I am learning Kotlin coroutines. I've read that runBlocking is the way to bridge synchronous and asynchronous code. But what is the performance gain if the runBlocking stops the UI thread?
For example, I need to query a database in Android:
val result: Int
get() = runBlocking { queryDatabase().await() }
private fun queryDatabase(): Deferred<Int> {
return async {
var cursor: Cursor? = null
var queryResult: Int = 0
val sqlQuery = "SELECT COUNT(ID) FROM TABLE..."
try {
cursor = getHelper().readableDatabase.query(sqlQuery)
cursor?.moveToFirst()
queryResult = cursor?.getInt(0) ?: 0
} catch (e: Exception) {
Log.e(TAG, e.localizedMessage)
} finally {
cursor?.close()
}
return#async queryResult
}
}
Querying the database would stop the main thread, so it seems that it would take the same amount of time as synchronous code? Please correct me if I am missing something.
runBlocking is the way to bridge synchronous and asynchronous code
I keep bumping into this phrase and it's very misleading.
runBlocking is almost never a tool you use in production. It undoes the asynchronous, non-blocking nature of coroutines. You can use it if you happen to already have some coroutine-based code that you want to use in a context where coroutines provide no value: in blocking calls. One typical use is JUnit testing, where the test method must just sit and wait for the coroutine to complete.
You can also use it while playing around with coroutines, inside your main method.
The misuse of runBlocking has become so widespread that the Kotlin team actually tried to add a fail-fast check which would immediately crash your code if you call it on the UI thread. By the time they did this, it was already breaking so much code that they had to remove it.
Actually you use runBlocking to call suspending functions in "blocking" code that otherwise wouldn't be callable there or in other words: you use it to call suspend functions outside of the coroutine context (in your example the block passed to async is the suspend function). Also (more obvious, as the name itself implies already), the call then is a blocking call. So in your example it is executed as if there wasn't something like async in place. It waits (blocks interruptibly) until everything within the runBlocking-block is finished.
For example assume a function in your library as follows:
suspend fun demo() : Any = TODO()
This method would not be callable from, e.g. main. For such a case you use runBlocking then, e.g.:
fun main(args: Array<String>) {
// demo() // this alone wouldn't compile... Error:() Kotlin: Suspend function 'demo' should be called only from a coroutine or another suspend function
// whereas the following works as intended:
runBlocking {
demo()
} // it also waits until demo()-call is finished which wouldn't happen if you use launch
}
Regarding performance gain: actually your application may rather be more responsive instead of being more performant (sometimes also more performant, e.g. if you have multiple parallel actions instead of several sequential ones). In your example however you already block when you assign the variable, so I would say that your app doesn't get more responsive yet. You may rather want to call your query asynchronously and then update the UI as soon as the response is available. So you basically just omit runBlocking and rather use something like launch. You may also be interested in Guide to UI programming with coroutines.
Related
I would like to improve my async-await in coroutines.
This is my solution
val coroutineContext: CoroutineContext
get() = job + Dispatchers.IO
[...]
lifecycleScope.launch(coroutineContext) {
async {
client = viewModel.getClientItem(clientId)
}.await()
if (inEditMode != null) {
changedEditMode = true
}
[...]
#Marko Topolnik wrote in Why does this coroutine block UI Thread?
Note: this post dates back to the pre-release version of coroutines. I updated the names of dispatchers to match the release version.
runBlocking is not the way to start a coroutine on the UI thread because, as its name says, it will block the hosting thread until the coroutine is done. You have to launch it in the Main context and then switch to the Default context for the heavyweight operation. You should also drop the async-await pair and use withContext:
button1.setOnClickListener {
launch(Main) {
withContext(Default) {
Thread.sleep(5000L)
}
textView1.text = "Done! UI was not blocked :-)"
}
}
withContext will suspend the coroutine until done and then resume it in the parent context, which is Main.
Why doing it with Main Dispatchers and then withContext using Default dispatchers is betters solution than async. It will also block main thread and work the same what is the difference. And how to handle this approaches?
Greetings
EDIT:
My other solution is this below or withContext(Dispatchers.IO)
lifecycleScope.launch(Dispatchers.Main) {
withContext(Dispatchers.Default) {
client = viewModel.getItem(clientId)
}
[...]
}
Now as I read I do not use runBlocking and don't block main thread?
In that configuration it works the same as with async-await.
In your opinin that's better solution
?
Why doing it with Main Dispatchers and then withContext using Default dispatchers is betters solution than async. It will also block main thread and work the same what is the difference
withContext doesn't block, it suspends. The terminology might should similar but the behaviour is very different. While Thread.sleep is executing on the Default dispatcher (and thus on one of this dispatcher's threads), the main thread can keep running other code. The coroutine launched via launch(Main) here is in a suspended state, which means the main dispatcher knows it can execute other things instead. The linked answer here explains why runBlocking was a bad idea, because no matter what you launch in it runBlocking has to block the current thread while it's executing coroutines inside.
Now back to your code. Note that async { doStuff() }.await() makes little sense. It is equivalent to simply doStuff().
Another thing is that you're passing a job explicitly to launch via the context here. This doesn't seem right, and could lead to problems. If you want to use Dispatchers.IO, you don't need to pass a job too.
That said, even Dispatchers.IO seems like a stretch here. You should instead make viewModel.getClientItem a suspend function (if not already), and leave the responsibility of using the right dispatcher to this function. Maybe you don't even need IO at all here, or maybe it's an HTTP or DB call that already has its own thread pool anyway.
So I would replace your whole code with:
lifecycleScope.launch {
client = viewModel.getClientItem(clientId)
if (inEditMode != null) {
hangedEditMode = true
}
}
I'm trying to use coroutines in my code since in one of my function, I need to do multiple network calls and wait for its result. Below is a portion of my activity code:
class SellerDeliveryRegister : AppCompatActivity() {
lateinit var sellerDeliveryVM:SellerDeliveryVM
sellerDeliveryVM = ViewModelProviders.of(this).get(SellerDeliveryVM::class.java)
var uploadVehicleImageResult = sellerDeliveryVM.uploadVehiclesImages(uploadMotorImage1Url,uploadMotorImage2Url)
}
And below is a portion of my sellerDeliveryVM ViewModel code:
class SellerDeliveryVM: ViewModel() {
suspend fun uploadVehiclesImages(uploadMotorImage1Url: String, uploadMotorImage2Url: String): Unit = withContext(Dispatchers.IO){
var uploadMotorImage1Result = "-1"; var uploadMotorImage2Result = "-1";
viewModelScope.launch(Dispatchers.IO) {
uploadMotorImage1Result = withContext(Dispatchers.Default) {
NetworkRepository.instance!!.uploadFile(uploadMotorImage1Url)
}
uploadMotorImage2Result = withContext(Dispatchers.Default) {
NetworkRepository.instance!!.uploadFile(uploadMotorImage2Url)
}
return#launch;
}
return#withContext
}
}
Please take note that previously uploadVehiclesImages is a 'normal' function that doesn't use coroutine, so now I'm converting it to use coroutine.
Below are the problems I'm facing:
Line var uploadVehicleImageResult = sellerDeliveryVM.uploadVehiclesImages(uploadMotorImage1Url,uploadMotorImage2Url) inside my SellerDeliveryRegister class returns this error:
Suspend function 'uploadVehiclesImages' should be called only from a coroutine or another suspend function
Initially I want to return Boolean from uploadVehiclesImages, so I have return true in place of the return#launch and return false in place of the return#withContext, but then I will get the error return is not allowed here, and Android Studio suggested me to make the changes above, although I really have no idea what the changes meant there.
So what should I do to fix this problem 1, and can anyone enlighten me more on what's really happening on the problem 2?
So what should I do to fix this problem 1
Remove the property. uploadVehiclesImages() returns Unit; there is no value in having Unit in a property. If your objective is to call uploadVehiclesImages() when the viewmodel is created, put a call to it in an init block, wrapped in a suitable coroutine launcher (e.g., viewModelScope.launch {}).
This assumes that you are going to keep the function in its current form — your next question suggests that this function may not be the right solution.
Initially I want to return Boolean from uploadVehiclesImages,
More importantly, you seem to want it to return values more than once. That is not how functions work in Kotlin. One call to uploadVehiclesImages() can return one Boolean value, but not one now and one sometime in the future.
If you want to be emitting a stream of results, a suspend function on its own is not the correct solution. For example, you could:
Use LiveData, with the suspend function updating the backing MutableLiveData, or
Use a function that returns a StateFlow or SharedFlow
For part 1, you cannot use a coroutine to initialize a property. Coroutines return some time in the future, but properties have to be initialized immediately at class instantiation time. You'll have to change the strategy so you launch a coroutine that calls the suspend function, and then does something with the result when it arrives.
For part 2, you have an awkwardly composed suspend function. A proper suspend function typically isn't launching other coroutines unless it is using them to break down multiple simultaneous asynchronous actions and then waiting for them.
The convention for a suspend function is that it is safe to call from any Dispatcher. It's not proper to be sending off these background actions by launching a coroutine in a specific coroutine scope. Usually, a coroutine that calls a suspend function should not have to worry that the suspend function is going to launch some other coroutine in another scope, because this breaks support for cancellation.
Also, you can use async instead of launch to run suspend functions that you need a result from. That will avoid the awkward variables you've created to store the results (and you neglected to wait for).
Assuming you want to return both of these image results, you'll have to wrap them in another class, such as List. So your function could look like below. It returns something, not Unit. It uses aysnc to run the two requests simultaneously.
suspend fun uploadVehiclesImages(uploadMotorImage1Url: String, uploadMotorImage2Url: String): List<ImageUploadResult> {
return listOf(uploadMotorImage1Url, uploadMotorImage2Url)
.map { aysnc { NetworkRepository.instance!!.uploadFile(it) }
.awaitAll()
}
I just put ImageUploadResult to stand in for whatever this uploadFile function returns. Maybe it's just Boolean.
Whenever you do want to call it, you would use either lifecycleScope (from an Activity or Fragment) or viewModelScope (from a ViewModel) to launch a coroutine that calls it. For example:
fun doUpload(url1: String, url2: String) {
lifecycleScope.launch {
val results = uploadVehiclesImages(url1, url2)
// do something with results here
}
}
To change the thread in a function I use either CoroutineScope or withContext. I don't know's the difference, but with CourineScope I can also use a handler.
Examples:
private fun removeViews(){
CoroutineScope(Main).launch(handler){
gridRoot.removeAllViews()
}
}
private suspend fun removeViews(){
withContext(Main){
gridRoot.removeAllViews()
}
}
I call this function from a coroutine that works on background thread (IO). Is any more appropriate than the other?
These two are actually radically different and you just happen to have a use case where you don't experience the difference:
CoroutineScope(Main).launch(handler){
This launches a concurrent coroutine that goes on independently.
withContext(Main){
This is a function that completes only when the code inside it completes, and returns its result. This is the way you should be doing it.
The first approach, with CoroutineScope, has another deficiency in that it circumvents structured concurrency. You create an ad-hoc coroutine scope that has no parent and thus won't be automatically cleaned up if it takes a longer time to complete and your GUI is dropped (user navigates away from the current Activity).
You should actually never use the CoroutineScope(Main) idiom, I don't think there's a single instance where it would be appropriate. If you explicitly want to avoid structured concurrency, it is still better and cleaner to write
GlobalScope.launch(Main + handler) {
and has pretty much the same effect.
If you want a concurrent coroutine that fits into structured concurrency, use
fun CoroutineScope.removeViews() {
launch {
gridRoot.removeAllViews()
}
}
Note I removed the handler argument, a child coroutine ignores it because it forwards any failures to its parent coroutine, which is exactly what you want. The parent coroutine should have an exception handler installed.
Technically both are same but when it comes to use case both are different and has big impact on the different use cases so be careful while using them
Coroutine Scope:
CoroutineScope is a starting Point of Coroutine. CoroutineScope can have more than one coroutine within itself, which makes coroutine hierarchy.
Lets think, Parent has more than one children. Think CoroutineScope is a parent and this parent can have more than one child which are also coroutines. These childrens are known as job
private val coroutineScope = CoroutineScope()
coroutineScope(IO).launch{
val childOne = launch(Main){}
val childTwo = launch(Main){}
}
see that childOne and childTwo? why we need these? because we can't directly cancel the coroutine there is no such way the coroutine can be cancelled directly, either the coroutine gets completed or it gets failed. But what if we wanna cancel it? in such cases we need job. But thing to be notice here these job children are totally associated with parent. And Parent is (IO) and childrens are (Main), this parent is started in IO Disptacher but when it comes to those childrens they are gonna switch to (Main) and do their thing but the parent will still be at (IO) switching the Dispatcher of childrens not gonna effect parent.
But what happens if something wrong happens to either of the children,
in that case we will watch this summit:
https://www.youtube.com/watch?v=w0kfnydnFWI
This summit about coroutine exception and cancellation. watch it, its amazing...
withContext:
What is withContext?
withContext should be inside any Coroutine or suspend fun because withContext itself is a suspending function.
withContext is use to switch the context in different situation
but how?
suspend fun fetchFromNetworkAndUpdateUI() {
withContext(IO){
println("Some Fake data from network")
}
withContext(Main){
//updating Ui
//setting that Data to some TextView etc
}
}
see the code, we are fetching the data asynchronously from network cause we don't wanna block the MainThread and then we switch the context, why? cause we can't update UI related stuff in IoDispatcher that's we have change the context to main with withContext(main){} and update the UI.
and there are other use cases like liveData, we are fetching the value using retrofit using IoDispatcher then in next step we have to set it to the liveData by using withContext(main){} cause we can't observe liveData's value in background thread.
yeah, I hope this helps. comment if there is any question.
From the Antonio Leiva article about coroutines:
The coroutine context is a set of rules and configurations that define
how the coroutine will be executed
withContext is a function that allows you to easily change the context of a suspending function, in order to be sure that that function is executed in a particular thread (E.g. Thread from IO pool). To do so you can force a suspending function to execute its body within a particular thread pool, for example:
suspend fun getAuthenticationStatus(): AuthenticationStatus = withContext(Dispatchers.IO) {
when (val result = repository.getAuthenticationStatus()) {
is Result.Success -> result.data
is Result.Error -> AuthenticationStatus.Unauthorized
}
}
This way, even if you're calling this suspending function from a UI scope (MainScope), you are 100% sure that the suspending function is executed in a worker thread and you can update the UI with the returned result in the main thread, such as:
MainScope().launch {
userIdentityVM.getAuthenticationStatus().run {
when (this) {
is AuthenticationStatus.Authenticated -> {
// do something
}
is AuthenticationStatus.Unauthorized -> {
// do something else
}
}
}
}
To sum up, by using withContext you can make your suspending function "Main Safe".
The difference between scope and context is basically the intended purpose.
To launch a coroutine you normally use launch coroutine builder, defined as an extension function on CoroutineScope.
fun CoroutineScope.launch(
context: CoroutineContext = EmptyCoroutineContext,
// ...
): Job
The context specified as parameter on the coroutine scope is merged to coroutine scope by plus operator and takes precedence on the "default" context specified by coroutine scope. This way you can execute the code in a "parent" context. To go deep I suggest you this article by Roman Elizarov (Team Lead for Kotlin libraries #JetBrains).
My understanding is that Kotlin's coroutines are libraries, which leaves the only language-level feature of concurrency in Kotlin as the suspend keyword.
I'm still wrapping my head around coroutines in Kotlin, but I'm wondering if that may be overkill for my problem, which is updating a text view as soon as an HttpsURLConnection returns data. exception handling makes callbacks ugly enough that I want to avoid those if possible
does the suspend keyword simply mean that the runtime may suspend a function that takes a while to complete? or is suspension only enabled inside a coroutine? as a hypothetical, can I write
suspend fun getStringFromNetwork(): String {
val request = URL("https:stackoverflow.com").openConnection()
val result = readStream(request.inputStream)
request.disconnect()
return result
}
//and then elsewhere
foo()
val s = getStringFromNetwork()
bar(s)
baz()
and know that if getStringFromNetwork downloads 1 GB of data that baz() will be called in the meantime, while bar(s) waits for s to be populated by getStringFromNetwork?
The "and then elsewhere" part calls getStringFromNetwork(), so it won't compile outside a suspend function (including suspend lambdas), and they can only be executed inside coroutines.
that baz() will be called in the meantime, while bar(s) waits for s to be populated by getStringFromNetwork?
No, if you write it this way, baz() will only start executing after bar(s) returns. But of course bar(s) can start a new coroutine which will do the actual work.
I feel like i'm missing some crucial part to my understanding to how this code below is working:
private fun retrieveAndStore() {
launch(UI) {
val service = retrofit.create(AWSService::class.java)
val response = service.retrieveData().await()
store(data = response)
}
}
private suspend fun store(data: JsonData) {
val db = Room.databaseBuilder(applicationContext, AppDatabase::class.java, "app-db").build()
db.appDao().insert(storyData)
}
This is the error i get when run:
java.lang.IllegalStateException: Cannot access database on the main thread since it may potentially lock the UI for a long period of time.
I don't understand why the network code via retrofit works but the store function fails. I'm hoping someone can tell me what's going on?
Interestingly if i wrap db call with async {}.await it works, does that mean coroutines can only call other coroutines?
Coroutines aren't about running in either foreground or background. They are about the ability to get suspended, just like a native thread gets suspended by the OS, but on the level where you are in control of that behavior.
When you say launch(UI) { some code }, you tell Kotlin to submit "some code" as a task to the GUI event loop. It will run on the GUI thread until explicitly suspended; the only difference is that it won't run right away so the next line of code below the launch(UI) block will run before it.
The magic part comes around when your "some code" encounters a suspendCoroutine call: this is where its execution stops and you get a continuation object inside the block you pass to suspendCoroutine. You can do with that object whatever you want, typically store it somewhere and then resume it later on.
Often you don't see the suspendCoroutine call because it's inside the implementation of some suspend fun you're calling, but you can freely implement your own.
One such library function is withContext and it's the one you need to solve your problem. It creates another coroutine with the block you pass it, submits that coroutine to some other context you specify (a useful example is CommonPool) and then suspends the current coroutine until that other one completes. This is exactly what you need to turn a blocking call into a suspendable function.
In your case, it would look like this:
private suspend fun store(data: JsonData) = withContext(CommonPool) {
val db = Room.databaseBuilder(applicationContext, AppDatabase::class.java, "app-db").build()
db.appDao().insert(storyData)
}
I'll also add that you're better off creating your own threadpool instead of relying on the system-wide CommonPool. Refer to this thread for details.