:$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".
Solution 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.
Solution 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).
Solution 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.
Comments
-
Benjamin Basmaci about 2 years
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.onSurfaceTextureAvailableand 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.GONEBut this does not work properly. As I understand it, the
runBlockingdoes not start a new thread, asAsyncTaskwould, which is what I need it to do. But using thethreadcoroutine, I don't see a reasonable way to get notified when it finished. Also, I can't putprogressBar.visibility = View.GONEin 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.