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
}
}
Related
I am learning about threading and kotlin coroutines to load my UI and repository concurrently, but then "withContext()" as an alternative to async-await.
If I understood "withContext()" correctly and it executes one task after another waiting for the previous task to finish, why ever use it? Is there another concept I'm missing?
withContext is a suspending function that allows to execute a specific piece of code in a different coroutine context. It is in particular useful when you want to execute something in a different dispatcher.
For instance, you could have some code run in the default dispatcher with multiple threads, but then use a value produced by that code to update some UI in the UI thread. In that case, you're not looking for concurrency, the computation on the default dispatcher has to happen before updating the UI because you need the result:
val result = someComputation()
withContext(Dispatchers.Main) {
updateUI(result)
}
Of course, even if the computation and the update of the UI are not concurrent, their sequence can be concurrent with other pieces of code:
scope.launch(Dispatchers.Default) {
val result = someComputation()
withContext(Dispatchers.Main) {
updateUI(result)
}
}
If you need to execute concurrent things, you can use coroutine builders like launch and async. However, using async { ... } immediately followed by .await() defeats the purpose of concurrency, because the code between the async and await() calls is precisely what will run concurrently with the async's body:
val deferred = async { computeSomeValue() }
somethingConcurrentWithAsyncBody()
val result = deferred.await()
You can read more about how to organize your calls to achieve concurrency in the part of the doc about composing suspend functions.
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).
I'm putting together a simple demo app in Kotlin for Android that retrieves the title of a webpage with Jsoup. I'm conducting the network call using Dispatchers.Main as context.
My understanding of coroutines is that if I call launch on the Dispatchers.Main it does run on the main thread, but suspends the execution so as to not block the thread.
My understanding of android.os.NetworkOnMainThreadException is that it exists because network operations are heavy and when run on the main thread will block it.
So my question is, given that a coroutine does not block the thread it is run in, is a NetworkOnMainThreadException really valid? Here is some sample code that throws the given Exception at Jsoup.connect(url).get():
class MainActivity : AppCompatActivity() {
val job = Job()
val mainScope = CoroutineScope(Dispatchers.Main + job)
// called from onCreate()
private fun printTitle() {
mainScope.launch {
val url ="https://kotlinlang.org"
val document = Jsoup.connect(url).get()
Log.d("MainActivity", document.title())
// ... update UI with title
}
}
}
I know I can simply run this using the Dispatchers.IO context and providing this result to the main/UI thread, but that seems to dodge some of the utility of coroutines.
For reference, I'm using Kotlin 1.3.
My understanding of coroutines is that if I call launch on the Dispatchers.Main it does run on the main thread, but suspends the execution so as to not block the thread.
The only points where execution is suspended so as to not block the thread is on methods marked as suspend - i.e., suspending methods.
As Jsoup.connect(url).get() is not a suspending method, it blocks the current thread. As you're using Dispatchers.Main, the current thread is the main thread and your network operation runs directly on the main thread, causing the NetworkOnMainThreadException.
Blocking work like your get() method can be made suspending by wrapping it in withContext(), which is a suspending method and ensures that the Dispatchers.Main is not blocked while the method runs.
mainScope.launch {
val url ="https://kotlinlang.org"
val document = withContext(Dispatchers.IO) {
Jsoup.connect(url).get()
}
Log.d("MainActivity", document.title())
// ... update UI with title
}
Coroutine suspension is not a feature that magically "unblocks" an existing blocking network call. It is strictly a cooperative feature and requires the code to explicitly call suspendCancellableCoroutine. Because you're using some pre-existing blocking IO API, the coroutine blocks its calling thread.
To truly leverage the power of suspendable code you must use a non-blocking IO API, one which lets you make a request and supply a callback the API will call when the result is ready. For example:
NonBlockingHttp.sendRequest("https://example.org/document",
onSuccess = { println("Received document $it") },
onFailure = { Log.e("Failed to fetch the document", it) }
)
With this kind of API no thread will be blocked, whether or not you use coroutines. However, compared to blocking API, its usage is quite unwieldy and messy. This is what coroutines help you with: they allow you to continue writing code in exactly the same shape as if it was blocking, only it's not. To get it, you must first write a suspend fun that translates the API you have into coroutine suspension:
suspend fun fetchDocument(url: String): String = suspendCancellableCoroutine { cont ->
NonBlockingHttp.sendRequest(url,
onSuccess = { cont.resume(it) },
onFailure = { cont.resumeWithException(it) }
)
}
Now your calling code goes back to this:
try {
val document = fetchDocument("https://example.org/document")
println("Received document $document")
} catch (e: Exception) {
Log.e("Failed to fetch the document", e)
}
If, instead, you're fine with keeping your blocking network IO, which means you need a dedicated thread for each concurrent network call, then without coroutines you'd have to use something like an async task, Anko's bg etc. These approaches also require you to supply callbacks, so coroutines can once again help you to keep the natural programming model. The core coroutines library already comes with all the parts you need:
A specialized elastic thread pool that always starts a new thread if all are currently blocked (accessible via Dispatchers.IO)
The withContext primitive, which allows your coroutine to jump from one thread to another and then back
With these tools you can simply write
try {
val document = withContext(Dispatchers.IO) {
JSoup.connect("https://example.org/document").get()
}
println("Received document $it")
} catch (e: Exception) {
Log.e("Failed to fetch the document")
}
When your coroutine arrives at the JSoup call, it will set the UI thread free and execute this line on a thread in the IO thread pool. When it unblocks and gets the result, the coroutine will jump back to the UI thread.
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.
Deprecation alert
This code uses the old Coroutines Api. If you're using kotlinx-coroutines 1.1.0 or newer, this code won't be useful to you
The original question was:
I'm finding that this particular code in my Android App blocks the UI Thread:
runBlocking {
async(CommonPool) {
Thread.sleep(5000)
}.await()
}
textView.text = "Finish!"
I've been using coroutines for several tasks, and they never block UI Thread, as can be read in the documentation:
. Coroutines provide a way to avoid blocking a thread and replace it with a cheaper and more controllable operation: suspension of a coroutine
But curiously, this code:
runBlocking {
async(CommonPool) {
launch(CommonPool) {
Thread.sleep(5000)
runOnUiThread { textView.text = "Finish!" }
}
}.await()
}
behaves as expected; does not block, waits five seconds then prints the result (I need to update the UI after, and only after the sleep is completed)
The documentation says that async and launch can be used independently and don't need to be combined. In fact, async(CommonPool) should be enough.
So what's really going on here? why does it work only with async+launch ?
Update (2021)
[Deprecation alert] This code uses the old Coroutines Api. If you're using kotlinx-coroutines 1.1.0 or newer, forget about this code
My full sample code:
class MainActivity : AppCompatActivity() {
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
button1.setOnClickListener {
runBlocking {
async(CommonPool) {
Thread.sleep(5000L)
}.await()
}
textView1.text = "Finally! I've been blocked for 5s :-("
}
button2.setOnClickListener {
runBlocking {
async(CommonPool) {
launch(CommonPool) {
Thread.sleep(5000L)
runOnUiThread { textView1.text = "Done! UI was not blocked :-)" }
}
}.await()
}
}
}
}
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.
As documentation tells us:
[runBlocking] runs new coroutine and blocks current thread interruptibly until its completion. This function should not be used from coroutine. It is designed to bridge regular blocking code to libraries that are written in suspending style, to be used in main functions and in tests.
As quoted, it’s often used in tests that work with regular coroutines and also in main methods to wait for the completion of coroutines.
Also, this tutorial will help understanding its use cases.
I'm sorry for this late answer, but I hope you find it helpful.
1- Because in the first situation
runBlocking {
async(CommonPool) {
Thread.sleep(5000L)
}.await()
}
the runBlocking{} block will block the main thread until the code inside it finishes, then when we go inside the runBlocking{} block we find that you awaited on the async{} block, so we must wait until what's inside async{} finishes so we must wait for the 5 seconds. That why this code blocks the main thread.
2- But in the second situation:-
runBlocking {
async(CommonPool) {
launch(CommonPool) {
Thread.sleep(5000)
runOnUiThread { textView.text = "Finish!" }
}
}.await()
}
You didn't await on the launch{} block (via using .join()), so you the only thing you did inside the async{} block is launching the coroutine without waiting for it to finish. It is like that the async{} block is empty, so the async{}.await() and runBlocking{} will not wait for anything to finish. That's why this second situation doesn't block the main thread.
I hope this answers it.